brake issues

[hardhead]

Took our 66 for the first ride from a 3yr rebuild and the right rear is locking up under moderate braking. Everything is new except for the proportioning valve, new hard lines, rubber lines WC, MC, Calipers and hard lines. We used 72 disc brakes and a used 72 prop valve, S10 8" booster and MC. I'm pretty sure it's plumbed up right. My son took the plug out of the little shuttle valve and said it was spring loaded and he could push it with his finger. I heard talk about a button on some of them but ours doesn't have one. Any ideas? We are hoping to get it registered and hit cruisin the coast in MS in a couple of weeks.