Which tranny? The 722.0/1/2 trannies have a band piston that brakes or blows its seal. It can be fixed from the outside.
The 722.3/4/5/6 trannies don't use a band for rev they use a clutch brake. (multiple clutches, splines to the trans housing). These trannies will need disassembly usually; for lack of reverse.
Bosch Master, ASE Master, L1
33 years MB technician