Brake master cylinder

[dstates]

I’ve read a few brake cylinder threads on here and am starting this new one so I don’t get confused with previous issues. I have the master cylinders that had the longer parking break levers (now removed) and the lock-o-seal. During my annual inspection my A&P and I found my right brake was not building pressure. After attempting to bleed it from the wheel it made no difference. So I disassembled the master cylinder and replaced the lock-o-seal and the piston o-ring. I adjusted the lock-o-seal to 0.040” gap from the piston to lock-o-seal washer with the piston spring compressed. After re-assembly, there was no difference. The brake pedal had full travel with no resistance. After checking it again, I put in a second lock-o-seal (I had purchased an extra to have around). Still no change. Now I have the master cylinder completely out and will do a thorough cleaning.

What else should I try?

Doug

[johneeb]

Doug, it the bleeder on the top hole or bottom hole on your brake caliper? Very hard to bleed from the bottom if the bleeder is in the top hole.

[dstates]

Doug, it the bleeder on the top hole or bottom hole on your brake caliper? Very hard to bleed from the bottom if the bleeder is in the top hole.

Top hole of brake caliper, but we were using a brake bleeding system that you pump up. Would that location still matter?
Doug

[c170b53]

I use this exact pump, bleed screw on bottom, use a tygon tubing between the pump and bleed screw with a high spot for bubbles. Super easy to pump from bottom and remove all air.

[GAHorn]

From another thread:

PLain old ordinary plastic Garden Sprayer cost's $5 at WalMart and works beautifully! Cut the nozzle off the end of the wand and slip a piece of clear plastic hose on the end, and push the hose directly onto the bleeder fitting.

https://www.walmart.com/ip/RL-Flo-Maste ... r/13376324

(My earlier post was in reference to the Lock-o-seal only.). I have had only good results with using a garden sprayer converted to a hydraulic service tool.

[dstates]

From another thread:

PLain old ordinary plastic Garden Sprayer cost's $5 at WalMart and works beautifully! Cut the nozzle off the end of the wand and slip a piece of clear plastic hose on the end, and push the hose directly onto the bleeder fitting.

https://www.walmart.com/ip/RL-Flo-Maste ... r/13376324

(My earlier post was in reference to the Lock-o-seal only.). I have had only good results with using a garden sprayer converted to a hydraulic service tool.

My A&P has this garden sprayer like bleeder. Any other ideas besides trying to bleed it again?

[gfeher]

Have you also replaced the piston O-ring (item 9 in your diagram)? Might as well put a new one in since you have everything apart.

[dstates]

Have you also replaced the piston O-ring (item 9 in your diagram)? Might as well put a new one in since you have everything apart.

Yes, I did replace the piston o-ring. I forgot to mention that.

Doug

[johneeb]

Doug, it the bleeder on the top hole or bottom hole on your brake caliper? Very hard to bleed from the bottom if the bleeder is in the top hole.

Top hole of brake caliper, but we were using a brake bleeding system that you pump up. Would that location still matter?
Doug

With the bleeder on top of the caliper you might try bleeding from the top down, like we usually do auto brakes, Keep the reservoir full and have a mate pump the brake peddle while you open and close the bleeder.

[Bruce Fenstermacher]

With the piston pushed to close the .040” gap, the lock-o-seal must seal to the piston. Otherwise no presume will be built. The lock-o-seal and the piston create a one way valve. It must seal when the piston rod is pushed in, the seal must open as the piston rod is returned out by the spring.

You can test this without installing the master cyl. In the air craft. Put a cap on the exit line. Pushing in on the piston shaft should produce pressure. If not your lock-o-zeal is not sealing. Assuming of course the other o-rings are installed correctly and still in working order.

Btw it is not .040 from the seal to the piston. It’s .040 from the edge of the piston shaft to the piston.

After reading Doug's comment below and relooking at the image, he is right and my observation is wrong. It is .040 from the washer of the lock-o-seal to the piston

[dstates]

With the piston pushed to close the .040” gap, the lock-o-seal must seal to the piston. Otherwise no presume will be built. The lock-o-seal and the piston create a one way dealbreaker it must seal when the piston rod is pushed in, the seal must open as the piston rod is returned out by the spring.

You can test this without installing the master cyl. In the air craft. Put a cap on the exit line. Pushing in on the piston shaft should produce pressure. If not your lock-o-zeal is not sealing. Assuming of course the other o-rings are installed correctly and still in working order.

Btw it is not .040 from the seal to the piston. It’s .040 from the edge of the piston shaft to the piston.

Bruce,
Per the image from the service manual I shared in my original post, the .040" is from the seal washer. The cross hatched part the dimension goes to is the seal and the shaft is not hatched in the image. Let me know if this is an error in the manual.

Doug

[MoonlightVFR]

Dear dstates

You can do this. You already know some stuff.

My A/P does amazing work w the simple little pump oiler filed w aircraft brake fluid.

Helps w two persons working together but not really necessary.

A key factor may be to have a clear line from top plug hole of the brake Master cylinder to a small glass jar setting on cockpit floor beside the brake pedal.

You advise person down by the wheel brake to crack 1/4 Turn and pump. And then lock. If there is an air leak it pumps brake fluid w a Stream of bubbles. Obviously you do not want bubbles. You pump brake by hand and observe when the bubbles STOP coming .

Person down by wheel can see brake clamping action - cannot rotate wheel by hand.

MOst pilot owners do not have a command of Lock O seal. You will be able to go to a White board and explain the functions to group of pilots. We will be deeply impressed when Part Numbers roll off your tongue as you speak.

Do This

[GAHorn]

OK. Boys.... Let’s play nice.... I’m the only one licensed to pizz people off indiscriminately even when I never intended it.

[c170b53]

How about this; brake pedal at rest; the upper spring causes the piston to contact the compensating sleeve, the piston rod is pushed further up by the lower spring (adjustment) ( the adjustment is really an adjustment of the upper spring compression) and creates a .040 gap between the lock seal and piston which allows fluid to fill the lower chamber. Loosening nut 12 should create a larger gap.
Brake application, Pushing the pedal, causes the piston rod to extend through the sleeve, the lock seal on the end of the piston rod, seals against the piston sealing the lower chamber. Pushing further applies the brake. Is the lock seal binding on the compensating sleeve bore and or is the pedal not fully returning to neutral ?
Or what Bruce said.

[dstates]

How about this; brake pedal at rest; the upper spring causes the piston to contact the compensating sleeve, the piston rod is pushed further up by the lower spring (adjustment) ( the adjustment is really an adjustment of the upper spring compression) and creates a .040 gap between the lock seal and piston which allows fluid to fill the lower chamber. Loosening nut 12 should create a larger gap.
Brake application, Pushing the pedal, causes the piston rod to extend through the sleeve, the lock seal on the end of the piston rod, seals against the piston sealing the lower chamber. Pushing further applies the brake. Is the lock seal binding on the compensating sleeve bore and or is the pedal not fully returning to neutral ?
Or what Bruce said.

Jim,

I agree with your explanation and Bruce's comments on how it should work. That is how I understood it as well. I don't believe the lock-o-seal is binding on the compensating sleeve because with the cylinder assembled I could feel that .040" movement at the start of pressing on the brake when I imagine the rod is moving to push the lock-o-seal up against the piston. I'll confirm no binding by making sure the compensating sleeve will slide over the lock-o-seal.

I'll likely be back at the hangar this afternoon. I have the cylinder out of the plane and will clean it and make sure it is sealing when I push down on the rod. If that doesn't work I'll adjust it so there is .040" clearance from the piston to the piston rod shoulder like Bruce mentioned instead of how I read the service manual. I'm tempted to take out my other cylinder rod to see how it is adjusted, but it is working so I don't want to touch it

Hopefully I can get that where I feel like it is sealing when pushing down on the rod and then install it and try to bleed it again.

Moonlight,
When we were bleeding the cylinder, I had a clear line connected to the fill port at the top of the cylinder running to a cup. I was the one in the cabin and my A&P was at the wheel. We did not move a lot of fluid through the system through the clear line and into the cup, but we didn't have bubbles. How much fluid do you expect to need to push through the line to verify no air (how much should be in the cup)? I feel like that is one thing I could do is try to run a lot of fluid through the line.

The other option is try to bleed from the top down like johneeb suggested.

Doug