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Cub brake servicing

longwinglover

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I posted the following on the yahoo! J3 site (http://groups.yahoo.com/group/piper-cub-builders/) back in March 2002. It was then reprinted in the Cub Club news letter and I have posted it on the Super Cub website (www.supercub.org). Hopefully it contains some information that will help you with your brakes.

At the time I wrote this, the Grove brakes were not available. I welcome any additions/inputs/comments from knowledgable Cub mechanics!

Hey guys (and gal),

I've read all the recent postings about problems with Goodrich brakes
and thoughts of changing over to Clevelands. After reading all this
I wonder if the problem is brake servicing, not worn out or "bad"
brakes??

The Cub brake system is really simple. Pressing on the heel brake
lever presses on a diaphram that in turn forces brake fluid in the
line to inflate the expander tube which in turn forces the brake pads
aginst the brake drum on the wheel. There are two places to service
the fluid: at the reservoir on the brake pedal (the master cylinder)
and a "bleeder valve" at the connection between the brake line and
the wheel.

In my experience two things can cause weak brakes in the Cub: worn
brake pads or air bubbles in the system.

If you remove a wheel you can see the brake pads. If you look
closely there is some sort of wear indicator on the pads themselves.
I forget if it is a groove or a notch on the end of the pad or what.
All of the pads should be worn the same amount. No, you can't put in
a couple of less worn ones and get better results - you would only be
rubbing on the newer ones for braking! I've heard that in the "good
old days" people would "shim" the brakes to get longer life either by
wrapping electrical tape around the expander tube or putting aluminum
shims under the pads.

Air bubbles in the system will cause weak brakes because the air will
compress, robbing the fluid (which doesn't compress) of the
mechanical (hydraulic) force to apply to the expander tube. To get
good brake effectiveness you need to get all the air out of the
system and keep it out.

How does air get in the system? There are two main culprits:
the "filler cap" on the master cylinder and the fittings in the brake
line. Look closely. The "filler cap" on the "master cylinder"
should have a washer/gromet that seals the system opening. Check
that this is in good condition.

Do you have any leaks or seeps of brake fluid that you notice on
preflight? Look where the brake line connects to the wheel at the
bottom of the landing gear "V". Look where the flexable line
connects at the top of the "V". Look where the flexable line
connects to the "master cylinder". If any of these areas are "wet"
either you have a loose connection or a cracked fitting. Obviously
try tightening things first. Clean up the area and look again after
a flight or two. If you are still getting seepage, you may need to
replace the fitting.

If fluid can be forced out, it means that air can get sucked in.
Keep air out!

Here are my thoughts on the BEST way to service your Goodrich Cub
brakes. You will need a small hand pump oil can filled with brake
fluid, some clear tubing, an adaptor to the bleeder fitting (I think
I made one from an 8x32 screw, drilled through its center and the
head ground off) and a screw type band clamp big enough to go around
the brake pads.

Drain the system by opening the bleeder valve and pumping on the
brake. Open the "filler cap" on the "master cylinder" and drain out
the residue.

Remove the wheel and put the band clamp around the brake pads and
tighten. This compresses the expander tube, forcing out any fluid
and AIR BUBBLES.

Service the system "from the bottom up" by connecting the pump can to
the "bleeder valve" with the adaptor and clear tubing (the clear
tubing helps you not put any air back in the system because you can
see the bubbles). Pump until the fluid fills the "master cylinder"
(it helps to have a buddy watch and let you know when it is full).

Close the bleeder valve. Release the band clamp from around the
brake pads. This should allow the expander tube to relax from its
compressed state, drawing in fluid (but hopefully no air) from the
line.

Top up the "master cylinder" of the fluid drawn into the expander
tube either "from the bottom" or from the top and replace (and
tighten) the "filler cap".

Theoretically, you have just completely serviced the system without
any air bubbles. Practically, you may have to do this a time or two
to work all the air out of the system.

If your brake pads are worn thus requirind a lot of pedal travel with
the system propery serviced, you can cheat a little. After you have
serviced as above, use the pump can to force a little extra fluid
into the system through the "bleeder valve". This "pre-expands" (my
word) the expander tube, making up for some brake pad wear. Have
someone spin the wheel as you do this and stop when you just hear the
pads start to rub. Careful, it is possible to put too much extra
fluid in, causing the brakes to drag!

Please remember to wear eye protection so you don't risk getting
brake fluid in your eyes if the tubing pops off during servicing!!

I see in the latest issues of the Cub Club (remember - this was written in 2002 JS)
newsletter that Univair is searching for someone to manufacture new
expander tubes. Is this the part that Cub brakes REALLY need? I'm not
convinced. Rubber deteriorates from exposure to ozone and. I think that the
expander tubes are pretty well protected all closed up in their
assemblies, even if removed from the plane and "lost" in the back of
the hanger. I think the parts that we really need are brake PADS.
The parts that wear out! These should be easier for someone to
produce than expander tubes.

I hope this helps some of you that are suffering from poor brakes. I
hope it also keeps some of you from converting to Clevelands! I
think it takes away from the looks of the Cub!

Best of luck,

John Scott
 

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