I am midway through installing a power brake booster and new master cylinder on a 1960 F250. I got a universal setup from Speedway with a dual chamber master cylinder and an adjustable proportioning valve. The wheel cylinders and all other components have been replaced. So far so good, but I am having an issue with the rear lines. I have been hunting for a leak somewhere, because I am getting a squishy pedal after multiple good bleeds using a vacuum canister. I have been suspecting that pumping the brake pedal introduces air into the lines through a small leak somewhere, so I have replaced all of the fittings and double checked everything for tightness. Which brings me to a question: If the point of a dual reservoir master cylinder is to have 2 separate circuits for safety through redundancy, why would a leak in the rear circuit cause my brake pedal to go to the floor? Isn't the point for the front to still have the ability to build pressure? If I completely close down the proportioning valve on the rear the front circuit gives me a solid pedal. I can bleed the rear circuit with a vacuum canister and get all of the air out. However, as soon as I pump the pedal it is squishy. If I pump the pedal while the vacuum canister pulling fluid I start to get air through the lines again. I am starting to suspect there may be an issue with the new master cylinder or proportioning valve. Or there may be a gap in my understanding of what is happening with that proportioning valve. Any wisdom for a guy who just wants to drive (and stop) his truck this spring? Pic included because I know we all like to look at trucks.

 

Did you bench bleed the master cylinder? If not it may still have air in it.

 

I followed the procedure that was explained on the paper that came with the master cylinder- but since then I have inadvertently run it dry a few times. Can you tell me more about that? If it wasn't done properly or 'completely' on the bench would the problem persist even once it is pushing fluid through the system? I have run gallons of brake fluid through it at this point with the vacuum canister and pumping the brakes... would it still be in need of more bleeding? Can this be done from an installed position or does it need to be removed again?

 

In a vise you would route the outlet back into the reservoir and push the cylinder till no bubles. You can do the same thing on the truck with the brake pedal. after try to get the tubes attached with as little leakage as possible. then you still need to bleed the system start with the farthest away point (right rear).

You could have a bad new master.

 

I agree, it needs to be bench bled first, with the important part being that it is bled before being connected to the brake lines in the truck. It can be done without this, but it takes forever.

Don't let the reservoir go dry while bleeding (or otherwise). If you do, you may as well bench bleed it (again?) before continuing.

Regarding the possible leak: If you hold moderate pressure on the brake pedal for a few minutes and it doesn't move and no brake fluid leaks out, I'd bet you have no leak.