1974 C20 Brake problem

[mntoy]

I have a problem bleeding the brakes, the master and vacuum power booster are both new and came bolted together as a single unit. (rear drums front disc brakes) I bleed the rear brakes and the pedal feels firm and all the air goes away when bleeding. Then I move to the front brakes and I can hear a swooshing sound from near the Poportion valve (sounds like lots of air) and I cannot get all the air out of the front brakes and the pedal gets soft and hard as I continue to bleed the brakes, but never as hard as when I did the the back brakes first. After a quart of bleeding I finally gave up and started the truck and pushed the brake pedal down, it went right to the floor and did not return and a groan came from the vacuum booster. After I shut the truck off a minute goes by and the pedal returns to normal position. When the truck is off the pedal seems firm, but when I return to bleeding I cannot get the air out of the front brakes. Any ideas how to fix this problem, any help would be great. Mike

[boxrodz]

Did you first bleed the master cylinder before bleeding the rear and front brakes? If not, you may want to bleed the mc before bleeding the brakes. I'm guessing the reservoir for the disc brakes is introducing air into your front brakes. I didn't replace my mc or booster, but when I replaced my braking system, I had to bleed the brakes a few times before removing all the air out of the system (which is not uncommon). I also had to hold the metering valve on the combination/distribution block open when bleeding the front brakes.

[mntoy]

I bled the MC prior to installing in the truck but I can try it again while installed on the truck. How do you hold the proportion valve open while bleeding the brakes? Mike

[boxrodz]

I'm not sure if your '74 c20 has one, but on the '83, I wedged a block of wood to depress the metering pin. (Don't forgot to remove the wood block once you're done.)

[mntoy]

Yes my Pvalve does have a pin like yours, what does this do when I wedge a block next to it? does it allow me to bleed the brakes past the pvalve? Is the air noise I am hearing the pvalve stuck to the front or back position, if so how do I center it again?mike

[boxrodz]

Yeah, I figured GM combine the metering valve starting in the early 70s.

The metering valve is used to delay the application of brake pressure to the front brakes until the some brake pressure rear brakes have been established during a braking operation. Otherwise, the fronts would brake first. Depressing the metering valve allows you to bleed the front disc without having to build-up the pressure in the rears. I used a block of wood to depress the metering pin inward. (Worked for me.) Be careful no to rupture the rubber boot covering the metering pin.

I wouldn't think that the combination valve is defective. I'm thinking that you still have air in the system. The valve shouldn't cause you problems while bleeding the brake system. BTW, is you brake warning light on the dash come on when your truck is running? (What problems were you having with the brake system before you replace the mc and the booster?)

[mntoy]

This all started when I put in a new motor and when I started the engine the brake light came on and the pedal went to the floor. I tried bleeding the brakes and at that point could not get the air out of the rear brakes, then every time I pushed the brake pedal the motor increased in RPM so I thought I had a leak in the vacuum booster, so I replaced both booster and mc. After installation I could not get the air out of the front brakes. Sometimes the light goes on when the truck is running and sometimes not or it goes off after the first push. If I open the rear brake bleed and slowly have someone push the brakes after a few I hear the click which I think is centering the poportion valve but I still have no brake pedal or if I try to bleed I immedately hear the air by the poportion valve again. This is so frustrating, the new motor sounds great but I cannot even pull my truck forward and I never even did anything to the brakes. Sometimes after I start the truck and push down the pedal the pedal stays down and after a few seconds say 15 I hear air release in the mc and the pedal comes up. I even hear the engine increase in RPM when the truck is running and I push the pedal in with the new boost and mc. So many things are happening but the main thing is I am not able to get the air out of the front brakes. what is going on?

[boxrodz]

I copied your post in your secondary thread:

Originally Posted by mntoy

Never got my brakes fixed, I bench bleed my MC again, and leveled the truck and did it on the truck too. Back brakes bleed out great and stiff but the front brakes stll have lots of air, not sure why. I have bleed them manually by pushing the brake pedal and used a speedy bleed but lots of air keeps coming out even spurts large bubbles. When I go to fill the reservori small bubbles rise out of the front brake reservoir only(the one for the front brakes is what I mean). while the truck is off the pedal is great but when I start the truck pedal goes straight to the floor and does not come back up. What am I doing wrong, I have been through two quarts of brake fluid bleeding and it has not gotten any better. the brake light does not come on and I have tightened every fitting for the front brakes. Any help out there?

It sounds from what you described that you may have a leak in your system. If you checked all the lines, brake fittings, piston seals and wheel cylinders then you may have an internal leak around the piston seal in the MC causing the air in your front brakes system. Since the MC is new (or a reman) it may have a faulty seals or they may have been damaged during the install.

After saying this, the faulty metering valve could also cause excessive pedal travel, but that doesn't explain the air bubbles.

I assume that your brake system were in working order before you did the engine swap.? Is the assembly correct?