Brake Master Cylinder

[robrien]

Lots of posts on brakes - I did a quick search but did not see this - apologies if I missed it (please send link).
Issue:
I have cleveland brakes on my 170B, and having issue with air in the system - the brake is solid on the ground after being bleed from the bottom up (fluid pushed up from the brake bleed screw), and after about 30 minutes flying, the pedal goes to the floor. Another 60 seconds of furiously pumping the pedal, and I have pressure again. This will hold, but then there might be one pedal pump during a subsequent landing rollout, and then the brakes are back to solid. Is this something commonly seen? What is the cause, and what should be done to resolve this? Thanks!

[Bruce Fenstermacher]

Probably not air but the Lock O Seals need to be replaced in your master cylinders. If the seal does not seal there will be no pressure when you push the brake pedal, just like it would seem the master cylinder has air in it. And this can be sporadic. Pressure nearly every time but when you need it.

See this thread: viewtopic.php?f=24&t=12516&p=118268&hilit=lock+o+seal#p118268

[robrien]

thanks Bruce - I also saw another post of yours referencing the seal replacement - I can leave the cylinder in place and just diassemble the pedal from the shaft/piston asym and simple remove it. Is it that simple?

[robrien]

Here's an image of the master cylinder that's currently installed - is this the original from 1952? I will have an A&P rebuild for me, but nice to have the info. Notice the fluid at the bottom - the filler screw (with I assume vent) is at the top. Not sure what this plate does, as there is a spring that fits on this - any insight? I looked at the exploded parts diagram you provided a link to, and did not see this part?

[Bruce Fenstermacher]

thanks Bruce - I also saw another post of yours referencing the seal replacement - I can leave the cylinder in place and just diassemble the pedal from the shaft/piston asym and simple remove it. Is it that simple?

Yep, it's a pain but it can be done.

Yes it is a pain, yes you can disassemble just the top leaving the bottom and hoses attached. More of a pain is removing the bottom end as well.

[c170b53]

Not sure what this plate does, as there is a spring that fits on this - any insight?

The plate that has the shaft going through it ?: is for a parking brake cable /system. I think there a few different styles of these levers most of which are removed for safety purposes. Just about everyone has removed theirs; suggest you search and read related posts.

[robrien]

thanks Jim - the emergency brake bracket is not hooked up to anything - appreciate the info.

[Bruce Fenstermacher]

It is the tabs on the cylinders that can be activated and they should be removed when you go through the trouble of disassembling the master cylinder in my opinion. Disconnecting them from the control cable is only the first (and easiest) step.

[robrien]

Bruce - looked through a bunch of posts, and saw the reference to the tab left on the cylinder - can you provide any links to steps to remove this, or diagrams? I did not see any posts in my initial search.
I would assume that when rebuilding the cylinder, all that's required is to slide it off the shaft assembly - or is there more to it?
From your, and other comments, it seems that this can move and stick in position, essentially locking the brake shaft on the master cylinder, which will, in turn, lock the brake on the pilot side, correct?

[robrien]

Perfect - thanks!

[Bruce Fenstermacher]

The tab works by jamming the shaft after brake pressure is applied by the cable lifting the tab. removing the cable only removes the possibility of the cable lifting the tab unintentionally as the peddles are pushed. The tab by itself can still make contact with fire wall material and be unintentionally and unknowingly engaged. Removing the tabs at disassembly is easy if your disassembling. If your not planing to disassemble to insure the tab is not activated they should be wired down.

Note: all this tab business does not apply to early master cylinders with serrated shaft as that system is different.

[MT_Don]

Ok. I have a dumb question. On my 1948, I cannot locate the master cyl. On later models, I note it is behind each pedal. Mine pushes a tube down to a torque tube that travels into the center. Is it in there? The belly access plate reveals only the heater tube passing.. I'm replacing the O rings, and need to bleed the brakes. Thanks! Don

[sfarringer]

Well, your '48 must be different than my '48!

The master cylinders on mine are directly behind the left side set of rudder/brake pedals.
The right side set of rudder/brake pedals are connected to the left side set of rudder/brake pedals by the torque tubes that you speak of.

You're not looking behind the right side set of rudder/brake pedals, are you?

[GAHorn]

Well, your '48 must be different than my '48!

The master cylinders on mine are directly behind the left side set of rudder/brake pedals.
The right side set of rudder/brake pedals are connected to the left side set of rudder/brake pedals by the torque tubes that you speak of.

You're not looking behind the right side set of rudder/brake pedals, are you?

Right side? LOL. (Dual meanings)

Co-pilot/starboard pedals use the torque-tubes to activate the pilot-side (port side) brake cyls.

Trivia Note:

Left/port/RED all have fewer letters per word than
Right/Starboard/Green.