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I have a 1941 Desoto Custom my brother in law and I are restoring. The car has been in his family since about 1948 when his great grandfather bought it at an auction. The car was driven until 1961 and then parked in a building until a few years ago when we got it out and started working on it. We got it running and driving, at least in the driveway. We cannot figure out the problem with the brakes. The problem is the pedal will stay up for two or three days and then it will go to the floor unless we push the pedal every day. We have replaced everything about the brakes and still have the problem. New brake master cylinder, lines, rubber hoses, wheel cylinders, and shoes. I have worked on a number of vehicles over the years and never had a problem like this unless there was an obvious leak. There are no leaks on this system. We first rebuilt the master cylinder that was on it and same problem. If anyone has an idea of what may be going on I would appreciate the help.
Thanks
Mark
Hi Mark, as a fellow 1941 DeSoto owner, I am glad to hear another one is on it's way to being back on the road!
If I am following correctly, you tried to rebuild the existing master cylinder that was on the car, then you tried replacing the master cylinder entirely with a new one, but still had trouble. The master cylinder has a valve in it that seats against the plug end of the master cylinder where the brake lines connect. The spring in the master cylinder keeps just enough pressure on that valve to keep it closed and seated to retain a small amount of pressure in the lines, but not so strong to over pressurize the lines and lock up the brakes.
If there are no obvious leaks or puddles of brake fluid, although everything in the system is new, I would still suspect something is not right with the master cylinder.
Here are some items to check:
- First thing to check, make sure the valve and rubber seat are positioned properly inside of the master cylinder. (See below for a detailed explanation)
- Inspect the master cylinder bore for any pitting, allowing it to seal enough while bleeding and pumping the brakes, but allowing brake fluid to seep past the rubber cups when not in use.
- If the master cylinder was honed to try to clean up the bore, it might have been over honed and the rubber cups aren't sealing fully. You might need to try a size up in rubber cups.
- Inspect the master cylinder piston rubber cups for any tears.
- Brake pedal push rod length, generally this should never need to be touched, but if someone in the past tried to correct something and adjusted the length of the push rod, it might not be allowing the piston in the master cylinder to have it's full travel.
- Unlikely, but check the brake return spring resistance at each set of brake shoes. It is possible that the brake return springs are too strong and are overcoming the spring pressure of the valve and spring in the master cylinder.
Regarding the master cylinder valve and seat. The exploded diagram of the master cylinder in the factory shop manual can be a little misleading depending on what type of valve & seat assembly you are using. The original type of valve assembly is all one piece with a tiny spring inside of it and is different than what is provided in modern master cylinder rebuild kits and reproduction master cylinders. If you follow the assembly order outlined in the original parts or shop manual, it will not work correctly. The rebuild kits available now require a rubber washer, which is usually provided, to be inserted before the metal valve can be installed. Without that rubber washer, the master cylinder will never pressurize. Often times the rubber washer that is supposed to pair with the metal valve gets placed in between the solid rubber cup and the metal piston, which is incorrect.
1936-54 Brake Master Cylinder internal pieces diagram using metal valve that requires a separate rubber seat that looks like a washer (Commonly found in all reproduction master cylinders and master cylinder rebuild kits):
Carl thank you for the reply and advice. I have taken the new master cylinder apart and even rebuilt it with a new kit. I think I put everything in the right way but I will take it back off and check it. I have the front floor section out so it is easy to get to. Thanks also for the diagram. I hope I find something in the wrong order. If not I will try a larger cup. The one in it seems to fit really well and being a new master cylinder the bore looks good. I know the rubber washer is in it.
I like the picture of you with your car. A few years ago we went to Charlotte and met Dean Mullins’s when he had a green 41. I think he sold it since then. I attached a picture of ours the day we picked it up. I will let you know what I find in the master cylinder
Mark
