Why the **** won’t my rear brakes work??

[Nick_R_23]

Need some serious help here, about to drag the truck out in my driveway and burn it over as much trouble as these brakes have been giving me. I’ll give a recap of what’s been done on this truck.

1978 Chevy K20. D44HD front end. Corporate 14 bolt rear end.

Originally a vacuum boost truck. Originally had 11” rear brakes.

Swapped from vacuum to rebuilt hydroboost unit from a 1983 CUCV.

Installed a new Master Cylinder for a 1993 Suburban K2500 (aluminum body/hydroboost)

Installed new spindles, rotors, pads, calipers, hoses, and lines on the front.

Sourced 13” backing plates and changed over to 13” brakes. Installed new wheel bearings, drums, shoes, wheel cylinders, hardware kit/springs, parking brake kit, and all new parking brake lines in the rear. Also replaced the rear brake lines and brake hose.

Went to adjust the rear shoes. I can tell the new drums are slightly out of round, as they drag slightly on the shoes on only about 1/2 of the drum. Both sides do this. However, at the adjustment sight windows at the bottom of the backing plates, the shoes have a small gap and don’t touch the drum. I could likely crank on the adjuster and force them to, but the drum would be locked up. It feels like the shoes touch the drum at the top before the bottom. The driver side is slightly better than the passenger side. Currently, I have it adjusted where there is some slight drag.

After attempting to bleed the brakes, the front worked fine. I could not get consistent pressure to the rear, although the rear brakes did work really well once sporadically. I determined the proportioning valve was shot. Ordered a new proportioning valve and the bleed lock tool for it. Installed both of those and had better results, but still inadequate pressure in the rear.

Warrantied out the brand new master cylinder for another one and rebled the system. Now I have good pressure to the rear. However, the hose from the frame to the axle was machined incorrectly, and the hard line wouldn’t seat and kept leaking. This damaged the new passenger side hard line. I warranted out the hose and reflared the hard line and replaced the fitting. Went to install and the drivers side threads pulled right out without even tightening down, damaging the driver side hard line. Replaced the hose again, with a different brand, and reflared the other hard line and replaced that fitting. Installed the new line and rebled the system again. Removed all the air and the pedal feels good.

However, if I have the pedal pressed I can still turn the rear drums by hand. They definitely drag more than what my adjustment was, but they definitely don’t work. Front seems to lock up solid and work great. For an extra brain scratcher, if I manually pull the parking brake cables by hand, both rear brakes will work perfectly via the parking brake.

I’m completely at a loss here.