Brakes !!!!!!!!!!!

N

Nick in Fayetteville

Senior Member
Car: 1939 Ford Deluxe Coupe...Stock brake system...New master cylinder, new wheel cylinders, new hoses. Problem: I drive it 3 or 4 miles and the brakes lock up. Brake pedal is at top and hard as a brick. Put the car on a lift, run the brakes out loose...3 or 4 miles later, same problem. Any ideas...Are all brake fluids the same? It's like the fluid builds up pressure and pushes the pedal up and the brakes out.....ANY IDEAS??????
 
Typically this is because the brake pedal isn't letting the piston in the master return all the way. This blocks the fluid from returning to the master cylinder. The fluid heats up, expands, and the brakes lock up.

Check the pedal return and the length of the rod that goes into the master cylinder. There should be about 1/16" clearance before the rod contacts the piston. If there is clearance, I would suspect the new master cylinder.
 
Not enough free play in the pedal preventing the valve in the MC from letting fluid back in after braking thus building up pressure.
 
some thing is keeping the fluid from returning to the master cylinder, maybe the piston in the master cylinder is to long, wrong part installed when master cylinder was rebuilt
 
NO clearance will prevent the piston from moving forward enough to let the fluid return. More than 1/16" will let the fluid return, but too much might let the rod fall out of the master cylinder, and it will increase pedal travel.
 
I had the same problem with my '50. Same symptoms, rock hard pedal etc. The only way I could free up the brakes was to bleed off some pressure at one of the wheel cylinders. Two pumps and they were rock hard again. Turned out that the master cylinder was missing the spacer that goes under the snap ring between the snap ring and the front of the piston. I haven't had a '39 unit apart so don't know if it's the same, but mine was also a rebuilt, installed by previous owner. I found that a washer 3/32-1/8" thick, with a couple of thousands under the ID of bore and a hole in center that clears the rod, fixed the problem. In my case, the piston position wasn't letting the fluid bleed back. I didn't have my parts book at the time but the brake section clearly shows the spacer that mine was missing. Hope this helps.
Howard
 
Remove the M/C top and push on the pedal, release and you should see the piston return. It should return past the compensating port, in the base of the reservoir, to allow the fluid to return to the reservoir. If not, adjust the push rod until the port is open with the pedal up.
 
x2 what fordor41 says. When you look down into the MC you should see the little hole in the piston line up with the hole in the MC casting. This is how the fluid gets back to the reservoir. As stated above, disconnect the pushrod from the pedal and watch for the piston to come back far enough to see the holes line up. Then adjust the clevis so that there is some freeplay before the pedal begins to push the piston.
If the piston doesn't come back far enough for the holes to line up, something is wrong in the MC.
 
Could it be you need a rpv vlave front and rear…….speedway 910 31345…..page 50….now that is just a guesss……….
 
..New master cylinder,
Did you bench bleed the master cylinder before you installed it? Might be you have air in the line.
 
If you loosen the mounting bolts for the master cylinder when the brakes are locked up and they release the problem is a pedal to master cylinder free play problem, if it stays locked up with the master cylinder loose in the mounting the problem is in the master cylinder

with the master cyl empty you should see 2 holes, one large and one that gets small---if that small hole is blocked the system will build up pressure ---it could be blocked by dirt, or mechanically by the piston being pushed some, and I have seen the hole missing in a re-sleeved cylinder that was just "rebuilt" along with missing and wrong parts, also sometimes contanimated fluid swells the rubber to cover the port(oil in the brakefluid)

If just one front brake, or both rear lock up look at the hoses ---if loosening any bleeder relaxes the brakes it's probably not a hose ---I have had bad new hoses that locked up wheels
 
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