Cherokee 180 soft brake - causes?

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.

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 *
************************************************** ***********************

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

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

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.

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

"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.

"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

"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

"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