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No rear brakes on 1st pump

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No rear brakes on 1st pump

Old 12-11-2018, 08:00 PM
  #1  
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Join Date: Jun 2013
Location: Upstate NY
Posts: 62
Engine: TPI 327 SBC Turbo MS2
Transmission: 200-4R CKP Stage 2
Axle/Gears: 3.73
No rear brakes on 1st pump

I have a '90 GTA with factory rear discs that I'm working on. As the title indicates, I have no rear brakes on the first pump and the brake light will come on at the same time. If I pump the brakes, then light goes off and the rear brakes begin working. How well do they begin to work? Not sure, but they do begin working. Also the pedal will raise slightly on the second pump. I put new brakes on the front in the spring due to poor braking (and old brakes). The issue in the front was 100% caused by the brake lines. I changed the pads and calipers first with no improvement. After changing the front brake lines all issues were gone at that time.

This issue being a little different now than it was in the spring I figured it was in the master (after bleeding all the brakes a couple of times). New master cylinder made no change. So now I'm scratching my head. Could it be the rear brake hose(s)? I'm assuming the combination valve is good due to the brake warning light seems to work correctly - it only comes on when the brakes suck on the 1st pump. When bleeding the brakes I noticed that the rears do not have much pressure when bleeding as compared to the fronts, and the pedal won't travel to the floor when the rear bleeders are cracked during the bleeding process.

Lastly, the rear brakes do not have a brake hose going to the left caliper, where the right rear caliper has a hose? The brake line on the left looks like someone butchered something in there, but I can't tell for sure. My question is would the factory rear disc setup have had a brake hose on each side, or just the passenger side?


Looking for any help and thanks in advance.
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Old 12-18-2018, 08:51 AM
  #2  
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Join Date: Nov 2004
Location: Lansing, MI
Posts: 289
Car: 1988 IROC-Z
Engine: 5.7 TPI
Transmission: 700R4
Axle/Gears: 3.70
Re: No rear brakes on 1st pump

The proportioning valve has a safety feature. When your light comes on, that means the valve has shifted blocking the fluid flow to the rear circuits. Switch picks up on this, and light the "Brake" light. A good stop on the pedal once everything is fine because the equalized pressure on both sides shifts it back to center opening both circuits.

Rear brakes on these cars do not hold like the fronts. The spring in the valve absorbs some of the pressure that should go to the rear circuit. This also protects from keeping the rears from locking up during hard braking and causing a tail spin.

So when bleeding, go slow so you don't shift the valve. I personally like using a vacuum bleeder. No pedal push, so it usually won't shift the valve.

Drivers side is hard line to the caliper, passenger side is a host. There is also a flex hose on top of the axle to the drivers side.
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