The Stovebolt Site Forums The Parking Lot Big Bolts 1954 GMC Master Cylinder Slop

I got my truck running the other day, now its on to the brakes.

The brake pedal was stuck when I got the truck so I was thinking that the master cylinder was seized up. I pulled it off and to my suprise the pison is free and so is the pedal.

I'm thinking about just reinstalling the cylinder, but it seems like the piston push rod has too much slop. It moves in and out about 3/4 of an inch between the Piston stop plate and the piston.

Is this a normal amount of slop or is my piston seized halfway through the cylinder?
My second question is, are the master cylinders the same for all the trucks of that year? I can't find one for a two ton, but the half ton cylinders look identical.

The Master Cylinders may look the same, but they will have different bore sizes. The 1/2 ton may be 7/8" or 1" while your 2-ton may be 1-1/4". See if the part number and bore size are cast into the side of the cylinder.

Mike B

I pulled it apart and I definitely need to rebuild, the slop was caused by rust in the cylinder that stoped the piston from fully retracting.

The piston is labled 1-1/8 but my manual says it should have a 1-1/4. Will the smaller bore be sufficient?

It is possible someone rebuilt it with some incorrect parts or that a incorrect master cylinder was substituted at some point. Clean the exterior of the master cylinder well, there should be manufacturer name and part number on there if it is original to truck or was replaced some time ago. Foreign made newer reproductions are not as likely to have manufacturer name or part number.

No, you want to use the correct MC in your truck.

The smaller bore (less volume) MC's require a longer push of the piston to move the volume needed to expand the wheel cylinders. This will show up as the brake pedal being closer to the floor than was designed...less safety margin.

Mike B

The castings on the side of the master cylinder say it is a 1-1/8 bore also. someone must have swaped it out at some point.

I can get a rebuild kit for $15 and a new correct master cylinder is around $115.
I understand that the smaller diameter will give me less stoping power, but is it enough to make the truck unsafe to drive? If the bigger bore will make a huge difference, it's worth the extra money. But if I wont gain much, i think the rebuild is the way to go.

For your safety and the safety of those around you fix it right...you'll sleep better!

Mike B

The smaller cyl. will not give you less stopping power, just less pedal effort at the expense of less volume of fluid pushed out to the wheel cylinders. Get the proper size, cheap.

Ed

as said above, get the 1 1/4. 100$ extra is a lot cheaper than running over someone because you could not stop and living with the thought that you might have been able to prevent that for 100$.

Point taken, I agree. New cylinder will be on order soon and if all goes well the truck will be driveable!

Ambulance-chasing lawyers just love it when they can prove somebody knew a part was wrong, and used it anyway. One incident I recall with an aftermarket air brake slack adjuster on a Freightliner tractor caused J. Crewe to lose a trailer load of merchandise when a drive axle hub caught fire. The trailer had 3 MILLION DOLLARS worth of cargo in it. The lawsuits were epic over that one! I was scheduled to testify in a 3-day trial in Orlando Florida, but after re-reading my inspection report, somebody's insurance company decided to settle the claim. I don't think they wanted me on the witness stand!
Jerry