Hi,

My 88 rear disk brakes will not hold the wheels stopped when running the car in drive on jack stands (engine at an idle). It won't hold with the parking brake or with the pedal. So far I've installed the "recall kit" in both calipers and bled the system. I've also taken the master cylinder off and bench bled it.

My current car is actually a v6->v8 swap and a swap of rear ends from my wrecked 88 (which came with the delco moraine rear disks) to the 92 (which originally came with drums). Here's what's in my brake system as of now.

88 Firebird master cylinder
88 Disc combo valve (at least I assume it is a disc valve)
92 drum brake booster (is there a difference between drum and disk boosters? maybe the rod is longer?)
88 rear delco moraine calipers with recall kit.
88 parking brake cables and lever (p-brake has been adjusted)

Any ideas on what could still be wrong? Is there a difference between the drum and disc boosters? Maybe I have a bad combo valve or master cylinder?

It doesn't make sense that the parking brake doesn't hold the wheels in drive at an idle either. Perhaps the pads are "glazed"?

Thanks!

 

The Delco moraines are weak at their best. Check your e-brake cables for binding. With your cam, you are losing enough idle vacuum to significantly reduce power brake assist at idle. Also, be sure that the proportioning valve isn't stuck at one end, closing off fluid to the rear brakes(this should turn the brake warning light on). Because of their cheesy design, combining the parking brake with the hydraulic piston, if you haven't set the calipers by pressurizing them hydraulically, the parking brake won't work. if you suspect that the PV is stuck to one end, pump the pedal, rapidly to try to dislodge it. This generally works for me. Or press firmly on the rubber capped "reset button" on the end of the valve to return the valve to center, then repeat bleeding.

 

I have 15" of idle vacuum in park and 10" in drive holding the brake. Is that enough for power brakes?

I checked the "reset button" which seems to be on the back side of the combo valve. I took the rubber cap off and found what looks like a little pin sticking out. I pushed the pin all the way back in. I'll try bleeding the brakes again tonight.

 

Sounds like two problems (I'll throw in some guesses.)

First, in 88, the car was equipped with a tunnel-mounted adjustment piece as seen below:

https://www.thirdgen.org/forums/atta...e-adjuster.jpg

The 92 would have had a self-adjusting parking brake, so there would be no tunnel adjustment. Some who have done a forward swap (88 to 92) got the tunnel piece, say from the 88--as well as the parking brake handle and correct cables as you did. Apparently that worked.

Second, something to consider if the brake light is on.

Here's how the switch works, taken from the following site:

http://auto.howstuffworks.com/auto-p...ter-brake2.htm

"Pressure Differential Switch
The pressure differential valve is the device that alerts you if you have a leak in one of your brake circuits. The valve contains a specially shaped piston in the middle of a cylinder. Each side of the piston is exposed to the pressure in one of the two brake circuits. As long as the pressure in both circuits is the same, the piston will stay centered in its cylinder. But if one side develops a leak, the pressure will drop in that circuit, forcing the piston off-center. This closes a switch, which turns on a light in the instrument panel of the car."

BTW, according my shop manual, the only way the light can be turned off (short of disabling it) is to repair the failure and apply a pedal force to develop approximately 450 psi line pressure.

JamesC

 

The proportioning valve does more than differentiate pressure and turn the light on. It also shuts off fluid to the front or rear if either develops a gross leak. It has a rubber cap on on the end that covers a stem which extends from the valve. By depressing this stem, assuming the piston isn't being forced forward from pressure on one side, you can open the valve allowing fluid to enter the closed side of the system. I've had some success this way. The alternate method is to rapidly pump the pedal which will cause some fluid to bypass the valve. If no leaks remain in the system, you will eventually achieve balanced pressure on the two sides.

 

I bled the rear brakes some more after pushing the pin all the way in. The brake warning light was on after I did this. Pumping the pedal several times with the bleeder valves closed turned the warning light back off. Not sure if that indicates anything?

 

It means that you achieved pressure balance to recenter the prop valve piston. If you dont have good rear brakes now, you may have glazed pads. As I said, your cam reduces idle vacuum quite a bit. The Delco Moraines are weak. With the ZZX cam in my 355, it took all I had to stop the car sometimes. That's why I went to Ed at FlyN'Bye for the excellent brakes in my sig.

 

I will try de-glazing or even out and right replacing the rear pads (since they are cheap).

If I'm understanding you correctly, the rear delco moraine calipers inherent weakness combined with glazed pads could mean sufficient force to slow the wheels down to a crawl when with the motor idling in drive, but not enough to hold them still?

 

Yes, that's essentially my point. They simply don't provide enough braking force to effectively stop the rear wheels. They rely heavily on the added force provided by the vacuum booster. When you lose significant idle vacuum, you in turn lose significant braking force. The worst part is that when you really need to stop quickly, they won't be there for you. This can be real scary when you're in traffc and someone stops quickly in front of you.

 

I finally have the rear brakes stopping the rear wheels. I ended up changing the spring in the front of the proportioning valve per the thread below (I still have 4 more 1NCR5 springs since they are sold in packs of five):

https://www.thirdgen.org/forums/brak...e-upgrade.html

I have to push the pedal pretty hard, but it does stop the wheels.

Thanks to all who replied!

 

got an extra spring you could send me?

 

PM me.

I've also found the same pack of 5 is available from a couple other vendors.

http://www.drillspot.com/products/43...ression_Spring

http://buy.gissn.com/Compress-Spring...K5-p/1ncr5.htm