The Stovebolt Site Forums The Shop Area Driveline 1951 1/2 ton original brakes- repair

I'm having trouble with the brakes on my all original 1951 1/2 ton. The pedal is firm if you hold hard pressure on it, but if you hold moderate steady pressure (as you would at a stop light) the pedal sinks slowly to the floor. When you pump the pedal it goes back to normal. I have had some mild fluid leaks at 1 rear wheel cylinder. I bled the brakes and it was no worse or no better. Could it be the master cylinder? When I bought the truck 7 years ago I replaced all wheel cylinders and all 4 rubber hoses at the wheels. Any help appreciated.

Sounds like the main master cylinder piston cup seal/s are bad.
1948 - 1951 Chevrolet Truck Shop Manual
You can Replace or Rebuild Enlarged.
And fix the mild fluid leak at that 1 rear wheel cylinder.

After you pump does it hold or does the pedal eventually start to sink?
Could be air in the lines, but not likely.
If main master cylinder bore is pitted your better off to replace.

When I pump its firm at first then slowly starts to sink. I thought it may be air in the lines (maybe it leaked in when the fluid leaked out) but it didn't get better when I bled it. Are the major retailers (classic parts, lmc, Jim carter) making good replacements? Or should I try to rebuild my old one?

Its definitely the master cyl bad. Most of the time you can rebuild them. If the bore has pits in it, it will need to be replaced. You can get a hone at the Chinese tool store or your local parts store. Usually you can get the repair kit or a new master cyl. at the local parts store.

I have not bought from the retailers you mentioned, so I have no experience.
But they better be making good replacements.
It's the brakes!

Should you replace or rebuild?
If the bore is pitted, replace it.
If not rebuild it.
Although it is a lot less trouble just to replace it with a re-manufactured one, but will cost more.

Your local parts store is the best source.
No shipping cost.
Easy return ability.
You will probably have to give them the old master cylinder, otherwise there will be a core charge.

Group number 4.650 CYLINDER ASSEMBLY
Part number 5454384
1 1/8" Diameter cylinder Bore

Why not fix the leaky cylinder and see if that resolves the issue?

A solid pedal that bleeds down with steady pressure on it is a
bad master cylinder, simply rebuild or replace it like George
posted.

dg