1990 Nissan Pathfinder

I have a 1990 nissan pathfinder, while driving I had a transmission fluid leak and immediatly pulled over and had my vehicle towed (lost about 2 litres of fluid) the mechanic serviced it replaced the oil pump seal. He took it for a test drive. All was going fine untill he heard a pop and then no gear engagement at all.
He pressure tested ports all were at about 0-6 lbs except the reverse port which was at 60 lbs indicating to me that the pump has not failed Is it possible that there is a solenoid or valve that is stuck and can this be checked without dropping the transmission again?
November 7, 2006.

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