The Stovebolt Site Forums The Shop Area General Truck Talk Master cylinder questions

Hello all.possible silly questions not sure. I'm planning an update of the master cylinder to a dual unit.i have a 49 gmc 1-ton with stock drums.original master was 1-1/4 bore. First I've. Been searching out replacements and found a raybestos with a 1-5/16 bore would the 1/16th difference be ok to use. And secondly.what I'm seeing is two different ends on the cylinder piston where the pedal rod meets. One is a "shallow" end and the other a deeper cup where the rod goes in further. Would either work? Thanks for any help. Also as a side note. Truck never hauls any weight. Just mainly low speeds 40-50 mph. Maybe go with 1-1/8 bore? Will put on residual valves

Hy Johnny N, while going larger on the master cylinder bore will create more force at the wheel cylinders (force equals area times pressure) it will not adversely affect braking action. Going smaller than the original bore is not a good thing. If the original application for the master cylinder was disc/disc or disc/drum then residual pressure valves will be required in the circuits that were designed for disc brakes, you can install residual pressure valves in the circuits to alleviate any problems that may occur because the master cylinder is mounted below the level of the wheel cylinders. You can use whichever bore in the master cylinder piston you like it must be able to consistently push the master cylinder piston without the possibility of slipping, remember to leave a small amount of clearance between the pushrod and the piston with the master cylinder at rest, hope that helps.

Thanks for the response.all the masters I see in The original bore size of 1-1/4 seem to have the shallow piston cup.so I guess ill go with the 1-5/16. That one I saw has the deeper cup. I just seem to be skeptical about using the shallow one and having the pedal rod slip out of place. Maybe possible. Maybe not. I think I'm just paranoid. Lol

If you use a larger bore MC it will take more pedal pressure to stop it. You will have a higher pedal though. It seems you would use the piston to match the old cyl. & rod. If you use a shallow holed piston you will want just a little play in rod to the piston & a stop to keep the pedal from going back any further.