68 GTO Front Disc Brake Conversion Help - Rears locking up - Not right.

[old66tiger]

My buddy bought a front disc brake conversion from the Right Stuff for his 68 GTO. Now he has power front disc and drum rears. He opted for the factory correct hold off valve for the front brakes. Apparently, this valve holds off about 80 psi until the rears catch up with the front action. The kit came with the corvette style master, new lines, hoses, hardware, pads, rotors and spindles.

The car brakes pretty good except that the rears lock up on a mildly aggressive stop. He has tried bypassing the factory hold off valve, replaced the master and the rears still lock up. He even plugged the port for the rear brakes and the with just the fronts working, the front brakes will not lock up.

After driving it, I agree that the rears should not be locking up. Let me put it that way, if you were driving down the road at 20 mph and a squirrel ran out in front of the car, a quick stop would result in both rear drums locking up without the fronts locking up.

Anyone else having this problem? Thinking about installing an adjustable rear pop valve to take some pressure off of the rears.

[MonzaRacer]

Well in drum brakes you need a 10 psi check in back brakes, and make sure the brake shoes are adjusted up so you have 2 hand no strain drag against the drums, too loose drum brakes will grab bad.
And if your doing brakes an adjustable prop valve is best way to do brakes, then you simply make it stop , if the backs lock, take out/put in 1/4 to 1/2 turn at a time till car stops good. There should be no check in front lines UNLESS you need it to alleviate long pedal travel, also you need to make sure you have a disc/drum master cylinder, not a disc/disc master.
If they are stock front disc, a stock 68 master would suffice honestly.
OH yeah if it has a large bore Corvette master cylinder its never going to lock them up, it may require a smaller diameter bore master cylinder to help build pressure faster/higher in the fronts, and MAY even need the 2 psi check int the front lines to maintain the "takeup" in the fronts.

[Apogee]

You mentioned that he is running the "factory correct hold-off valve" to slow the actuation of the front disc brakes. Is this part of a combination valve block with a hold-off valve, balance switch and disc/drum proportioning valve or a stand alone unit as were used on a few applications in that era? The reason I ask is because if it is part of the combination valve block, then you already have a fixed proportioning valve in the system and should not add another whether it be fixed or adjustable. The hold-off valve should be fine once everything else is sorted out.

The fact that you can't lock up the fronts at all indicate a lack of pressure at the calipers which could be a result of an oversized MC bore, underpowered booster (low vacuum?), crappy pads, or too little pedal ratio. I would suggest focussing on getting adequate pressure to the front brakes rather than reducing the pressure to the rears, at least based on the information provided thus far.

Te 2 psi residual pressure valves (RPV) are typically only used to prevent brake fluid backflow when the calipers are higher than the master cylinder. In all honesty, 2 psi X 7 square inches is only 14 pounds of force on the caliper piston...hardly enough to make a difference if you have "take-up" issues. If you've ever removed a piston from a caliper, then you'll know what I mean. There is a lot more friction from the piston seals than that.

In the rear, you will want to add a 10 psi RPV for the drum brakes so that you have some pressure on the wheel cylinder cup seals during retraction. The C4 Corvette disc/disc master cylinder does not have an RPV in the port for the rear brakes like an OE disc/drum MC, so you'll need to add an inline aftermarket unit or swap MC's. Without the RPV, the seals can "burp" air past the seals when the wheel cylinder pistons retract. There are certain wheel cylinders that use mechanical expanders inside the seals, however GM did not use them on their mid-size applications in that era to the best of my knowledge.

Hope that helps,
Tobin
KORE3

[old66tiger]

One other thing that puzzles me is the progression of disc brakes in the A-body cars. This is the way I see it - correct me if I am wrong.
*
1967 - 68 was the first year for the disc brakes. They used*2 piston calipers, introduced the*hold off valve for the front brakes and only had a distribution block with*no proportioning valve in the rear.
*
1969*kept most things the same as 68 but eliminated the 2 piston calipers for single piston calipers.
*
1970 kept most things the same as 69 but now added*a rear proportioning valve back by the rear axle. Probably because 69 system was a disaster???
*
1971 did away with the previous architecture and went to a hold off/ valve combo unit at front of the car - I am sure it was a cost cutting move on the part of GM to get rid of the separate hold off and proportioning valve.
*
I would say that his car is setup much like a 69 would have been - single piston calipers, the hold off valve by the booster and no rear proportioning valve. Was 69 a crappy year for brakes and GM was learning from their mistakes as they evolved the brake system?