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Hi y'all. I'm a old member of this site and haven't been logged in for awhile. I forgot all my login info so I just re registered. Ok, please bear with me. My vehicle is a 1953 M-37, runs great but a have a serious braking issue. It's been going on for years now and I can't find whats wrong. Vehicle runs great but routinely my brakes lock up. And it seems to happen at every parade. At the last parade i drove 20 miles to get there, no problems. As the parade started my pedal got hard and brakes locked. I pulled over and cracked a bleeder screw and fluid shot out. after that all was fine. I figured there must be an issue with the MC, something happening to cover the relief port maybe. I changed the MC (3rd time). Pressure bled system, shoes adjusted to spec, changed all WC's. Made sure MC vent clear all the way to air cleaner. Changed all hoses. Fluid level in MC at 3/4 inch below fill as per spec. Adjusted MC clevis rod so the pedal goes halfway down until I get a firm pedal. All was good, I thought. Two weeks later I go into my garage and my brake lights are on. Pedal rock hard all the way up. I didn't drive the vehicle or touch anything. Jacked rear up and couldn't turn wheels. I bled one bleeder screw and again fluid shot out and pedal returned to normal.
PLEASE CAN SOMEONE GIVE ME A CLUE !!! Thanks in advance guys.

Hi Bill,

It sounds like something is definitely preventing the fluid returning to the master cylinder. I am assuming that you’re running a stock brake system?
-have you checked for free play at the pedal? There should be a small amount of free travel before the pedal engages the master cylinder (maybe 1/2 inch? The manual should specify). This prevents holding any pressure in the lines.

Cheers,
Rob

I would look for a blockage in your lines or hoses. Old hoses deteriorate over time and there could be a piece floating in there that is allowing fluid flow in one direction only.

Two things cause this. (Normally) Your MC pushed is adjusted too long and is not letting the piston back. Improper parts or assembly of MC.
Open the filler port and VERY SLOWLY depress the brake pedal. You should see a bit of a squirt from the port at the bottom of the reservoir as the piston travels past the port. If you don't see it. See above causes.

Thanks guys! Checked everything. And all your ideas are valid. But it does not explain why the brakes lock up just parked in my garage with no one touching the brake pedal. Oh, and one more thought. Am I turning the adjustment nut the right way? I thought by turning it clockwise, thus making the threaded portion showing smaller would be correct. Am i wrong?

Ok, I think I got it. I just came back from a 40 mile round trip to Augusta GA to be in a parade riding around all the hospitals in the medical district including the VA Hospital with signage thanking them. All went well until I pulled into my drive and brakes locked. I decided to pull the MC actuator rod out. It was adjusted with only 3/4 of a inch left showing threads on the clevis. When I pulled the clevis it shot forward about half an inch. I guess that was the problem. What i thought was free pedal was not really free. Now it has about an inch of "free" pedal.
Thank you everyone for the help!

Pix of the "Locking Brakes" truck.

Greg, (Just Me) has you on the right track. I can’t explain lock up while vehicle is idle. I agree with Greg, your pedal travel to free play may be a bit short on return travel. Why not adjust the brake pedal to an extreme 3” free play travel and see if the lock up disappears. You can always re-adjust the free play again later. As always, I suggest, try the cheap thing first before investing big $$ on other solutions.
JB