The Cessna 120-140 Association Forum

Left brake pedal is intermittently spongy.. Have bled several times but still the same so looking to change the master cylinder plunger o ring...

Is there a preferred way to tackle this...

Should I remove the cylinder...if that's easy to achieve..

Anyone have some images of the steps, that would be great..

Many thanks.

What kind of brakes do you have? If Clevelands, there are only two "O" rings involved. One small one in the master cylinder and one larger one at the piston in the wheel. Very easy to change. Are you leaking any fluid at the wheel? Bleeding is not hard if you force fluid up through the system at the drain valve until the master cylinders are about 2/3 full. (Assuming Clevelands)

Yes they are clevelands..

I replaced the piston O ring some time ago in the caliper... Theres no fluid on the floor, so looking at the master cylinder as the culprit?

Half the time when you press the pedal its stiff as it should be and then it will be soft, with the pedal able to be pressed all the way and little braking action. Guessing fluid leaking past the o ring?

I replaced the master O-rings and fluid a few years ago. Was a pain but EZ. I just removed the bolt that connects the plunger to the pedal. Unscrewed the top and pulled out the plunger (put rags all around the work area). Took a cup and 1/4" hose and removed all the fluid in the reservoir. Some carb cleaner and paper towels to clean out any grunge in the reservoir. New O-ring, screwed the top back on, installed the plunger to the pedal, removed the vent cap and filled (3/4 full) fluid from the caliper up to the master. I went from NO brakes to solid as a rock brakes L&R. Took about 2 hours. The hardest part is the pedals.

If you have the original masters, there's only a single o-ring to fail. There is a pin, vertical, that, when the piston tops out, pushes down on one bit of the o-ring to cock it and break the seal, so the fluid can move in and out of the reservoir. After 40 or so years the o-ring takes a set and doesn't seal properly when you press on the pedal. A new $0.69 o-ring fixes it.

Another thing to check, depending on the brake model you have, is the clearance between the piston rod and piston. See attached diagram, it should be set to .040" My understanding is that this gap allows the piston to 'lag' - to allow displaced fluid to return to the bottom side of the piston. A new o-ring (Lock o Seal) and setting this clearance to .040" fixed a spongy/non-leaking brake for me.

My Goodyear master cylinders with Cleveland brakes developed a soft right brake that always had to be pumped up with the brake pedal to be even and effective. Finally found that the spring was installed upside down and the small end was broken. Found replacements at Smokin' Rivits Aircraft Salvage. Up and running.

I have a brake fluid leak that does not seem to be coming from the master. It is within the fuselage somewhere and is a slow leak requiring a top up of fluid every 20h or so. A drip forms on the belly every so often. What are the potential leak points downstream of the master cylinder but still within the fuselage? 1946 140. Clevelands.

Thanks.

There is a flexible hose at the base of the master cylinder that connects to a hard line that runs along the side, above the floor, IIRC, to the gear box and exits at the base of the gear leg. It could be the fitting on the master, the hose, where the hose connects to the hard line, or the fitting at the gear leg, or possibly the hard line itself.

atypicalguy wrote: ↑Thu Jun 10, 2021 11:04 pm I have a brake fluid leak that does not seem to be coming from the master. It is within the fuselage somewhere and is a slow leak requiring a top up of fluid every 20h or so. A drip forms on the belly every so often. What are the potential leak points downstream of the master cylinder but still within the fuselage? 1946 140. Clevelands.

Thanks.

atypicalguy, it sounds to me like you've got the same problem I have had. Twice now I have had to remove the RH gear leg and replace the 6 inch long horizontal brake line that connects to the top of the gear leg brake line. It is very difficult to access and very difficult to fabricate and get the angles just right. In my case the root cause of this problem was a PK screw which fastens the gear leg fairing to the fuselage puncturing through that six inch long horizontal brake line. Needless to say I am no longer using a PK screw through that particular hole.