72 K-10 Rear Brake Problem...

[azsmokey]

We converted our 72 K-10 to rear disc brakes during our rebuild and we are now having some problems. We ordered the disc brake conversion from Summit Racing and the fit is great. We also get the Booster, Master and Disc/Disc combo valve from Summit. After bleeding the system, the front brakes lock hard whereas the rear brakes do not. With the front end jacked up and the brakes depressed, the wheels will not budge. The rear wheels, under full braking, can be turned by hand??? What gives?

[azsmokey]

Any ideas?

[rsavage]

Is your proportioning valve adjustable? If so, have you tried adjusting it? When you bleed the brakes, are you getting the same flow of fluid from the rear calipers as from the front?

[D.PASSMORE]

Rear brakes are used for drag and the fronts are mainly for stopping. But, when you mod, you have to get all the right components or there will be little issues. Funny, I was thinking about doing the mod on my SWB 4x4.

[azsmokey]

The prop valve is not adjustable. It is a disc/disc combination valve. It seems that I get the same fluid flow when bleeding the rear as well. The parking brake cables are not installed yet. Would that have any effect on the rear caliper operation during normal braking?

[BurnoutNova]

Having the e brake cables unhooked should not have an affect on the caliper. I wonder if you have the wrong prop valve. Any time you are doing 4 wheel disc brakes, I prefer an adjustable prop valve. With the differences in master cylinder bore size, and caliper bore sizes available an off the shelf prop valve is almost never correct. I have a cheap ($40 ish) knob style wilwood valve on my K10 that I converted to rear discs.

What kind of calipers does the kit have? I assume like a metric Cadillac rear since it has the e brake built in. I have seen some of those with different piston sizes ( 2 1/2" and 2 3/4"), so if it is a small bore piston, you will have less clamping force on the rotor. That, combined with possibly having the wrong output pressure from the prop valve and that's enough to do what your describing.

I'd try a different valve to start out. Maybe call Summit and ask them to verify your valve is correct?

[BurnoutNova]

The other thing is.... make sure you have all the air out. I know you probably bled it and bled it, but sometimes it takes a lot to get all of the air out. I like the pressure bleeders so you can force fluid all the way through without having to pump the pedal the whole time.