The Stovebolt Site Forums The Shop Area Driveline Brakes Locking On

I recently fitted a master cylinder kit to my '40 pickup and all seemed well. After sitting for a few weeks, I took it out & after half a dozen stops, the pedal went rock hard and the front brakes locked on. After an hour's rest it was all good until the same thing happened yesterday. Have I done something wrong, or is it possible that there is a blocked hole in the unit which is not allowing the fluid to return to the reservoir?

Adjust your actuating rod to proper specs for a start.

Riggles Look in your master cylinder,if fluid is clear take a tag wire and carefully "feel" in the tiny compensating hole to see if it is letting fluid come back after a stop. May need to adjust pedal linkage if rubber cup is in the way.

May need to replace the rubber brake hoses. The inside rubber comes loose and plugs the inside of the line. The brake fluid can't return to the master cylinder and it takes a while for it to bleed back around the plug.

I had the exact same problem with my 49'. My first thought was my master cylinder wasn't releasing back. Turned out to be the front wheels cylinders weren't moving freely from lack of be driven. A combination of some rust and brake dust oil/sludge buildup. They would expand out upon braking but not return back. If I let the truck sit for a bit all was fine as they eventually crept back in until I drove it again and tada the fronts would lock up. I took the wheel cylinders apart and cleaned the rust and crud off of the cups, a little WD40, and all was good again.

Same problem I had. Twice.
1. The first time was because my pedal pushrod was not properly adjusted. It didn’t have free play before contacting the master cylinder plunger, so the MC plunger was not able to fully return. If I cracked open the bleeder I’d get a squirt of brake fluid and the problem went away for a little while until I made a dozen or more stops.
2. The second time was because the new brake booster intermediate rod was not properly adjusted. So it was not allowing the MC plunger to fully return. This one was hard to diagnose because I was 100% certain my pedal pushrod was properly adjusted with the right amount of free play.
3. As others have mentioned, you could have a collapsed brake hose somewhere.
4. Dirt in the MC passages could cause this problem but less likely that the first 3.

I've checked the M/cylinder (which I should have the first time), and the reservoir was caked up with hard varnish type stuff on the welsh plug end. It was also a bit of a push to get a pin through the little return hole. My local brake man bead blasted it all nice & shiny, and I put it all together, adjusting the pushrod on assembly. As the hoses have been there for a bit over 40 years, I'm replacing them while I'm at it. I cannot budge the hose from the rear of the backing plate, last resort is to angle grind it off, then use a socket & impact driver to get it off. It's not a left hand thread is it? Any helpful hints would be greatly appreciated as usual. As for the wheel cylinders sticking, the truck gets used reasonably often, so I'm doubting that's the problem. They were also all replaced with stainless steel sleeves & new rubbers a few years ago. Many thanks guys, Riggles.

I'm all set to put it all together and need the length of the piston push rod. Can someone give me the measurement or point me in the right direction please?
Many thanks, Bob

You just want a bit of free play after installing the push rod. This allowed the system to release all the way. I’m sure there is an exact measurement listed somewhere but I’ve always made sure I can wiggle it 1/4 inch or so where it contacts the piston.