Hiya. Been stalking this forum for years, gathering information. Thank you all. I own a '69 conv. with 351 4bbl, Auto, and power disc front brakes. The poor girl holds down the garage floor all year and gets started and checked over once a year. This year, she had no brake pedal at all. A look into the master cyl. revealed zero brake fluid. Checked all 4 corners to find no leaks. before i go any further, where the hell did the fluid go? I can only assume the real seal in the master cylinder failed and the fluid has run off into the brake booster. Any ideas or suggestions here would be great. Thanks again for the years of info.
Welcome and It sounds like you are on the right track.
Also, if it is in the booster, can it be saved or is the booster ruined?
That depends, you can remove the master cyl and see how much fluid is in the booster and you might be able to sponge most of it out, then you can test the booster to see if it will hold vacuum.
I feel sure that you will get more reply’s.
Often the brake master cylinder rear seal fails causing it to leak fluid into the brake booster. The wheels stay dry.
The other common leakage point is the proportioning valve. It’s the cylindrical object clamped to the LH inner fender below the hood hinge. If that leaks it removes paint from the inner fender. The wheels stay dry.
I had a similar problem and was able to suction out the brake fluid from inside the booster when I replaced the master cylinder. I understand the booster’s life has been shortened, but mine is still going a year later.
As you probably already know brake fluid when it pulls moisture out of the air is corrosive to steel so the sooner you can clean it our the more likely your booster can be saved.
Update: Master Cyl rear seal gone. The fluid leaked between the MC and booster instead of in. New Mc and have pedal again. Going to replace the lines as well. The line going to the rear looks wet and bloated. . . bad sign. Can anyone tell me if that’s a proportioning valve on the rear axle or just a splitter for the lines? Thanks, it was good to be able to drop her in gear again.
The fitting on the rear axle is just a straight connection for the hard lines to the rubber hose.
Replace the rear rubber brake hose ASAP. Even if it looks good from the outside, they tend to swell closed, and then you will have little to no fluid flow to the rear brakes. We have seen this many times with original brake hoses.
15psi is not enough to move drum brake shoes much. A typical hold off pressure for drum brakes is 2 Bar or about 30 PSI.
The spurt will come from the rear chamber first. The front chamber port will not spurt until the front piston moves. That piston is moved by the pressure from the rear chamber pushing on the front piston, so it has a delay until the rear builds enough pressure. In a properly adjusted and well bled brake system, that lag is short.
In figuring out how the master cylinder should behave, keep in mind the pressure difference between the front and rear chamber is small. The front pressure will be less than the rear pressure due to the cup seal drag and the front return spring pressure. Typically that will be < 3 Bar or < about 45 PSI. When the system is working properly the front piston is hydro-locked to the rear piston.
In the event of a leak on the rear chamber circuit there is a rod like feature that will contact the front piston as the pedal is pressed in order to build pressure in the circuit that isn’t leaking. Similarly the front piston will “bottom out” in the front bore, if that circuit has the leak while the rear piston builds pressure against the bottomed out front piston.
Thank you for the replies. You were correct. The rear brakes have little to no flow. replacing that rubber line this weekend. Will let you know
The rubber hose with the splitter was the culprit. I only had 1 frozen tube nut to repair and off we go. Thanks again.