Reverse issue

I think something is wrong with my transmission. It's the original 904 but with around 10,000 miles since it was rebuilt. This only happens when its been sitting for a while. I would say at least a week without moving. Yesterday for example I went to drive it. When I shifted into reverse The car instantly shifted and the rpms dropped as normal. When I let off the brake it didn't backup. My garage floor is flat. I let it sit for a few seconds then it started to move. I was thinking I was low on fluid. After a good drive I popped it in neutral to check it and the line was almost down to "add 1 pint" Is my trans about to take a crap? I sure hope its okay being it was rebuilt a few years ago. When driving it shifts fine through all gears. If something is wrong can it be fixed with the trans still in the car?

Any info is greatly appreciated. Thanks