Brake Light On, Pedal Feel not Right, Front Brakes not Bleeding Anymore

[badpeanut]

When I bought my '72 Chevy 4x4 the brake pedal went to the floor and both the rear reservoir on the master cylinder and the rear wheel cylinders leaked. I replacing those and the rear hard lines from the rubber Y to the wheel cylinders (they were rusty and I damaged them disconnecting them front the wheel cylinders). After bench bleeding the master cylinder and vacuum bleeding the lines the brake pedal felt okay so I took it out on my gravel road and sneaked out on the highway. Brakes still didn't feel 100% so I went home and looked things over closer. There were some burned out lights in the cluster (one being the brake light) so I replaced them and now the brake light is on.

I vacuum bled some more and couldn't get the light to go out, so I had my wife help while I opened and closed the bleeders. The rear brakes seam okay and push out lots of fluid. The front brakes don't push out much fluid and the pedal has gone soft again. Now with the vacuum pump I can't get any fluid out the driver's front and very little out the passenger's front. I pulled out the front bleeder screws and verified they aren't plugged.

I don't have enough courage to drive it until I get the pedal feel back.

What should I check next?

[Kerry P]

I'd change the rubber flex lines. I've encountered several over the last 10 years that have collapsed internally. When that happens little or no pressure makes it to the calipers/wheel cylinders.

[dznucks]

Your proportioning valve has tripped. Its a safety feature. when you loose either the front or rear brakes, the valve will trip to stop brake fluid from flowing to the side that is experienceing lower pressure.

Some of them you can reset and some wont. I have heard of a special tool you can buy or make. I have read of people hitting thier brake pedal with a hammer. It may require a new proportioning valve.

[ERASER5]

If the proportioning valve does not reset, it is broken. Pretty rare event though.

Go to http://www.mpbrakes.com/. They have a tech suport page that details the reset proceedure. No extra tools required, only the line wrench you are using to loosen/tight the tube nuts. Oh, and your foot on the pedal. Basicly, you open the oposite line a bit and stomp the pedal. The pin will move.

The centering valve is made to shut off the flow of fuid in the case of major failure thereby stopping all the fluid from draining.This is to retain the braking power of the "good" side.

While you are at MPbrakes, you might want to read the rest of the tech sheets. Loads of information about brake systems. Stuff like when you need residual valves and when you do not. How a vacuum booster works and how to tell if it is broken.

[Stocker]

Assuming it's the prop. valve, they can sometimes be re-centered by pushing the brake pedal very hard several times.

[cdowns]

reading the GM factory service manual allows you to learn how to bleed the brakes properly on these trucks

[slikside]

Originally Posted by Stocker

Assuming it's the prop. valve, they can sometimes be re-centered by pushing the brake pedal very hard several times.

This is how I got mine to recenter after a complete brake system overhaul. I did it with the help of the booster....started the truck with the booster hooked up, and pressed hard on the pedal...light went out.

Bleeding procedure calls for clamping the pin of the combination valve open when bleeding the brakes.

[badpeanut]

I ordered new rubber flex lines for the front today.

[badpeanut]

Fixed!

The rear brake shoes (which were on the truck when I bought it) were either defective or the incorrect part number because some of the holes were off slightly causing insufficient contact with the star wheel adjuster.