The Cessna 120-140 Association Forum

Has anyone figured out a way to stop the brake fluid from coming out the tops of the master cylinders? I just replaced the O-Rings and filled the reservoirs about half way up. I have fluid coming up through the shaft hole and the lock slot of both cylinders when I push them.

Is it when you press down, or when you let up? The question is less of how to stop the fluid from coming out than why it is doing that. There are no seals, but the fluid should not be getting up there anyhow. Are they working properly? Absent any additional information, I'd guess the fluid is squirting up out of the cylinder rather than pressurizing the brake line.

I'm not sure if this is possible, but, you might try adding a little more fluid. Maybe, at about half, the fluid is not covering a port that fluid is supposed to flow out of at the top of the piston's stroke, and a little more fluid would damp that out. I always fill them to about 3/4" from the top (and have never seen fluid come out the top).

I did a little searching over the net and found some info about a guy having the same problem in a 170. The replies were about drilling a small hole in the filler plug. The guy I bought the plane from replaced the plugs with new brass plugs from the hardware store, and they don't have holes in them. The brakes are working but appears to be splashing fluid up the shaft. The missing vent holes might be the problem.

The correct part is an MS20913-1 plug, which is cross drilled for safety wire. You drill up from the base of the plug into the cross drill creating a "T" shaped vent. I guess if your plugs are unvented then the cylinders are venting through the space around the shaft, and carrying fluid with the air.

https://www.aircraftspruce.com/catalog/ ... /an913.php

We probably just "assumed" you have the little 1/8 pipe plugs threaded in the top of the master cylinder.

Yes they should have about a .040 hole right through the center of them. Do you have that?

I drilled a set yesterday and installed them. I'll have to see if that fixed the leaks up the shaft. The guy was filling the reservoirs before I came to get the plane and dropped them down the black hole, so he replaced them with shiny new brass plugs from the hardware store.

Sounds like maybe you shouldn't let that guy mess with your airplane...

4390 wrote: ↑Sat Mar 30, 2019 9:14 am I drilled a set yesterday and installed them. I'll have to see if that fixed the leaks up the shaft. The guy was filling the reservoirs before I came to get the plane and dropped them down the black hole, so he replaced them with shiny new brass plugs from the hardware store.

And that "Is" the black hole on these aircraft. Darn near impossible to get anything once it rolls under the pilot/co-pilot floor boards. They are riveted in place and it's a small gap, all the way back to the fwd gear box bulkhead, even if the part/hardware etc is magnetic, it's still practically impossible, unless you want to tip the airplane up on its nose and shake it.........

I normally wouldn't let anyone work on my airplanes but this was the guy I bought the plane from. He was getting it ready for me to ferry it back home. I wasn't there to oversee the work. I was thinking about running a small vacuum hose down in the black hole just to see what pops out.
Now it looks like I have a mushy right brake. When I flew it home the right brake needed pumping up every time I wanted to use it. I replaced both L&R O-rings, the left works fine but the right is mushy. There was a lot of crud in the right master that I cleaned out and the plunger has corrosion on the aluminum part but everything else looked fine. I didn't see any corrosion in the master. I'll have to check for air in the line or maybe the R master is bad.