1991 Chevy C2500 4x2 brake problems

[skyline2250]

I have a 3/4 ton 1991 Chevy C2500 4x2 cab and a half with an 8 foot bed and a 5.7L 350 engine RWAL JB6 braking system 7200 GVW. I started doing the front brakes yesterday. I replaced the Calipers, Rotors, and Brake pads. Started bleeding them, and it seemed to be going fine, but when we were done bleeding the fronts, the pedal went straight to the floor. So, we started bleeding the back brakes thinking that it could be the problem to no avail. While bleeding the back brakes, we replaced the wheel cylinder because it was not budging. I started replacing brake lines thinking that there was a clog or something in them. After replacing about 95 percent of them, and testing out the other 5% to make sure they work, we RE bled the system again, and had the same results. Finally, we thought that the Master cylinder was bad. So, I went and bought a new one and replaced it. I bench bled the new Master Cylinder and installed it. Rebled the whole entire system to no avail. Let the truck sit over night thinking by some strange chance, that something needed to settle down. Nothing changed. I tested the power booster, and it tested out fine. I have back brakes, I can feel them being engaged when i press the Brake Pedal, but it does not feel like i have anything coming from the front. I was reading a few other posts, and was not able to come up with anything. I am at a total loss here and everyone that I talk to seems to be stumpped as well. If anyone has any ideas, please help.

[og25]

i'm thinking it may be the proportioning valve, i'd hate to see you spend anymore money on a guess, so wait and see if someone else chimes in.

[z07]

How was the fluid flow when you were bleeding the front calipers? Was it a nice strong squirt with each pedal push?

If the combination valve were stuck in front bypass mode then the rears would get all the power. Or if there is a height sensing proportioning vale that is inop it could do the same thing. You could open both rear wheel cylinders and see if you get back front brake pressure.

[skyline2250]

The brake fluid level kept going down. I keep it full as we were bleeding tho. There was a stream when bleeding the front brakes, not as strong as i think it should have been. When you say open, you mean the bleeders? Im new to mechanics, and i know a little bit about it, but i bought this truck to be a learning experience!

[z07]

yeah that's what I mean. Just open the bleeders to simulate a wheel cylinder failure.

This is my thinking:

You opened the fronts to replace calipers and then to bleed.

The combination valve is designed to sense this via a pressure differential between front and rear circuits. When in front bypass mode the rears receive all the power and the brake light should come on.

I was thinking that your combination valve may be stuck there either from a worn internal seal or from air being in the front circuit.

The height sensing proportioning valve is installed on some trucks and varies the force sent to the rear brakes inversely with the distance from the frame to the rear axel. That is to say that when you have a load or a trailer the rear brakes get more force than when you are empty.

[skyline2250]

Thank you. I shall try that tonight.

[leolkfrm]

did you change the rubber flex hoses on the front?

[Case 7230]

You still have air in the system. Did you bench bleed the new master cyl. before you fitted it? Make sure all air is out of the M/C first. Also try to bleed the RWAL module by loosening the outlet pipe. I put 5qts of fluid thru mine before I got brakes. I will make a pressure bleeder the next time.

[skyline2250]

I did that case. I feel like an idiot for saying and posting this, BUT I FIGURED IT OUT. I had the calipers on the wrong side. So there was a lot of air in the system. But hey, I am a beginner to auto mechanics, so Im glad that i did this on my truck, so now i know not to do it again. But they are working like a champ, and Im happy. My monster and I are back on the road!!!!!

Originally Posted by skyline2250

I have a 3/4 ton 1991 Chevy C2500 4x2 cab and a half with an 8 foot bed and a 5.7L 350 engine RWAL JB6 braking system 7200 GVW. I started doing the front brakes yesterday. I replaced the Calipers, Rotors, and Brake pads. Started bleeding them, and it seemed to be going fine, but when we were done bleeding the fronts, the pedal went straight to the floor. So, we started bleeding the back brakes thinking that it could be the problem to no avail. While bleeding the back brakes, we replaced the wheel cylinder because it was not budging. I started replacing brake lines thinking that there was a clog or something in them. After replacing about 95 percent of them, and testing out the other 5% to make sure they work, we RE bled the system again, and had the same results. Finally, we thought that the Master cylinder was bad. So, I went and bought a new one and replaced it. I bench bled the new Master Cylinder and installed it. Rebled the whole entire system to no avail. Let the truck sit over night thinking by some strange chance, that something needed to settle down. Nothing changed. I tested the power booster, and it tested out fine. I have back brakes, I can feel them being engaged when i press the Brake Pedal, but it does not feel like i have anything coming from the front. I was reading a few other posts, and was not able to come up with anything. I am at a total loss here and everyone that I talk to seems to be stumpped as well. If anyone has any ideas, please help.

[skyline2250]

Oh and BTW... THANK YOU ALL for all of your help.