1996 Chevy Astro 4.3 Vortec V6.

Rear brake line replacement done by unknown shop.
Master cylinder and wheel cylinder replacement.
Rear brakes are inop. was told by repair shop that ABS pump assembly needed to be replaced.

I've tried bleeding rear brakes. they gravity bleed just fine. Pedal is firm and feels normal. raise van into the air and apply brakes when in drive and the rear wheels slow down but just keep spinning. It there a proportioning valve inside the ABS pump assembly that could be the culprit?

I did a tune up on my 01 Astro van cargo thinking that was the cause of the shaking which never stopped, it seems only on cold start it shakes less once it warms up it gets worse, so this is what I've done- changed the 02 sensor, didn't help, did full tune up didn't help scan showed misfire. Put new injectors still didn't do anything still get obd P0303 checked pins on harness all fine,compression of engine is fine, i'm confused on wot it is causing the shaking when Ive done all measures to the 303 for #3 misfire im confused at this point what to do next.