94 LT-1 Firebird rear disc brakes release after application

[rixtrix1]

Got a 94 Firebird in the shop. Customer got it from a kid who deleted most everything factory and replaced necessary parts with aftermarket, trying to make it a lightweight"streetrace car". In the brake system, he removed the ABS unit and replaced it with a Summit adjustable prop. valve block. Calipers are new oem style as is the master cyl. Hoses are braided steel. Problem is that the rears will release after application and roll while the fronts maintain pressure. Any ideas why, or is the prop valve just working as designed? Thanks.

[Apogee]

An adjustable proportioning valve shouldn't release any pressure, it just reduces the amount of pressure going to the rear brakes above the knee-point relative to the front brakes.

Up to the knee-point, front and rear pressures should be 1:1. Above the knee-point, the rear pressures will be slightly lower than the fronts, but still increasing, just at a slower rate or shallower slope if you were to graph it. The adjustment changes the pressure at which the knee-point occurs. If you set the adj PV to it's minimum setting, it won't add any proportioning to the system, so it's like it's not there. Pressures should be 1:1 all the way. If you set it full the other way, you should see rear pressures that are equal to ~57% front pressure per the armature design of the valve.

Tobin
KORE3

[rixtrix1]

Thanks, Tobin, but it doesn't help with the loss of pressure to the rear. Could it be a run of bad master cylinders? Customer said he has put 2 on car since he got it and the rear brakes still won't maintain pressure.

[Apogee]

It could be a master cylinder, but the odds of getting two consistently bad ones in a row is pretty low...especially to have them fail in the same way. Have you or the customer inspected all of the connections in the rear circuit? It doesn't take much of a leak to cause a problem.

If everything looks good and you're still drawing a blank, I'd put a guage on the outlet ports of the MC to rule that out first, then move to the calipers and see what's happening there. Sorry I can't be more help. I'm curious to know what you find.

Tobin
KORE3

[rixtrix1]

I'll let you know.

[rixtrix1]

Finally got to do some checking on the '93 Formula. Standing on the brake pedal as hard as possible, we have 1750# of force from the rear calipers. This is with the proportioning valve wide open. Since the car originally had ABS, could it be that the bias is already in the MC, although it's had weak rear braking from the git-go? But, after 2 new MC, 3 new ABS units, new stock rear calipers and ditching the ABS for a differential block with an adjustable prop. valve, we still don't have enough brakes to hold from just off idle in gear. we did just adjust the park brake lever per the manual, to no avail.

Don't have a guage to check the pressure at the MCJust have a "transducer"style set of hydraulic guages. Will have to try the fronts when we have some time to see the pressure there.