I was always told to start a fresh 727 in neutral, then go into forward, then reverse. A 727 doesn't actually pump in Park. That's why it says to check level in neutral. Anybody that does a 727 rebuild, knows how to pressure check everything before they put the valve body in. The valve body itself, or casing could of cracked from being over tq'ed. Or it could of been warped and not setting flat if tightend wrong. A valve sticking in the VB or things of the sort. I would of thought one of the pro's would have chimed in for you by now. The wrong type of seal will also cause havock. I am just a novice, but 40 yrs playing with mopars and watching friends build them has been a plus. Seen lots of times a 50 cent o ring will stop it dead