LS3AEROCPE

Replaced rear drums with disc. Stops well but how have soft pental with a long throw?

chrisfrost

iTrader: (8)

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 .

TTLS1GP

iTrader: (10)

MC May not be large enough.

1981TA

iTrader: (7)

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)

oldgoat69

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

2nd Gen Fl 'bird

iTrader: (13)

This exactly^^^^^. Master cylinders for 4WD have to push a lot more volume to get correct pressure to caliper to work correctly.

spacemanspif

iTrader: (5)

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.

LS3AEROCPE

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?

mantycarguy

You will need to use both a dedicated disc/disc master cylinder and proportioning valve to make this system work properly. Check out mpbrakes.com. Tons of good info and troubleshooting.

TTLS1GP

iTrader: (10)

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!

Lost_in_calgary

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.

LS3AEROCPE

I used a basic kit from "Right Stuff" to match orginal wheel.

Lost_in_calgary

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.

1981TA

iTrader: (7)

True, but an 80s era G-body most likely did have a different prop valve. It's not an absolute must, but the primary indicator for "do the switch" will be if (after complete bleeding) your rear brakes lock up too early. I found this out the hard way road racing my TA. The rears did waaaaay too much of the work, while the fronts did very little.

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.

LS3AEROCPE

I too have talked to Right Stuff and they also said check the parking brake, and I do not need a different prop valve if adjusted right?

1981TA

iTrader: (7)

Factory prop valves from this era normally are not adjustable.

N20zuki

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

Anubis403

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

1981TA

iTrader: (7)

Wow, that's pretty slick.

bczee

iTrader: (12)

I have a Right Stuff MC.. I am going to check for the rubber piston as I told them it was for a Disk/Disk setup.. Thanks for sharing..

BC