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Old 11-08-2020, 03:49 PM   #1
kda406
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Join Date: Apr 2013
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Exclamation C20 Rear Lockup after Disc Conversion

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
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Old 11-08-2020, 10:52 PM   #2
Captainfab
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Re: C20 Rear Lockup after Disc Conversion

Are the lines connected to the correct port on the master cylinder? As in the rear line to the correct port and the front line to the correct port? Before you bled the brakes, did you remove the sensor in the prop valve and install the little tool to hold the shuttle valve in the center while bleeding the brakes?
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Old 11-09-2020, 03:51 PM   #3
kda406
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Re: C20 Rear Lockup after Disc Conversion

Since I'm using the Suburban's factory lines between the MC and the proportioning valve, it seems unlikely that I could have them backwards - but I'm going to check!

I did not use the little tool in the valve when I've bled the brakes in the past. The truck always seemed to bleed just fine without installing that tool. But with that being said, something is wrong, so I'll bleed them again with that tool installed just in case.

Thanks for the suggestions. I've been beating my head against the wall and really needed some outside advice.

-Kyle

Last edited by kda406; 11-09-2020 at 04:03 PM.
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Old 11-09-2020, 10:41 PM   #4
Captainfab
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Re: C20 Rear Lockup after Disc Conversion

Just bleeding brakes and starting with lines that do not have any brake fluid in them are two different procedures.
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Old 11-16-2020, 03:11 PM   #5
kda406
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Re: C20 Rear Lockup after Disc Conversion [SOLVED]

For posterity, it was the Master Cylinder.

First I verified the MC was not plumbed backwards. I finally broke down and bought a new (instead of another remanufactured) MC. I bench bled the MC and mounted it. I used the screw in tool in the proportioning valve and a power bleeder to get all the air out. I bled them forever, until all the fluid was clear (new).

I really didn't think that would solve anything, but was shocked when I drove her and I had normal brakes. No build-up delay, proper toe-play, and the rear brakes no longer lock up at really low speeds. I don't know what was wrong with that MC, but I'm glad to have the confounding issue behind me.

Thanks for the help, guys!

-Kyle
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Old 11-23-2020, 06:44 PM   #6
64shortbox
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Re: C20 Rear Lockup after Disc Conversion

kda406,
Thanks for posting your findings & your fix. I'm sure your info will help someone else down the road. I have a question for you - are you an Old Ham or CB guy?
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