C20 Rear Lockup after Disc Conversion

[kda406]

I'm seeking advice regarding the rear wheel cylinders on my 1966 C20 after a disc brake conversion on the front. The rears always lock up with minimal brake pressure and the FAQs don't seem to cover this.

Even at low speeds like 5-8 MPH driving in a parking lot, touch the brakes and the rears locked up.

I got the disc brakes and cross member from a 76 C20 Suburban donor truck with JB7 brakes. I used everything I could from the 76 including the power booster, proportioning valve, etc.

My rear brakes are like the ones shown in this thread: http://67-72chevytrucks.com/vboard/s...d.php?t=660022
This C20 has the full floating HD rear axle with coil springs. I believe the problem is to be caused by my truck's wheel cylinders being 1.125", while the Suburban appears to have been 1.000". Unfortunately, 1-1/8" seems to be the only size available for my backing plates.

To "fix" the problem, I tried the 1976 JB5 and JB6 master cylinders on my C20, but they had no affect at all. I put the JB7 master cylinder back on.

So I added a Wilwood adjustable proportioning valve to cut the pressure to the rears. I adjusted it until the brakes did not lock up when I touch the pedal, but I think it still does too much braking from the rear.

The BIG problem with the Wilwood is the brakes are hard to apply at first and then seem to being stopping a little later (there is a braking delay). Pumping the pedal does not seem to change this. I have bled the brakes many times, even using a pressure bleeder. The Wilwood seems to "slow down" the brake action instead of just keeping the pressure down on the rears.

All the FAQs make disc conversions out to be an easy change, but there is no mention of fixing front/rear brake bias problems after the conversion. Any help is appreciated!

-Kyle