The brake light on my dash came on and will not go out. I check the master cylinder resivor. Itwas a little low in the back chamber which I topped off. I have checked the switch on the brake peddle and  the parking brake and they are fine. This happened before and I bled the brakes and pressed hard on the peddle and the light went out. the light had stayed out until I recently had to quickly hit the brakes and the light came back on. I'm wondering if I have a bad proportioning valve. Anyone else have this problem or have any ideas?

How are the brakes?
The lights will only come on if the system has a problem on one end(an imbalance from front to rear), the wire(for the switch) is grounded/shorted, or the park brake is on.
If it came on when you had to get on the brake hard, most likely there is air in the brake system. It's possible the p-valve is faulty, but that's not all that common...it's actually pretty rare. I'd lean towards having air in the system at this point.

Do you use the parking brake often? I'd also check the switch for the e-brake. Once had a gummed up switch that got stuck in the on position....

I have not been able to get on the site for a few days, thanks for the replies. The more I think about it, the more I think the the p-valve may be ok. I will try the things you all have recommended and let you know the outcome. The brakes seem to be working ok, altough I seem to be loosing some fluid somewhere. I use the parking brake and the light would come on and go out as it should before the light started staying on.

[QUOTE=Jim]The brakes seem to be working ok, although I seem to be loosing some fluid somewhere. [/QUOTE]

Find the leak, and you've found the problem.
Be sure to unbolt the master cylinder from the booster, and see if fluid is leaking from the rear of the master...fairly common issue, but next to impossible to find unless you look. Most folks only look around the wheels/calipers for leaks...

There should be other post on the issue of rotor run out. if you find no leaks this could be a cause. I have not been able to fix mine yet I quess I will need new rotors. I need to bleed my brakes at about every 1000 to 2000 miles. simply if the rotor run out is off with our brakes it makes the pistons move in and out fast and they tend to suck air, you will notice after a bleed nice firm brake pedal the over time the pedal becomes mushy then one day you push on the brakes and the brake light will come on for the unequal pressure. You bleed, things are fine for awhile

if it was the p-valve you wouldn't have equal pressure between front and back. my p-valve went on my 75 and i am waiting for it to come in (backordered). when i stepped on the brake it felt like no pedal and the front brakes were locking up. pulled it out and was checking it out and it has what looks like a small freeze out plug (which was not there).