Rear Disc Converison 
Rear Disc Converison
hello , how are you today ? first question is did you change the combination valve that is located below the master cylinder to one from a car that had rear discs and is the same weight front and rear ? second question is did you bleed the system starting with the right rear , then the left rear , then the right front , then the left front .
Did you replace your proportioning valve when you did the brake swap? If not, you need to. This probably won't completely fix your problem, but it is part of the fix. I'd also make sure you did a complete bleed and see if things firm up before swapping out the master. I went through roughly the same thing on mine, and ended up putting a Wilwood adjustable prop valve on mine (which is manual, no vaccuum booster)
TECH Fanatic
Joined: May 2010
Posts: 1,081
Likes: 8
From: Atlanta, Ga
It is all about volume. What calipers did you use, and then what is the bore of the master cylinder. Did you swap out the master cylinder as well as the calipers? Silly question but are the caliper bleeders at the highest point?
Typical rule of thumb, MC too small = soft pedal
too large = very hard pedal
Typical rule of thumb, MC too small = soft pedal
too large = very hard pedal
Many G-bodies running around with rear disc using only a different proportioning valve. USUALLY the master and/or booster don't need to be changed. A long pedal sounds like air in the lines to me. I'd completely re-bleed the system after installing a new prop-valve.
What rear disc are you running?
I think flynbye.com and core3.com both have prop valves for G-bodies with rear disc.
What rear disc are you running?
I think flynbye.com and core3.com both have prop valves for G-bodies with rear disc.
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Teching In
Joined: Jun 2013
Posts: 24
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From: Camano Island
Prop All suggestions were done except one. I must change prop valve from drum to disc but may not be the best? I found an adjustable valve may be the best but may have to change the lines?
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Switching from drum to disc doesn't always dictate changing the proportioning valve.
It really depends on what calipers were used in the rear that does. Many automakers use the same valve for their drum and disc trim level cars on later model cars.
Ive done a couple of rear disc swaps using the "drum" proportioning valve. A different part number doesn't necessarily mean its internally different!
It really depends on what calipers were used in the rear that does. Many automakers use the same valve for their drum and disc trim level cars on later model cars.
Ive done a couple of rear disc swaps using the "drum" proportioning valve. A different part number doesn't necessarily mean its internally different!
Teching In
Joined: Jan 2010
Posts: 29
Likes: 0
What rear calipers are you using?
Some rear calipers use the park brake to "ratch" the pads closer to the disc to take up wear.
This ratching mechanism can seize (even on rebuilt calipers).
I had this problem on a 1980 T/A with 4wheel disc, so all the components were factory setup for 4 wheel disc. Good brakes but lower pedal.
Some rear calipers use the park brake to "ratch" the pads closer to the disc to take up wear.
This ratching mechanism can seize (even on rebuilt calipers).
I had this problem on a 1980 T/A with 4wheel disc, so all the components were factory setup for 4 wheel disc. Good brakes but lower pedal.
Teching In
Joined: Jan 2010
Posts: 29
Likes: 0
You made no mention of what car you have these on.
I searched Right stuff and if the calipers are the same as the ones pictured in most of their kits they are the kind that requires the park brake to adjust the rear pad to disc clearance.
A caliper picture would be great.
If you don't have park brake cables hooked up, you should. The other option is to crawl under the car and use a pry bar on the park brake lever to adjust up the rear brakes.
These calipers were a poor design and the adjusting mechanism often seizes. It is best to use park brake cable and use them EVERYTIME you stop to help keep the mechanism from seizing.
I searched Right stuff and if the calipers are the same as the ones pictured in most of their kits they are the kind that requires the park brake to adjust the rear pad to disc clearance.
A caliper picture would be great.
If you don't have park brake cables hooked up, you should. The other option is to crawl under the car and use a pry bar on the park brake lever to adjust up the rear brakes.
These calipers were a poor design and the adjusting mechanism often seizes. It is best to use park brake cable and use them EVERYTIME you stop to help keep the mechanism from seizing.
Switching from drum to disc doesn't always dictate changing the proportioning valve.
It really depends on what calipers were used in the rear that does. Many automakers use the same valve for their drum and disc trim level cars on later model cars.
Ive done a couple of rear disc swaps using the "drum" proportioning valve. A different part number doesn't necessarily mean its internally different!
It really depends on what calipers were used in the rear that does. Many automakers use the same valve for their drum and disc trim level cars on later model cars.
Ive done a couple of rear disc swaps using the "drum" proportioning valve. A different part number doesn't necessarily mean its internally different!
Rear drum brakes as originally equiped on most older cars require more pressure bias than rear discs to properly balance a car during a hard stop. Lock the rears while high speed cornering, and you'll be spinning in less than a blink. Then you'll be cleaning your pants out
Note that when I originally went to the Wilwood adjustable prop valve, I had a rear axle with WS6 rear discs. I had to dial the pressure down to about 70 percent to balance the brakes right. As mentioned by others, these older OE rear discs are royal pain in the can to maintain. I always had a slider pin bind or problems getting the e-brake to back off for pad switchouts.
I eventually switched to LS1 rear discs (dialed pressure down to 60 percent) and have been happy ever since.
TECH Resident
Joined: Jun 2009
Posts: 856
Likes: 2
From: Elizabeth City NC
The biggest problem with the G-body was the 12in single diaphragm brake booster that was used in 86 and under on the Gbody. 87+ and up uses a dual 9in diaphragm that is a must for decent pedal feel. For some reason my 88 had a older single 12in when I got it and my brakes felt like garbage after C5 fronts and LS1 rears. After swapping to the dual 9in made a world of a difference.
Also Speedway motors has a disc disc combination valve that is a must and isn't expensive, I couldn't get a good bleed with the stock combination valve as its suppose to keep pressure from bleeding off the stock rear drums.
Just some food for thought
Also Speedway motors has a disc disc combination valve that is a must and isn't expensive, I couldn't get a good bleed with the stock combination valve as its suppose to keep pressure from bleeding off the stock rear drums.
Just some food for thought
Teching In
Joined: Nov 2006
Posts: 38
Likes: 0
From: West Michigan
No need to buy a new proportioning valve or pay someone to modify it. You can do it yourself for free. Here is a link that may help. I've done this on my G body and it works great.
http://www.classicperform.com/Instru...structions.pdf
http://www.classicperform.com/Instru...structions.pdf
No need to buy a new proportioning valve or pay someone to modify it. You can do it yourself for free. Here is a link that may help. I've done this on my G body and it works great.
http://www.classicperform.com/Instru...structions.pdf
http://www.classicperform.com/Instru...structions.pdf
