Lost reverse?

I agree with 69MOPE, the rear servo can actually split, therefore trans fluid leaks through, no pressure to to engage rear band = no reverse
with the sump,filter removed, if you can remove the pin that the rear lever pivots on, (sometimes there is a tab on the case you may need to grind off)
Then compress the servo with a G clamp, remove the circlip, the piston will now drop out when you loosen the G clamp.
It is potentially possible to repair this without removing the box.