The Stovebolt Site Forums The Shop Area Driveline setting master cylinder rod length

1950 gmc.1/2 ton original huck brakes. new master cylinder and wheel cylinders. also new brake pads. How does one determine at what length the m/c shaft should be set at. Too long and the brake pedal gets hard and brake pads rub and heat. of course at this point the heat from rubbing pads causes the fluid to expand and increases the problem. Adjust the other way and I pump up the brakes ,but within 1 minute I lose brake pedal. goes to the floor. Whats up ? I've changed all but one brake line. The back one on the rear axle. It will be changed soon. I guess it could be delaminating ? Am i missing anything ? Thanks in advance for all help.

Read the service manual, instructions for adjusting that linkage so the pedal arm is a set distance off the firewall/floor.

Do you have a return n sprig in place to pull the pedal back up? It attaches to a bracket under the steering box.

Grigg

return spring is in place and seems to have good force. To clarify. when i lose brake pedal the brake returns to the up position correctly. I just don't have brakes until i pump up the brakes again.At this point they last only a minute.

Hy can216, do you have your brake shoes adjusted up correctly? There should be a clearance between the pedal linkage and the master cylinder plunger with everything at rest, should be 1/16" to 1/8". Replace the last line, adjust the brakes up, adjust the linkage to plunger gap, rebleed the system and see what kind of brakes you get, hope that helps.

Thanks for the advise. I will check all the above. My back brakes seem set correct but the fronts i will pull apart and inspect. Been awhile since i have been in there. But first the brake hose gets replaced. Thanks sgain. One more question. Just how touchy is the m/c rod adjustment. Would a half revolution of the rod make the difference of a hard pedal to loseing the pedal . Could it be just that fine of an adjustment. Could it be i just need to find the sweet spot ?

Hy can216, sorry I haven't been back to your question, the master cylinder piston must be allowed to return to its fully at rest position to uncover the compensating port. We try and give a minimum of 1/16" clearance between the master cylinder piston and the pedal linkage so that when everything heats up the master cylinder piston will still return far enough to uncover the compensating port, hope that helps.

Thought I had it working . But as it looks after an extended drive shortly after I stop the brake pedal gets hard. Which means the compensating port is not uncovered at that point . When I adjust the m. Cylinder rod out further then pedal releases and can be pushed to the floor with no force. Can be pumped up but only lasts seconds. Can I be adjusting the rod out to far? It all works fine when the shaft is adjusted shorter on short drives cause it does not have time to build heat pressure. Any ideas ? Thanks

can216 - what was your final solution on this? I have been struggling with my '50 3100's brakes for way too long with no success.

Where a master cylinder is concerned, "new" doesn't necessarily mean "good". The symptom you're describing is classic master cylinder piston seal failure. Pumping the pedal allows the cup seal to expand, but as soon as the pedal pressure is released the cup shrinks. If you adjust the linkage far enough forward to keep pressure on the brake lines, the cup stays expanded, but the brakes drag.

Keep changing the master cylinder until you get a good one.
Jerry

Your shoes need adjusting up close to the drum or you still have air in the lines. Huck brakes are tricky to adjust. There are 2 adj. on each wheel. The threads are all right hand treads. Adj. each adjuster till you get a heavy drag, then back it out about 5 notches. There still might be some drag but it will clear up when the shoes seat. Then if the pedal still is not good, bleed them some more. You must have a small amount of play in the push rod like the guys say.

Wrenchbender Ret - you are right. I pulled all four wheels and adjusted the shoes out until I got drag. Then I went and bought a pneumatic brake bleeder from Harbor Freight and after a lot of fluid running through I actually have pressure. Thank you and HotRod Lincoln for the advice.

Sorry I have been away from the forum for awhile. Update on my cylinder length problem. As before I was having problems with the brakes getting hot after a short drive. After close inspection all was set as should be rod length and also brake shoes. This is where It got tricky. I took a very tiny wire and pushed it into the relief port inside the master cylinder. I worked it back and forth a few times. After that all worked fine. I laugh now thinking that it took me months to realize that the port was plugged with a tiny bit of dirt. It all makes sense now . So before you do anything check that this port is clear. If nothing else I sure know how to bleed and set brakes now.over and over and over . Ha ha hope this helps some one. I would like to thank everyone on this forum for all the help and info. Keep on trucking....