The Stovebolt Site Forums The Shop Area Driveline 46 1/2T - no pressure after bleeding brakes

Rebuilt all wheel cylinders and master cylinder and installed new lines. Pressure bled the system. No leaks in the system. When I put mu foot on the brake pedal, it is like there is nothing there - no pressure against the pedal. Am I missing something or do I have a very faulty master cylinder?

Bob.

I'd double check the adjustment on the shoes and there is a good chance there is still air in the lines.....it's happened like this before. Did you make sure it didn't run out of fluid while pressure bleeding? Just saying.....

Prior to bleeding, tighten ALL of the shoes (2 adjusting wheels per wheel on Huck brakes) so that you can't turn the drums by hand. (You will loosen them to specs after you are done bleeding.
Make sure that you back off the parking brake cables completely.
With Huck brakes, loose shoes will cause too much wheel cylinder piston movement preventing the shoes from seating with the drums. You will run out of piston stroke in the master cylinder before the brakes will work. Multiply this X 4 and you got no brakes.
Bleed the brakes.
Adjust the shoes to specs.
Stop on a dime.

I kept a careful eye on the fluid level. I had changed fluid colors so that I new when the new fluid started flowing out the bleed valve - should have been no air in the system. In my experience, if there is air in the system, you can pump the brakes and you will eventually develop pressure. And I had adjusted the brakes so that the shoes 'just' cleared the drum. I am inclined to believe that the master cylinder did not get correctly assembled.

The push rod in the master needs to drive the piston far enough to allow the fluid to refill the chamber for the next stroke. There is a port that must be cleared.

Ed

+ 3 on This ^^^^^^^^^^^^^^. Plus if this is the problem.........pump ( a lot) the brakes with the brake master cylinder cap off. Watch the fluid flow back into the master after you stop pumping. As all that fluid moving the shoes out returns to the master it’s a lot back flow.

Ed,
I don't follow this - what port are you talking about?

I am pressure bleeding the brakes so I should not have to start with adjusting the brakes tight. I have used this method many times on the MG and never had a problem - in fact, this was the only way I ever could get the brakes fully bled. Why should this Chevy be much different?


Bob

Looking straight down in the m/c you should see a small hole. Probably about 1/16". Depending on the position of the piston inside the m/c, when the piston returns, it may not be letting this port do its job. There is a procedure to check this but can't state it off the top of my head. I had this same problem with a 47 Dodge car. The actual problem with it was the m/c was new and the port hole was not drilled all the way through.
.......sometimes a known procedure just don't seem to work. I ended up putting a hose from the bleeder valve into a jar with some brake fluid in it to cover the end of the hose and just pumped until no air bubbles. This is just yet another idea.
Keep us posted. We all want to know what you find.

Loosen the tube at the end of the master cyl. Push the pedal & see if you have pressure coming out of the line. If you didn't bench bleed the MC you may just be pushing air back & forth. Make sure you have a small amount of free play in the push rod in the MC. If it is to long it won't let the piston come back to open the port. Like the guys have said, tighten the wheel adjustments till each one is heavy drag. If you try to back them off till there is no drag it may be to far as new shoes will drag some till they are broken in/seated. Usually about 4-5 notches from heavy drag.
George

Last year working on my buddy's 36 Pontiac, we installed a professionally rebuild master and it would not pump any fluid. When it came back from the shop the second time, it worked correctly.