For quite some time I have been trying to solve a hard brake pedal with little braking. Some owner prior to me had converted to discs and I have been trying to troubleshoot some of the problems. I have low vacuum (12 inches at idle and up to 20 at 1500 rpms) so I wasn't sure about the booster and the MC is not original. I found yesterday that the wire to the brake light in the combination valve had been cut. Hooked it back up and the light stays on, so how do I reset the valve? I then need to determine if the rear brakes have not been getting fluid or what. Any recommendations? Any help is appreciated, I'm about ready to puchase new booster, MC and combination valve just to make sure everything is in good working order. Thanks in advance.

Is there no one that knows how to reset the valve? Come on guys help me out I know someone out there has had this problem.

What year Chevelle are you working on?

1969

You'll need a helper to do this.You'll also need to find which side of the valve has shut down.Most likely the rear but still have to check.If the valve has shut down one side of the brake system, you'll have little or no brake pressure to that side.You'll be opening the bleeder on the side that hasn't shut down.
Start the engine if their power brakes.Whether its front or rear,open the bleeder thats furthest from the master, then slam the brake pedal to the floor and hold it there.Immediately close the bleeder.Check to see if the brake light went out.You may have to repeat this once or twice but it does work.

I learned something from this thread. I had already typed a response saying that GM Failure Warning Switches are spring loaded and will reset themselves. That has, in fact, been my experience. I've never actually worked on a GM car where the valve wouldn't reset itself once the system had been repaired. I HAVE worked on Fords that had to be manually reset.

Then I got out "the book". My Delco brake service manual confirms what 71350 stated above: Some valves have to be reset manually. I thought that was just for Fords!

All of that said, I'm still betting on a pressure problem to one brake circuit or the other, and when you fix the pressure problem, the light will take care of itself. That's where I'd go if the car was in MY driveway.

Possible ways to identify a pressure problem: Find a safe place to do brake testing. Drive the vehicle, and make a couple of hard stops. Crawl under enough to check the brake temperature. If the discs are hot and the drums are just warm, or cool, the drums aren't working. Or the other way around, if the discs are cool, while the drums are hot, the discs aren't working. Still another possible way to verify operation is to look for wear on the pads/shoes. If the pads are worn and the rear shoes aren't, there may be no pressure to the rear, therefore the rear brakes aren't working, for example. (You'd have to know the history of the vehicle to rule out the pads or shoes being recently replaced and providing a false diagnosis.)

If front and rear brakes get hot, and appear to be working, I'd gently tap the warning light valve with a SMALL brass or plastic hammer, if it's spring loaded but stuck, the tapping might cause it to re-set. Be careful, if the valve stuck once, it could stick again. You might consider replacing it.

Schurkey,
Great advise and your right most valves do reset on their own.These valves are designed to close off the side of the system thats leaking so that the master doesn,t empty out and you have some braking left.The warning light comes on to alert you of the problem.Sometimes they need a little help getting back to center.
Just put a floor jack under one frame rail so you lift both wheels off the ground an inch or so.Then hold the brake pedal,if the valve is shifted you'll be able to spin one of the front or one of the rear wheels by hand.

You'll have fun spinning a rear wheel if the vehicle has a positraction axle. That's my rationale for using brake temperature. If they're hot, they're working.

Based on what I see in the diagrams of the Delco service manual, it does not appear that the warning valve can shift far enough to "block" a passage and prevent leakage. Looks to me like it can shift position enough to trip an electrical switch, but cannot move enough to interfere with fluid flow. There are cutaway drawings of only three kinds of warning switch, so I guess anything is possible for the versions not shown.

I've checked both rear cylinders and I'm not leaking any fluid. Both front calipers also seem to be OK. I'm not down on fluid, yet the valve has tripped and doesn't seem to reset. As far as the braking goes, I feel like it's the rear brakes that are not functioning. Due to the weather here in Idaho, I'll have to wait a while before I can take the car out of the garage and test it. It has been this way for quite a while and I really didn't discover that the wire on the valve had been cut until just lately. When I repaired it was when the light came on so I feel the previous owner knew about the problem and solved it with cutting the wire.

What causes the valve to shift? If you don't have a leak in the system what causes the fluid to shift the valve where the light would come on?
It seems that with pressure in the valve it wouldn't shift that much.

Do the back brakes bleed easily? I'm just curious if you have to fight it get fluid from the rears.

I'm going to try that the first of the week when the weather is a little better to crawl under the car and I have some help. I think I'm fighting a number of problems and have to sort them all out.

Some of these valves can be re-set by standing very hard on the brake pedal once the system is in good order.

OK, I found a small leak in one of the rear cylinders. Replaced the cylinder and bled the rear brake. Not a lot of fluid came out but I just barely cracked the bleeder. It did seem to flow freely. However the light is still on and the brakes are still almost to the floor with little pressure. It just seems that there is very little resistance to the pedal. I can't really pump up the brakes, they just keep going to the floor with little pressure and just before bottoming out, they start to work. Any ideas?

Shot master cylinder (or air trapped in it) would account for both problems: Low pedal, and light won't reset.

OK, today I decided to pull the master cylinder off just to check the size of the bore and found a small amount of fluid between the master and the booster. About 1/2 spoon full. The fluid hasn't dropped any that I have noticed so I'm guessing that it's a small leak. There was some fluid by the snap ring in the rear and some on the rubber (gasket?) just inside the booster. I don't believe that any got inside the booster but I believe that I need a new master. Some people have told me that a small amount of fluid is normal, but I'm not sure...Any ideas?

No,thats not normal.You should replace the MC.You need to replace it anyway so why not eliminate this as a possibility.Continued leakage here will ruin your booster eventually.
When you bleed your brakes the next time you can put a small piece of scrap wood under the brake pedal so you can't bottom out the new MC.Also make sure you bench bleed it good.