8 3/4 REAR - WHAT size brakes? 9" or 10-11"

I've never heard of 9 inchers on an 8.75

BigB are you in trouble yet again?
Tell you what tho, it would be best to go check out that W/C on account of there could be any of 3 sizes on there. And trying to take it off usually results in a torn hard line.You need a special flare-nut wrench, a good penetrating oil,and probably some heat.
So what I would do is block the brake pedal down some to close the compensating port, so you don't drain the fluid all out. Do that right away. As in right now.Then get into it. Get everything out of there right down to the wc. Pop the rubbers off and push the pistons out. Be prepared for a mess.Clean it all up, and check the wc. bore. Pitting on the inboard side is no biggie. If it's pitted out where the seals go, well, it's had a long life, but it's all over now. Kits are cheap. I would try to kit it if at all possible. You may need to clean the bore out with a small hone. You might as well do both sides.
All the best.