Transmission : B series hydro 5spd
Symptoms: The transmission acts like it's in neutral in 5th gear

Took the car out for the first time after winter. 5th gear started popping out under load. I topped off the transmission fluid, but it was still popping out. Drained the fluid and refilled it. Doesn't pop out anymore, but it freely revs in gear. Also, although I'm not 100% sure, it looks as if 5th doesn't fully engage into place after the fluid change. I'm using asmoil synchromesh. I've never had any problems with this stuff before.

-1-4,R gears function normally
-It isn't hard to engage any of the gears. Shifts like butter.
-No grinds, not even in 5th
-Reverse is also fine
-Motor mounts are solid
-Shift linkage is solid
-Clutch is still good

I can coast in 5th gear and give it maybe 5% gas. Other than that if any load is applied then it disengages.

 

internal, i want to say the teeth in the synchronizer are sheared but you wouldnt be able to tell without removing and disassembling it.

 

also be the 5th gear itself.