#1: You can just replace the rings on the valve pin. I believe they are 006 cast urethane 90 durometer. However, if you removed the valve pin assy during rebuild, I would take it back apart, clean reg pin assy and the hole it assembles into, inspect, generously oil, and shoot it a few dozen times. These things always take a while to seat right for me after I remove and reinstall them. Rarely, have I actually had to replace the o-rings.
#2: I don't think I've ever had a reg piston assy go bad on four different Automags in ~16 years of using Automags. That doesn't mean it can't happen. But since it was right after a rebuild, I would check the same thing as #1 and also the reg seat o-ring. Look at the animation at zdspb.com>marker tech>animations, and you can see how these o-rings, if bad or otherwise not sealed properly, could let high pressure air get to the reg piston assy, either directly or through the hole down the middle of the reg pin assy, which would bleed to keep pressure at a safe level.
Good luck.
Last of the Salzburg Clan