Thread: 1932 ford disc brakes locking up

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I have a 32 ford truck that I have been working on. My dad did a disc brake conversion from speedway motors in 2008 it has a 2lb residual pressure valve in the front and a 10lb in the rear for the mid 70,s ford nine inch drums on the rear end. The Speedway motors kit came with a 1978 thru 1988 Chevy mid size car caliper that are apparently used for modified race cars. The issue is after I replaced the master cylinder due to a bad seal I drive for about five minutes both front calipers start to lock up the more I drive the worse they lock up. After it sits for about an hour they release.
I have replaced the new master cylinder twice and also the residual pressure valve. One of the calipers has also been replaced. The caliper pins are lubed pistons are not sticking kind of at a loss ????

Did you verify that the push rod into the master is fully releasing and has no tension holding the master from releasing completely? Do you have a spring pulling the pedal back?

I don't have a spring pulling the pedal back. I did confirm that the is about 1/16 or so play in the rod. I also never had this problem until I swapped the master cylinder. I have since replaced it with two new ones and still the thing the master cylinder is a 1972 Chevy pick truck. It's just had to believe i have gotten 3 bad master cylinders. The only thing left is the brake lines I confirmed nothing is running by anything real hot. I even checked the fluid temp when the brakes we locked up and it wasn't even warm. The other weird thing is the pedal got real hard to push the worse it got. It only had maybe 1/2 to 1" of throw when the brakes were really locked up

a lot of modern master cylinders have residual valves built in to the outlet of the master and also front disc/rear drum oem systems use a combo valve that control flow rates and pressure differences to front/rear----


Also---did you bench bleed the master before installing??? and have the proper section of the master hooked to proper end of car?

Yes I bench bled the master cylinder and it's hooked up correctly. I wonder if the new master cylinder has the residual pressure valve in it so with my inline one I have to much pressure? The old master cylinder has been in the truck for at least 30 years so maybe it doesn't have the residual pressure valve? I was thinking of making a small piece of brake line to bypass the residual pressure valve just to eliminate it as a possible problem anyway so this may explain the reason why it's not needed????

Are you sure that you replaced the MC with the same model that was on it before? Disc pads ride very, very lightly on the rotors, but they do tend to build up heat in the calipers. Everything you've said points to the buildup of pressure due to heat, and the MC not allowing the excess pressure to bleed back. Seems to me that an easy test might be to drive it until it starts dragging, then reach down with your toe and pull back on the brake pedal, removing ALL pressure from the pushrod. You did not mention a power booster, so we assume manual brakes, and that your MC is designated for manual brakes, right? Also, in a '32 we're talking below the floor mounting for the MC, right?

You are correct manual brakes right under the driver side floor. I did try and pull the brake pedal to me when I stopped once to let the brakes cool on the way home. It didn't seem to help. I went to my local parts place they are a really good local parts guy not a chain we traced the casting numbers and didn't get anywhere. We finally matched it up by picture it was an exact match the second one was the same except it had two bleeder screws above the brake lines . That's why I am thinking its close to the one I pulled off but not exactly the same. Maybe the new one has the residual pressure valve in it??

probably and then maybe you got the lines hooked up backwards with 10 lbs residual to fronts---on disc/drum set ups there is a combo valve that works as proportioning valve to the rear and the front end is delayed to the discs until the pressure builds up enough to apply the rears----------look up brake combo valve -------------

The brake lines were already bent so you couldn't mix them up but I double check tomorrow just to be sure

Back the push rod off 1/16th and put a pedal return spring on it.
As yer driving the pedal is bouncing and creating pressure to the master.
Do that and problem solved.

One more thought!! The residual valves have a direction of flow. Pete

I have double checked the residual pressure valve is installed correctly.

A couple of "out there" thoughts. First, you've never mixed any DOT 5 fluid into your system, have you? It can cause you all kinds of grief. Second, what's the age & condition of your flexible hoses that run from chassis to caliper? Rubber breaking down in a flex hose can create a "check valve" of sorts, but it's a stretch for both of them to go bad at the same time...

Have you been able to determine if the drums are affected as well?
What about the rubber hoses and fluid - did you replace fluid with silicon? and the hoses have swollen so that when heat is generated at the caliper the pressure cannot escape? It becomes easier to apply the piston than to send fluid back to the master cylinder?

Originally Posted by DA34GUY

Back the push rod off 1/16th and put a pedal return spring on it.
As yer driving the pedal is bouncing and creating pressure to the master.
Do that and problem solved.

The weird part is it's worked fine for about 6 years without the spring. The only change is the master cylinder