The Stovebolt Site Forums The Shop Area Engine Shop Switched to power disc-now locking up

HELP!!!! I switched my 1963 Chevrolet truck to disc brakes with a power booster and a dual master cylinder. I bleed the brakes, repeated no air for sure. I can drive the truck throught town, stop at all the light, go shopping etc. everything great next day I drive 4-5 miles and the brakes lock up. A mechanic friend of mine said the rod coming out of the booster was not allowing the pressure to release on the brakes. We took the master cylinder loose from the booster and the truck started moving. This booster and master cylinder came as a set for Master Brakes> why would it work so good one day and lock up the next? do I need to adjust the pedal under the dash, or work at the booster. PLEASE help a locked up brain...

The rod not having free travel could very well be the problem. Another thing to check would be the seats in the master cylinder, the place the flare nuts on your brake lines screw in. If by chance it's a drum brake m/c then there are residual pressure valves built into these seats--something you don't want with discs if your cylinder is high mounted. Even a low mount cylinder with discs only uses a 2# valve so a 10# drum brake valve will make discs drag.

The rod should be adjustable from the pedal arm to the booster. Another thing to check is that the rod is pushing straight into the booster. Some aftermarket suppliers don't get the bracket angle correct and the rod will bind up. In some cases you may have to drill a different hole in the pedal arm and adjust the rod accordingly.
Fred

I just made the same brake change, power dual low mounted power brake unit with disc in front and drums in rear. Mine also holds the brake on for about 30 seconds after I release the pedal. After that time the brakes will free up. I've noticed that even the brake light will stay on for a short time after I release the brakes. I have a 2 lb residual valve in front discs and a 10 lb to the drums. Any thoughts?

Should the brake arm be modified from the 6 to 1 manual ratio to a 3 or 4 to one ratio? In other words redrilled lower in the brake arm.

I just measured the rod going into the M/C with the motor running and the petal pressed to the floor the rod measures 2 3/4" with the petal released it measures 2" but the hole in the M/C is only 1 1/2". Is this the problem? do you adjust this rod at the petal or is there an adjustment at the booster?

Dennis,
It can be on both depending on the Mfg. On some boosters the rod on the MC end has an adjustment, if not it can be replaced with a shorter rod. If the rod is too long it will certainly cause problems. I think you should contact the supplier before you go much further and make sure they are aware of the problem. Most have a tech support person. You also don't want to void the warranty by modifing something.

wags, your problem may be a result of having a residual built into the master and installing additional residual valves, you should check with your supplier to find out what they set it up for.

Fred

This is from Dendo,
I am proud to say I am un-locked and driving my truck again. Thanks to all of you.
My problem was, just in case you were fighting the same battle.
1. the rod from the brake petal to the booster was in a bind
2. the rod was also to long causing pressure to stay on the master cyclinder