Brake Proportioning Valve

[ED78CUTLASS]

Gentlemen,
How can you tell if your brake proportioning valve is out of whack?
I will get right to it.
Before; rear tires would lock up under hard braking.
After; no rear tire lock up. Car brakes like a anti-lock system but without the pulsing pedal feel.
Regular driving is unaffected, until, I have to avoid running a stoplight. Then the brakes kind of fade but the car stops. The master cylinder and booster are in good shape.
Thoughts?
I have been driving this way for awhile. I just try to avoid the heavy,high speed braking.

[88hurstolds]

Could also be a kink but in your case it's probably unlikely.
Sounds like your Prop. valve is out of whack, the valve could be jumping around causing this.
A new Proportioning Valve isn't much, you could grab one from the bone yard but I wouldn't take chances on one...

[ED78CUTLASS]

Is this an item that should be bought from GM or are the aftermarket ones durable?
Thanks,

[88hurstolds]

Cannot be purchased from GM anymore, at least the original stock type can't. They may have a replacement though...
I've seen some decent aftermarkets on eBay.