Cherokee 180 soft brake - causes?

[Nathan Young]

I have a 1971 Piper Cherokee 180.

I changed the oil earlier this week, and then taxiied around the
airport as part of my runup check.

During this process, I noticed that upon first application - the right
brake is soft. Pumping the brake pedal gets the pressure back, and
then it works ok. However, if I taxied for a while, and then tried
the right brake again, it would be soft. Pumping the brake would get
it back.

The left brake is fine. The parking brake exhibits the same soft
behavior, but pumping the handle allowed the parking brake to work
too.

I checked the fluid fill level on the reservoir and it is full. I
have not had any brake work recently, so I am not sure how air would
have been introduced to the system. I also did not notice any obvious
leaks or drips of fluid at either the master cylinders or the gear
mains.

The plane does not have copilot toe-brakes, so that is one less issue
to fight.

Question: Are the symptoms I describe part of normal brake wear? Ie
the pad wears down, and now the piston has to extend further? I don't
understand why this would cause softness though...

I have wheelpants with the K2U fairings, so it appears it is time to
remove the fairings, dig deeper, and have the A&P look at it.

[[email protected]]

On Fri, 24 Feb 2006 16:52:10 GMT, Nathan Young
> wrote:

snip

>Question: Are the symptoms I describe part of normal brake wear? Ie
>the pad wears down, and now the piston has to extend further? I don't
>understand why this would cause softness though...

When the master cylinders are static, fluid can flow through them to
make up the difference in volume from the pistons extending from
pad/rotor wear.

Once you apply toe pressure, the system closes, trapping the fluid
between the piston/s.

The hand brake cylinder is in series with the toe cylinders. The fluid
flows from the reservoir to the hand cylinder inlet, out of the hand
cylinder outlet to the inlets of the toe cylinders.

Sounds to me like you've got some air trapped in the one side, you are
compressing with the toe cylinder, and also with the hand cylinder
(via the route described above).

>I have wheelpants with the K2U fairings, so it appears it is time to
>remove the fairings, dig deeper, and have the A&P look at it.

Bleeding PA28 brakes can be a nightmare, lacking RH brakes makes it a
little easier. Find somebody that knows how to do it, and don't get
****y if they tell you they had to pull the toe cylinders and invert
them to get all the air out.

Regards;

TC

[Capt. Geoffrey Thorpe]

> wrote in message
...
> On Fri, 24 Feb 2006 16:52:10 GMT, Nathan Young
> > wrote:
>
> snip
>
>>Question: Are the symptoms I describe part of normal brake wear? Ie
>>the pad wears down, and now the piston has to extend further? I don't
>>understand why this would cause softness though...
>
> When the master cylinders are static, fluid can flow through them to
> make up the difference in volume from the pistons extending from
> pad/rotor wear.


>
> Sounds to me like you've got some air trapped in the one side, you are
> compressing with the toe cylinder, and also with the hand cylinder
> (via the route described above).
>

As Mr. Cutter suggests, air is the typical cause.

But, if you have a lot of runout in the disk it will push the pad back too
far and the first time you press the brake, it will take a lot of travel.
But you can usually also feel the brakes pulsing when it gets that bad.

Another posibility is something is loose (e.g. wheel bearing) and the rotor
wobbles when you go around a corner - again the first time you press, it
won't be firm. I had a van that would flex enough that when you cornered
real hard, the brakes would be soft the first time I went to use it. I guess
a 3/4 ton van just isn't intended to be driven like a sports car :-)

If the problem is rotor flex / movement, once you press the brakes they will
remain solid as long as you don't move the airplane. If they get spongy
after sitting still for a bit, the problem is most likely air - not wear.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

[streeter]

This happened to me in a -140. A&P bled the brakes and the problem "went away" for just over a year. Then when I brought it up at annual, a different A&P found it to be a cracked AN T-fitting connecting the hand brake to the two master cylinders. When the toe brakes retracted, air was being drawn in through the crack and eventially into the cylinders. It took about 20 min to spot and correct the problem.

I have a 1971 Piper Cherokee 180.

I changed the oil earlier this week, and then taxiied around the
airport as part of my runup check.

During this process, I noticed that upon first application - the right
brake is soft. Pumping the brake pedal gets the pressure back, and
then it works ok. However, if I taxied for a while, and then tried
the right brake again, it would be soft. Pumping the brake would get
it back.

The left brake is fine. The parking brake exhibits the same soft
behavior, but pumping the handle allowed the parking brake to work
too.

I checked the fluid fill level on the reservoir and it is full. I
have not had any brake work recently, so I am not sure how air would
have been introduced to the system. I also did not notice any obvious
leaks or drips of fluid at either the master cylinders or the gear
mains.

The plane does not have copilot toe-brakes, so that is one less issue
to fight.

Question: Are the symptoms I describe part of normal brake wear? Ie
the pad wears down, and now the piston has to extend further? I don't
understand why this would cause softness though...

I have wheelpants with the K2U fairings, so it appears it is time to
remove the fairings, dig deeper, and have the A&P look at it.

[Nathan Young]

On Fri, 24 Feb 2006 22:51:31 GMT, wrote:

>On Fri, 24 Feb 2006 16:52:10 GMT, Nathan Young
> wrote:
>
>snip
>
>>Question: Are the symptoms I describe part of normal brake wear? Ie
>>the pad wears down, and now the piston has to extend further? I don't
>>understand why this would cause softness though...
>
>When the master cylinders are static, fluid can flow through them to
>make up the difference in volume from the pistons extending from
>pad/rotor wear.
>
>Once you apply toe pressure, the system closes, trapping the fluid
>between the piston/s.
>
>The hand brake cylinder is in series with the toe cylinders. The fluid
>flows from the reservoir to the hand cylinder inlet, out of the hand
>cylinder outlet to the inlets of the toe cylinders.
>
>Sounds to me like you've got some air trapped in the one side, you are
>compressing with the toe cylinder, and also with the hand cylinder
>(via the route described above).

Thanks for the info, I appreciate it. Several years ago, when I first
bought the plane, it had air in the brake lines, and myself, my IA and
one of his techs spent quite a while chasing the bubbles away.

I am not looking forward to that experience again...

[[email protected]]

wrote:
: Bleeding PA28 brakes can be a nightmare, lacking RH brakes makes it a
: little easier. Find somebody that knows how to do it, and don't get
: ****y if they tell you they had to pull the toe cylinders and invert
: them to get all the air out.

I'd just like to add that I've heard the same thing from my A&P/IA. He had to
bleed the brakes on a friend's PA28. Even with force (bottom-up) bleeding, they ended
up having to disassemble the master cylinders. Apparently there's some sort of seal
in there that causes all the heartburn when trying to bleed.

I really should to replace the brakelines to the discs on my plane, but I've
been dreading the inevitable bleed. I'm in the same boat, too.... LH toe-brakes only.
Mine leak down very slowly somewhere so the parking brake doesn't hold for more than a
few minutes. It doesn't seem to use any fluid though, so I haven't cared so far in
the 3.5 years we've owned it.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[Mike Noel]

I've never had to do work on my Archer's brakes, but in the auto's I've
worked on this was a symptom of a leaking piston seal in the master
cylinder. For the auto you can either hone the cylinder and get a new O
ring for the piston, or just replace the entire master cylinder. I think
the problem can develop because of corrosion caused by water absorbed by the
brake fluid over time.

--
Best Regards,
Mike
http://photoshow.comcast.net/mikenoel
"Nathan Young" > wrote in message
...
>I have a 1971 Piper Cherokee 180.
>
> I changed the oil earlier this week, and then taxiied around the
> airport as part of my runup check.
>
> During this process, I noticed that upon first application - the right
> brake is soft. Pumping the brake pedal gets the pressure back, and
> then it works ok. However, if I taxied for a while, and then tried
> the right brake again, it would be soft. Pumping the brake would get
> it back.
>
> The left brake is fine. The parking brake exhibits the same soft
> behavior, but pumping the handle allowed the parking brake to work
> too.
>
> I checked the fluid fill level on the reservoir and it is full. I
> have not had any brake work recently, so I am not sure how air would
> have been introduced to the system. I also did not notice any obvious
> leaks or drips of fluid at either the master cylinders or the gear
> mains.
>
> The plane does not have copilot toe-brakes, so that is one less issue
> to fight.
>
> Question: Are the symptoms I describe part of normal brake wear? Ie
> the pad wears down, and now the piston has to extend further? I don't
> understand why this would cause softness though...
>
> I have wheelpants with the K2U fairings, so it appears it is time to
> remove the fairings, dig deeper, and have the A&P look at it.
>

[Jay Honeck]

> I really should to replace the brakelines to the discs on my plane, but
> I've
> been dreading the inevitable bleed.

Our A&P hates bleeding Cherokee brakes, but it's more of a repetitive pain
than a really difficult procedure. He just climbs under the wing, holds the
line (or brake assembly?), and has me pump the brakes until nothing but
fluid comes out. It can take several pumps, and uses a bit of brake fluid,
but it works well. (I've not seen what he's doing under there, since I'm
inside the plane, but he's catching the fluid in a container of some sort.)

We've had to disassemble the right brake a few times, because the piston is
scored and is starting to eat O-rings (which results in slow drip), so I've
helped him with this a few times. Next time I'll ask him what he's doing
under there in more detail.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Newps]

Jay Honeck wrote:

> Our A&P hates bleeding Cherokee brakes, but it's more of a repetitive pain
> than a really difficult procedure. He just climbs under the wing, holds the
> line (or brake assembly?), and has me pump the brakes until nothing but
> fluid comes out. It can take several pumps, and uses a bit of brake fluid,
> but it works well. (I've not seen what he's doing under there, since I'm
> inside the plane, but he's catching the fluid in a container of some sort.)

They can't be bled from the bottom up?

[Jay Honeck]

>> Our A&P hates bleeding Cherokee brakes, but it's more of a repetitive
>> pain than a really difficult procedure. He just climbs under the wing,
>> holds the line (or brake assembly?), and has me pump the brakes until
>> nothing but fluid comes out. It can take several pumps, and uses a bit
>> of brake fluid, but it works well. (I've not seen what he's doing under
>> there, since I'm inside the plane, but he's catching the fluid in a
>> container of some sort.)
>
> They can't be bled from the bottom up?

They can, but this is my A&P's method of preventing the OP's problem of
eternally soft brakes. It apparently ensures that no air is left hiding
anywhere in the system.

All I know is that it works. My brakes aren't mushy.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Newps]

Jay Honeck wrote:
>>>Our A&P hates bleeding Cherokee brakes, but it's more of a repetitive
>>>pain than a really difficult procedure. He just climbs under the wing,
>>>holds the line (or brake assembly?), and has me pump the brakes until
>>>nothing but fluid comes out. It can take several pumps, and uses a bit
>>>of brake fluid, but it works well. (I've not seen what he's doing under
>>>there, since I'm inside the plane, but he's catching the fluid in a
>>>container of some sort.)
>>
>>They can't be bled from the bottom up?
>
>
> They can, but this is my A&P's method of preventing the OP's problem of
> eternally soft brakes. It apparently ensures that no air is left hiding
> anywhere in the system.

I know, that's why you do it from the bottom up, no pumping the brakes
required. My mechaninic rebuilt my left main strut this week and since
that requires disassembly of the brake system on that side I helped him
today bleed the brakes. Reason #6458 why Beech has the reputation they
do for quality and good engineering and design. The reservoir is on the
firewall, no having to wiggle over the front seats to wait for the fluid
to come squirting out onto your carpet.

[[email protected]]

Newps > wrote:
: I know, that's why you do it from the bottom up, no pumping the brakes
: required. My mechaninic rebuilt my left main strut this week and since
: that requires disassembly of the brake system on that side I helped him
: today bleed the brakes. Reason #6458 why Beech has the reputation they
: do for quality and good engineering and design. The reservoir is on the
: firewall, no having to wiggle over the front seats to wait for the fluid
: to come squirting out onto your carpet.

Not to argue since all I know is heresay at this point, but my mechanic had
problems bleeding a guy's cherokee *even though* they were bleeding bottom-up.
Top-down, bottom-up, it didn't seem to matter.... they couldn't get all the air out.
Ended up having to replace some weird seal/gasket/valve thingy (don't know the
specifics). Apparently it's a common thing to have to replace... maybe why all the
FUD about bleeding Cherokee brakes. Perhaps it's only a PITA if this other thing is
broken. Sounds like Jay doesn't have any trouble.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[[email protected]]

On Sun, 26 Feb 2006 14:55:26 +0000 (UTC),
wrote:

snip

> Not to argue since all I know is heresay at this point, but my mechanic had
>problems bleeding a guy's cherokee *even though* they were bleeding bottom-up.
>Top-down, bottom-up, it didn't seem to matter.... they couldn't get all the air out.
>Ended up having to replace some weird seal/gasket/valve thingy (don't know the
>specifics). Apparently it's a common thing to have to replace... maybe why all the
>FUD about bleeding Cherokee brakes. Perhaps it's only a PITA if this other thing is
>broken. Sounds like Jay doesn't have any trouble.

really depends where the air is trapped in the system. the primary
problem with the Cherokee's is the fluid inlet to the toe cylinders is
on the bottom of the near-vertically mounted cylinder (and goes "up"
to the handbrake and reservoir). the outlet is on the top, and goes
"down" to the calipers.

pressure bleeding from the reservoir or from the caliper is
problematic due to this routing. a little air here or there can
usually be worked out, but if there is a lot of air trapped, the
quickest solution is to remove the toe cylinders (mechanically, not
hydraulically) and invert them. a pressure pot from the bottom will
then clear the lines fairly rapidly.

the nastiest one I ever worked was on a Cheyenne that wouldn't leak
fluid out static or under applied brake pressure, but would leak air
in when the cabin was pressurized. if I hadn't had seen it, I never
would have believed it.

TC

[David Lesher]

writes:



>> 've never had to do work on my Archer's brakes, but in the auto's I've
>> worked on this was a symptom of a leaking piston seal in the master
>> cylinder.


>I agree. The most likely cause of the symptoms you describe is a "leaking"
>master cylinder.


There's an easy test for air vs leaking master, at least in cars.

Pump up & stand on the brakes, hard. Hold for 1-2 minutes. If it is
the master, the peddle will sink toward the floor. If it is air; it
will stay the same.

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Aaron Coolidge]

Mike Noel > wrote:
: I think
: the problem can develop because of corrosion caused by water absorbed by the
: brake fluid over time.

Aircraft brake fluid is hydraulic OIL, and does not absorb water in the
same manner that automotive brake fluids do.
--
Aaron C.

[David Lesher]

Aaron Coolidge > writes:

>: the problem can develop because of corrosion caused by water absorbed by the
>: brake fluid over time.

>Aircraft brake fluid is hydraulic OIL, and does not absorb water in the
>same manner that automotive brake fluids do.

Wow; I'm surprised.... I'd think oil would absorb air far too easily.
That's SURE to cause spongy brakes...

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Dave Stadt]

"David Lesher" > wrote in message
...
> Aaron Coolidge > writes:
>
>>: the problem can develop because of corrosion caused by water absorbed by
>>the
>>: brake fluid over time.
>
>>Aircraft brake fluid is hydraulic OIL, and does not absorb water in the
>>same manner that automotive brake fluids do.
>
> Wow; I'm surprised.... I'd think oil would absorb air far too easily.
> That's SURE to cause spongy brakes...

I've never seen or heard of oil absorbing air.

[David Lesher]

"Dave Stadt" > writes:

>> Wow; I'm surprised.... I'd think oil would absorb air far too easily.
>> That's SURE to cause spongy brakes...

>I've never seen or heard of oil absorbing air.

Well, overfill a crankcase and watch the oil get whipped into a
froth, and rapidly lose its lubrication qualities...

Automotive brake fluid was, ISTM, the very first federal
automotive safety standard. It must have:

High boiling point
low air absorbation
compatability with rubber lines and seals

There were a few cars, but VERY few, that needed non-standard brake
fluid. One was a friend's Hillman Minx (Mynx?) that Goodyear or
Firestone or such had put DOT-3 in; every seal was gone. The other
was the Citroen DS with master hydraulic pump that ran brakes,
steering, suspension and transmission. It used exotic "green blood"
and every owner carried a spare 10 litres or so...just in case...




--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Dave Stadt]

"David Lesher" > wrote in message
...
> "Dave Stadt" > writes:
>
>>> Wow; I'm surprised.... I'd think oil would absorb air far too easily.
>>> That's SURE to cause spongy brakes...
>
>>I've never seen or heard of oil absorbing air.
>
> Well, overfill a crankcase and watch the oil get whipped into a
> froth, and rapidly lose its lubrication qualities...

That's not exactly what could happen in a brake circuit. We are talking
brake circuits not crankcase. Leave oil exposed to the atmosphere and it
does not absorb air.

> Automotive brake fluid was, ISTM, the very first federal
> automotive safety standard. It must have:

Traditional automotive brake fluid is alcohol based and is hydroscopic. A
can will become useless in a matter of hours if not tightly capped. It is
not oil based.

> High boiling point
> low air absorbation
> compatability with rubber lines and seals
>
> There were a few cars, but VERY few, that needed non-standard brake
> fluid. One was a friend's Hillman Minx (Mynx?) that Goodyear or
> Firestone or such had put DOT-3 in; every seal was gone. The other
> was the Citroen DS with master hydraulic pump that ran brakes,
> steering, suspension and transmission. It used exotic "green blood"
> and every owner carried a spare 10 litres or so...just in case...

Anybody that drove or drives a Citroen needs more than 10 litres of green
blood. A Priest, a tow truck and a credit card with no limit is a good
place to start.

> --
> A host is a host from coast to
> & no one will talk to a host that's close........[v].(301) 56-LINUX
> Unless the host (that isn't close).........................pob 1433
> is busy, hung or dead....................................20915-1433

[David Lesher]

"Dave Stadt" > writes:


>That's not exactly what could happen in a brake circuit. We are talking
>brake circuits not crankcase. Leave oil exposed to the atmosphere and it
>does not absorb air.

Nope, just water..... but in any case the key issue is having a
fluid that has very low compressibility even at high temperatures.
(Look at the NOVA series on the 777; the rejected-takeoff test has
the solid carbon rotors glowiing bright orange as the rims melt...now
THAT'S hot....). Even a small amount of suspended air makes it
compressable. And oil, unlike brake fluid, does not err ?stratify?
well.

>> Automotive brake fluid was, ISTM, the very first federal
>> automotive safety standard. It must have:

>Traditional automotive brake fluid is alcohol based and is hydroscopic. A
>can will become useless in a matter of hours if not tightly capped. It is
>not oil based.

Ahem.. it's not alcohol-based; it's glycol based. (Except DOT-5 which is silicon..)


>> High boiling point
>> low air absorbation
>> compatability with rubber lines and seals

>Anybody that drove or drives a Citroen needs more than 10 litres of green
>blood. A Priest, a tow truck and a credit card with no limit is a good
>place to start.

Not at all; you merely need 2 tools: a cell phone and a pickup
w/trailer. Just like any MG/Jag owner...

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Dave Stadt]

"David Lesher" > wrote in message
...
> "Dave Stadt" > writes:
>
>
>>That's not exactly what could happen in a brake circuit. We are talking
>>brake circuits not crankcase. Leave oil exposed to the atmosphere and it
>>does not absorb air.
>
> Nope, just water..... but in any case the key issue is having a
> fluid that has very low compressibility even at high temperatures.
> (Look at the NOVA series on the 777; the rejected-takeoff test has
> the solid carbon rotors glowiing bright orange as the rims melt...now
> THAT'S hot....). Even a small amount of suspended air makes it
> compressable. And oil, unlike brake fluid, does not err ?stratify?
> well.
>
>>> Automotive brake fluid was, ISTM, the very first federal
>>> automotive safety standard. It must have:
>
>>Traditional automotive brake fluid is alcohol based and is hydroscopic. A
>>can will become useless in a matter of hours if not tightly capped. It is
>>not oil based.
>
> Ahem.. it's not alcohol-based; it's glycol based. (Except DOT-5 which is
> silicon..)

Ahem..glycol is.....well why don't you look it up.

[David Lesher]

"Dave Stadt" > writes:


>> Ahem.. it's not alcohol-based; it's glycol based. (Except DOT-5 which is
>> silicon..)

>Ahem..glycol is.....well why don't you look it up.

Could be, I'm no chemist (and got a C in the subject to prove it...).
But everything I've read on it describes it as glycol. The other
one most of us think of is anti-freeze. Time was, that was
alcohol...well, it used to be it evaporated regularly.

To solve that {and corrostion issues...} Ethylene glycol as "permanent
antifreeze" came on the scene in the 1950's/60's. If that's an
alcohol, then I've learned something new. It sure doesn't burn like
one...



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Tauno Voipio]

David Lesher wrote:
> "Dave Stadt" > writes:
>
>
>
>>>Ahem.. it's not alcohol-based; it's glycol based. (Except DOT-5 which is
>>>silicon..)
>
>
>>Ahem..glycol is.....well why don't you look it up.
>
>
> Could be, I'm no chemist (and got a C in the subject to prove it...).
> But everything I've read on it describes it as glycol. The other
> one most of us think of is anti-freeze. Time was, that was
> alcohol...well, it used to be it evaporated regularly.
>
> To solve that {and corrostion issues...} Ethylene glycol as "permanent
> antifreeze" came on the scene in the 1950's/60's. If that's an
> alcohol, then I've learned something new. It sure doesn't burn like
> one...

A hint: glycols are two-valued alcohols (two hydroxyl groups).

--

Tauno Voipio
tauno voipio (at) iki fi