I just put a new master cylinder on my 96 GMC. I bench bled it before I installed it. I used a vacuum pump to bleed the brakes. I never got fluid at the rear wheels with out air and I ran 1 1/2 qt of fluid through it. I finally got my wife to pump and hold the pedal. I got clean air free fluid at all four wheels. But my brake light is still staying on and my back brakes aren't working at all. With the truck up on stands and in gear I can step on the brakes and nothing. I have changed master cyls before and I have never run into this. I never let the reservoir run low. What is going on?

It sounds like there is an air pocket trapped in the proportioning valve.

How would you get rid of the air? I would have thought that bleeding a quart and a half of fluid through the system would have cleared any air out of the system.

Okay, now that you have all the air out of the system, you need to check the rear brake shoe adjustment. Improper adjustment will cause the rear shoes not to set when the brake is depressed..
As for the brake light being on, does it have a low fluid sensor on the resovior? If so, unplug it and see if the light goes off.. if it does, plug it back in and tap slightly on the resovior and see if the light goes off..
Good luck

Hopefully Stan will chime in. I had a co-worker a few years ago that couldn't get one to bleed then he finally found out it took a criss cross parretn of some type. Once he found that out he had no more problems.

No fluid sensor on the resivoir. The rear shoes and wheel cylinders are about a month old. I did find broken spring on the passenger side yesterday. I went and got a whole new set of springs for both sides and put them in. I had no trouble bleeding the wheel cylinders last month when I put them in.
I thought the Master Cylinder was bad because when I would step on the brakes the light would come on. After I pumped them once it would go out. I could bleed the brakes and the light would go out for a couple of days. Then it would start coming on again.

Sounds like you still have air in the lines. Before you attempt to bleed the brakes, first check the shoe adjustment, yes it adjust automatically but you still need to pre adjust it for it to work properly. Once you a confident it is adjusted, start your bleed for the R/R, L/R, R/F, and then the L/F.
What's setting the brake light to come on is the proportioning valve because of eneven pressure from the front calipers and rear wheel cylinders.

Is this a remanufactured master or a new unit??? I service cars/trucks for a living and have had numerous reman units be defective out of the box.... Also what way are u bleeding the system, i found working from the wheel furthest away from the master(right rear) and then moving to the left front and then back to left rear and to your right front, hope that helps ya a bit...

hadude
it sounds like you have a bad master cylinder to me also,as far as bleeding the system after you do a bench bleed you start with the right rear wheel then the left rear then the right front and finally the left front,it sounds like the new master cylinder is bypassing to me,good luck. and Merry Christmas.

Thank's for the reply Stan. This is a brand new Master Cyl not a reman. I have been bleeding from the right rear, left rear, right front then left front.
My neighbor it turns out is the suspension/brake mechanic for a large Chevy dealer. He is bringing a scanner to hook to the ABS and we will try bleeding again tonight. He said that this ABS system can give you fits. If that doesn't work I will take the Cyl back and get another. I usually buy new parts to avoid the problems that go along with remanufactured. Plus you get a better warranty.
I know it is an old truck with a lot of miles but it still runs great, doesn't look bad, doesn't use any oil and I have no payments.

Will,
Sorry I didn't mean to ignore your post. I appreciate the comments from all. This is a great way to get help with a problem. I really appreciate the help and concern.

let us know how it turns out..Good luck they can be a pain sometimes.

Got it fixed. When he hooked the scanner and cycled the ABS system twice with the preasure bleeder on the system. After he did that we bled the back brakes again and the pedal came up. I have better brakes now than I did when I first got the truck 8 years ago. Thanks a bunch for your thoughts.