Thanks Mitchgo. I was wondering about pulling air... the sound of the pump cavitating sounded like it. I had already resealed everything between the check valve and the pump, which didn't help. I didn't do anything ahead of the check valve because I wasn't losing prime, so I figured it was ok... maybe.
I think I might go back and re-check for leaks again though. You recommend using teflon tape AND paste on the fittings?? I haven't tried that.
With the bleeder... do you leave it on all the time, pump running or not running? I was kind of hoping to get away from wasting the extra water. I've been contemplating running two zones at once to relieve backpressure, but it used to work before so I figure something must be wrong and I don't want to just cover up the problem.
Thanks for the replies guys... I'm gonna try to get some time this weekend to tear into it again. I will let you know what happens.