Lost Reverse... help

[mattyg41383]

'93 SS, 4eat. 150k miles. Backing into a space at the softball field Friday night and lost reverse. We took it to the shop and started a few diagnostics this evening. No abnormal bits in the pan. we hooked up the pressure gauge to the Reverse servo pressure test-point. Only pushed around 24psi when put into reverse (i believe that at idle RPM it should be around 80psi). We speculate that it might be an o-ring / seal that let go and allowing the fluid to get by. If the solenoid were stuck or faulty, it wouldn't let any pressure though, right?
Given this info, has anyone had a similar problem or any experience with getting in there to replace said seals? The Alldata schematics for the tranny are not that great, so if someone has a good scan or files of tranny data and exploded views, that would be amazing.

We do have an outback 4.11 tranny, but the rear diff was already taken. These damn 3.90s are hard to come by.