GMC Repair: 1990 GMC Sierra (K1500), gmc sierra, foot lock

Question
Hello, I was driving down the street the other day doing about 60km/h, my drivers side rear wheel decided to lock up solid on me......I pulled off the road, pressed te brake pedal and was on my way again, nothing happened the rest of the day, today on the way too work I was crawling through the grocery store parking lot and it locked up again this time I was only doing 5-10km/h.....I got to work, went to back into a parking spot, and it locked up again, pressed the brake pedal, it would go back a foot, lock up again, press the brake pedal go back a foot, I repeated this process till I was in the parking spot....what could be causing this too happen? It seems to be alot worse when reversing....when in drive it locks up randomaly, differnet speeds, in reverse it does it all the time, usualy changing from reverse to neutral to reverse allows me to back up more than a foot then locks up again.....please help me, it's booked in to the mechanic for thursday. But i'd like to figure out a estimated repair cost.......

Thanks

Brett

Answer
could be a leak in the axle seal causing the brake shoes to be contaminated or a faulty proportioning valve axle seal repair here at the dealer could well exceed $300.  not so sure about the proportioning valve due to parts are not available for that that year via GM any more