He stated that he just got the transmission and it has 1st, but no reverse. The solenoid will activate and go in, but there is not any reverse at anytime and the plunger will not come back out. It could be like Mark said that the case is needing the extra holes drilled to pressurize reverse (the case mods that I had seen were for extra dump for the Pro-brake) or the spool valve could have been left out completely.
Bill Mitchell