K5 Brake issue

Subzilla

Subzilla

Test Driver
Joined
Jun 13, 2011
Location
Triad - Mt. Pleasant, Concord & Georgeville
Trying to wrap up my axle swap project on the 75 K5 and having some spongy pedal issues.
Stats:
Installed 14 bolt full floater WITH disc conversion using 3/4 ton calipers
Replaced front axle but didn't touch or break open front brakes/lines.
Didn't replace old master cylinder but it was working fine prior to project. 75 K5 model MC
Bled all four corners many times.
Initially had brake warning light come on (was surprised to see it working in this old thing) but bleeding brakes another time turned it off....so I assumed the line pressures equaled out.
While trying to hard brake on a gravel road, the brake light came on for a second or 2.
Removed the lines from MC and plugged the ports. Brake pedal hardly moves which told me it was working.
Vehicle sat for probably 10 months without moving or engaging brakes.

With the initial bleeding, I had a very spongy pedal going to the floor with about 25% brake power. After more repeated bleedings, I now have about 75% brake power. The vehicle will stop but I can't get the 37's to hardly lock up on a gravel road. Seems like the fronts do better than the backs. Are the rear calipers asking for more pressure from the MC?? Any other causes? I don't see any leaks.
 
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Discs in general require more fluid volume than the wheel cylinders in drum brakes (especially with larger 3/4 ton calipers). You most likely need a master cylinder with larger bore diameter. This should firm up your pedal.
 
Had same problems with 78k10 when I went to 1tons and disc brakes in rear
I put a master cylinder from an early corvette 82 83 84 (can't remember) first year they had 4 wheel disc and fix my problems
 
The brake warning light was the proportioning valve sensing that there was a lack of fluid pressure going to the rear. A 1 ton MC from 73-91 GM trucks will help, as will the aforementioned Corvette MC. If that doesn't take care of it, you may need to investigate installing an adjustable proportioning valve.
 
Yeah, that proportioning valve has a check ball in it that closes when low fluid pressure is indicated, which sets off that light. I've had them stick before, giving a false indication of a problem elsewhere within the rear brake system.
 
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