I've had so many brake issues with my brakes all the way around, so I figured it wouldn't hurt to post the problems I've been having on a forum.
I have already replaced: Both rear brake cylinders, master cylinder, front calipers, front brakes, rear drum brakes, whole hardware kit & springs for both rear drums, and rear brake shoes.
I have bled the whole system out like 5 or 6 times and still have a super spongy pedal. If I'm going 50 miles an hour and smash the brake pedal with 150lbs+ of pressure. It'll just roll to a stop after about 15 seconds of standing on the brake pedal. I understand it have a anti-lock module but it should still at least stop a lot better than rolling 50ft till a complete stop with that much force being put on the brake pedal. If anybody has come past a problem like this I'd love to hear what you think of mine. I've already spent 570 dollars in the brake system alone, so I'm really not trying to spend anymore unless I know its going to maybe fix the problem. I'm getting really stressed about it cause I cant even haul anything with it, due to the terrible brakes.

 

are you getting good fluid flow out of the rear when bleeding? If not,,,,check for a proportioning valve. It may need to be opened. The abs pump may need an auto bleed with the scan tool.

did the master go dry when the wheel cylinders were replaced?

 

Did you replace the hoses? The hoses can separate on the inside acting like a valve causing the problem you are seeing.

 

I've pressed the proportioning valve in while somebody was in the truck pumping the brakes and no air at all. Is there a scan tool that goes on a 1990 OBD1 to auto bleed the abs pump?
No I made sure I kept an eye on the reservoir and kept it topped off while replacing them. I think I should just replace the proportioning/Combination valve and go from there....

 

Do you mean brake lines? Cause on my truck there is only solid brake lines throughout the whole truck. I've checked all the three way fittings and all of my brake lines to see if maybe I had a small leak somewhere not letting it get enough force/pressure to the back brakes. Cause when I stomp on the brakes trying to stop it just does a nose dive, making it seem like I'm not getting enough pressure to the rear. But I've bled the rear brakes countless amounts of times. Like I told the other guy that commented, I think I should just replace the proportioning/combination valve just to see if maybe a seal is collapsed or busted not letting it get enough pressure to the rear to turn on the antilock module