71 Brake problem

[bstedman]

I have read every brake post in the forum and tried all the suggestions. But, I still have a brake problem.

I have replaced the master cylinder and power booster (rebuilts), the rubber brake lines, brake fluid, calipers (now o-ringed VBPs), and bled the brakes so many times I can do it blind folded. There are no visible signs of brake fluid leaking.

I also bought the Motive Products power bleeder and bled the brakes with zero air coming out at the caliper bleed screws.

The only items that have not been replaced are the metal brake lines and the proportioning valve.

The brake pedal is solid as a rock with the engine off. With the engine running, when the brake pedal is depressed, it goes almost to the floor, dash brake light comes on, and there is a hissing noise from underneath the dash.

I suspect it is the master cylinder or power booster but would like some confirmation prior to taking it all apart again.

 

[Bob Chadwick]

Does the brake pedal pump up? If so, you probably have a bad master cylinder. Your post suggests that you haven't driven the car so air would not be introduced from the possible usual suspects of warped rotors, bad bearing adjustment, etc. that pumps the rear pistons and introduces air.

The power booster would not cause the brake light to come on. It is actuated by differential pressure between the front and rear brakes causing a switch in the proportioning valve to ground out. If your booster were going out the brakes would just be hard to push.

The master cylinder is fairly inexpensive and easy to change, particularly with all of the experience you have in bleeding the brakes. :L

 

[bstedman]

Are you running a motor with high horse power and an aftermarket cam ? Low vacuum may be your issue sense most other components have been replaced. Give us some more info.

Tony
VBP

No on the high horse power......the car has been parked/stored for about 3 years and I am now trying to get it back on the road. All of the original calipers were leaking which started this whole process.

 

[zwede]

If the brake light comes on it is not the booster. The light illuminates when there is a pressure differential between the front and rear brakes. One of your circuits is not working right.

Now don't be insulted, but when you bled the rear calipers, did you bleed both of the bleeders on the rear calipers?

 

[bstedman]

The brake pdeal does NOT pump up. This is the third master cylinder, the other 2 were rebuilt, the one on there now is new.

 

[Bob Chadwick]

There are at least two bleeders on the top of the calipers. Some of us have bleeders on the bottom as well. I have been told that these calipers are universal, in the sense that they can go on either side of the car. The bottoms can be ignored when bleeding as the air goes to the top and these can let air into the system.

Does the pedal pump up. This can be a sign of a bad master clylinder.

 

[Bob Chadwick]

The problem is elsewhere then.

You've got a duplicate post going in the same forum. I am going to merge them.

 

[bstedman]

Both rear bleeders were bled, insides first, and when that didn't change anything, then outsides first. Basically, every combination was tried when nothing worked.

 

[guido34]

I'm no pro but I had a similar problem. To me the hissing gives it away. The power booster you put on may have a problem. My old one hissed like a snake ready to strike. It leaked vacuum like a sieve. The pedal would go to the floor and the pedal felt spongy, never hard. I bled the brakes until I was blue in the face with no relief. I replaced the booster and adjusted the shaft that goes into the master cylinder as per the instructions that came with the booster (got it from Autozone) and my brake problems were solved. Maybe you just need to adjust the shaft. Mine said to take off the master cylinder cover and depress the brake pedal 1\4 inch and you should see fluid eject from the master cylinder. You should adjust the shaft (turning it clockwise while looking toward the firewall shortens it, counterclockwise while looking at the firewall lengthens it) until fluid ejects with 1\4 inch of brake pedal travel. Good luck.

 

[brusso]

I had the same problem last year. With the engine off, I would bleed the brakes thoroughly and get a rock hard pedal. Once I started the engine and pumped the pedal, almost down to the floor she went and the brake light came on. Sound familiar? It turned out that I had a steel brake line that was rusted and porous. I got underneath while some else pumped the pedal and found the fluid seeping out of the brake line. If you follow the front to rear steel line, mine was leaking right where the "Bird Cage" meets the main frame on the driverside. There is a nice little area that acts like a basin and that was where the fluid was pooling up. Thats why I didn't see any pudles.

How does this happen? Well with engine running and brake booster working, you probably get ten times the pressure in the lines. Okay, ten times may be a stretch but you get the idea.

 

[GaryS]

I would start looking under the dash. Pull your driver's seat out (only takes 15 minutes and four bolts), lay on the floor with your head as far under the dash as possible. Using a strong light, look for the offending line that is leaking. Sure sounds like a vacuum leak going to the pwr booster.

 

[jdp6000]

Looks like everything has been covered on this problem. Here is my two cents worth... there is a one way valve at the front of the booster....a hose from the engine runs to it. Is it functioning as a one way valve???

One other thing. You say the hissing sound comes from under the dash area. There are only two things under there run on vaccum. One, your headlight switch and two, the bypass valve for your headlights. Make sure these under the dash hoses or components aren't the problem. Its a stretch but a serious loss of vaccum may be cause your problem...

Jim

 

[bstedman]

Update to 71 brake problem

After checking every inch of the metal lines and finding NO leaks, I purchased some metal caps to close off the brake lines going to the calipers (SS with O-ring pistons), to take them out of the equation. Now the brake pedal is hard as a rock with and without the engine running. So, it looks like I have something wrong in the expensive calipers.

 

[brusso]

After checking every inch of the metal lines and finding NO leaks, I purchased some metal caps to close off the brake lines going to the calipers (SS with O-ring pistons), to take them out of the equation. Now the brake pedal is hard as a rock with and without the engine running. So, it looks like I have something wrong in the expensive calipers.

Sounds like you at least found the problem. Good for you!

 

[magoo]

Hi everyone. I'm new here though i've been visiting a long time. I hope I'm doing this right. I think I can help bstedman with his brake problem. I ran into this problem while converting a 69 GTO to rear discs. I believe your bleeder screws are not at the top of the caliper when the caliper is installed. Remove the caliper bolts. Rotate the caliper until the bleeder screw is the highest point of the caliper, place something between the pads if they come off the rotor, then bleed again. Without doing this, that little bit of air above the bleeder screw will not come out. I'm certainly no expert, but it worked for me. So as you say, that's just my .02. But I wish you luck.