Cherokee 180 soft brake - causes?

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.

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

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.

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.

Originally Posted by 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.

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...

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

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.

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"

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?

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"