Oh it stops!

[ah_5500]

Well kinda..
Ive had a local shop doing some work on my 72 K5 Im building. One issue after another and Im pretty much done and tackling it myself, undoing and redoing.
Currently my latest issue is they redid some brake lines, new calipers, and a master and booster. When I got it back the rear would lock up. I don't even know if the front was doing anything the rear would lock up so quick.

Upon inspection they didn't install a proportioning valve. So I sent it back.
This time they replaced all of the lines and added the prop valve.
I go to get the truck and what do you know, the rear is still locking up. Any speed they lock up, even rolling up to a speed bump at 5-10mph they lock and it tries to go sideways. The brakes don't seem to have any resistance like theyre sticking. The mechanic says to just drive it and they should stop, which seems odd to me and dangerous considering the issue.

Im hoping to get some thoughts and opinions and Im going to post a picture up to see if its ran correctly or not.

Additional Info:
The two front lines coming off the prop valve go to the front brakes and the rear to the rear.
The Blazer is 5.3 swapped ( which seems to have its own list of issues too :/ )