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GM Brake system expert help requested

I am attempting to diagnose a problem on an aftermarket GM 1-inch bore brake system (having been added to a classic car for safety purposes).

The braking system is a Corvette-style street rod brake setup, discs front, drums rear. Using silicone fluid. The entire system was purchased as an aftermarket kit and now has been replaced bit by bit without uncovering a particular problem, which is little or no braking power, particularly in the rear. The current project has been replacing the rear wheel cylinders, the last bit of the system that hasn't been re-done; in trying to bleed the system I'm finding little pressure making its way to the rear brakes. Just a bit of fluid is coming out with each purge, and the pedal never gains any back pressure (despite dozens of reps), and the drums can still be turned by hand with full pedal pressure.

Blocking off the master cylinder produces a rock-hard pedal, as I think it should. Vacuum is good. The pedal has no feel at all with the car off. With the engine on, there is just a bit more feel, but still going all the way to the floor, and the pedal is slower to

*rise* after pushing it down. There are no leaks in the system.

I am suspecting some issue with the booster and/or the interface between the booster and the master cylinder. The initial install seemed to indicate that there was too much travel out of the booster before the piston engaged; we tried adding some washers around the inner steel piston (small enough not to block the main piston) and that helped a bit, but not much, and doesn't seem like a viable long- term solution.

There is also a brass combination valve in the system that has not been replaced since the original install; these seem pretty foolproof but can something go wrong there?

Thanks in advance for any help.

JGM

Reply to
jgmclean0
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You blocked the line at the master cylinder and the pedal was rock hard. Move to the next point and repeat until you find where the pedal goes soft.

BTW, why DOT5 fluid? I hear nasty stuff about it, and if there is ANY chance that someone might put DOT3, 4 or 5.1 into the system, I'd avoid DOT 5 completely.

Reply to
Noozer

I agree, and the first place I'd try is the rear-wheel output of whatever type of combination valve (safety/proportioning) that system has. My bet is that the pedal drops to the floor at that point and that you've got a bad combo valve.

By the way- why bother with a power booster in a street rod? So far I've put disk/drum brakes on two 60s vintage cars, both of which weigh well above what most street rods would (3900# and 4200# respectively) and in both cases the best (to my taste, anyway) pedal feel and overall performance was achieved by using a MANUAL brake pedal and pushrod setup with the disk brake master cylinder. Some people don't like the higher brake pedal resting position and longer pedal stroke, but I find that it allows much more positive braking feel and control. Just a thought.

Gotta agree there, too. There are several moisture-resistant variations of DOT 4 fluid (Castrol GT LMA4, Valvoline, and others) that are almost as moisture-impervious as DOT5, without all the disadvantages such as lack of lubrication qualities and tendency to allow water to accumulate at low points in the system.

Reply to
Steve

Thanks, to you and Steve, that will be the next thing to check.

It's not a rod, it's a near-stock '60 Cadillac, with about a 5,000 lb. curb weight. The stock brakes wouldn't cut it in modern-day driving (though they are saved and set aside).

Not my car, but I think the idea is that it's better for the long periods of storage. Will pass along the thoughts; what does it take to change over to DOT 4?

JGM

Reply to
jgmclean0

My heavier one was a C-body Dodge, which is lighter than a '60 Cad. I drove it for many years with a single master cylinder and 4-wheel drum brakes (it's been in my family since 1967). Looking back, converting it to front disks was the single best modification I've ever made on any vehicle! The drums were better than average 60s drums and totally unlike Mopar 7" A-body drums which were woefully inadequate. But still, just having a dual/split system is such a huge advantage that it makes no sense to leave a single-cylinder system on the car, especially since its my regular driver.

I'm not really sure about what it would take to go "back" to DOT 4, but I think just a routine flush would do it. Better do some checking, though, it might indeed be better just to leave it DOT5, especially if its not actually going to be used much so the faster seal wear with DOT

5 won't come into play.
Reply to
Steve

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