Brakes

[rusty baker]

The brake redal on my 70 just suddenly went to the floor. The master cyl was still full so I assumed it was bad. Replaced it, no difference. Won't pump up. The flood level stays the same.,Traced the lines and wheel cyls, no leaks. Bleed them, no air. In fact when I bleed them, there is almost no pressure, Manual brakes. I'm stumped. When I bench bled the master cyl. pressure seemed normal.

[grs]

First bleed the master cyl. Then hook up the lines to the master cyl. (The master should be level when you bleed it.) Then bleed each wheel cyl. If that does not work maybe the new master is no good.

[rusty baker]

I bench bled the master before i installed it. The reason I doubt it is air is because I can't even pump up any pressure. But when I bench bled the new cyl it had normal pressure on the bench. Could the plunger be too worn and not be pushing into the cylinder far enough?

[grs]

If the brakes worked before you installed the new master cyl. I would doubt that the plunger is to short. I had a car once that had a bad master. I pressed on the brakes and they worked. I pressed on the brakes again and the pedal went to the floor. Pressed on the brakes again and they worked. No way to tell if your new master is bad untill you replace it again.

[AzDon]

Today I completed re-plumbing to a five-port proportioning valve for front disc. Since I had two new front brake lines on mine, I had a bit more complex bleeding job than you should require......
I have a hunch where your problem is..... Remove the master and bench bleed it again using fittings with clear hoses into a bottle of brake fluid. Use a phillip screw driver that is long enough to fully depress the piston. Once complete cap the fittings and install the cylinder. Hook the brake lines in place of the plugged fittings quickly.....
NOW--
Have somebody pump the brakes three to five times S-L-O-W-L-Y and hold the pedal depressed while you loosen the brake lines at the cylinder and watch air bubbles come out. Tighten the lines before allowing your helper to release the pedal. Repeat the process as many times as it takes to get liquid without air bubbles. If you havent disturbed the rest of the system, all the air should be right there. If you still have no pedal, repeat the process at the next downline location where fittings can be loosened. Remember that your helper is pressurizing the system, but must not release the pedal unless the fitting being bled is tightened, otherwise air will be drawn in. You probably will get full pedal before bleeding all the way to the wheels... Good luck!

[rusty baker]

I already bench bled the master cylinder just as you said. I just suddenly had no brakes to start with, but the master cyl never lost any fluid. That is why I changed the master. I was a certified mechanic years ago and went thru all the usual stuff but still no brakes. When I bench bled the new master, the pressure felt normal. No leaks in the lines or wheel cyls. It shouldn't go from no air to full of air suddenly with no loss of fluid.

[Monte Carlo Man]

There should be either a stub on the end of the proportioning valve or a rubber bulb. If you press it in or hit it with a hammer a couple of times, the valve will go back into position and the brakes should work again.

The valve was designed so that when you lost fluid from one end, it would still hold pressure to the other end of the vehicle so you could get stopped.

Usually the proportioning valve would get stuck or was made so that it would not go back to it's original position so that you don't loose all your fluid when brake failure happens along the road.

[rusty baker]

Thanks, I'll try that.

[rusty baker]

This valve has 2 lines going in, 2 lines going out and the brake light switch. Nothing else. No bulb, no stud.

[Sub71]

It doesn't sound like a proportioning valve issue to me, even if it was stuck in one position you would still have pedal pressure and either the front or rear would work.

Are you sure that you got the correct master cylinder? The depth of the hole can be different, if it's too shallow you'll actually push the cylinder as you tighten it down and it'll never work. If it's to shallow you never build pressure.

Other than that I would say to replace the master cylinder again and/or bench bleed it again.

[rusty baker]

Since the master cylinder is less than $20. I think I will replace it again. The thing that had me puzzled is that this one behaves just like the one I took off.