ajme, looking through my manual, no reverse indicates that either the B3 clutch is worn out and the servo does not engage it anymore, the B3 seal is leaking, there are actually 2, the big teflon ring around the pistion itself which is most likely fine, or the rubber guide seal, that might be your problem right there. If the reverse went out slowly, then it's your band that is slipping and you will have to replace it, or get a bigger pin, but replacement is probably a better option. In order to replace the seal, everything must come out, if you look through the manual, the B3 seal, is the very first thing you install on an empty bell housing. Lastly, the pump clerance might be too big, but since forward gears work, that is probably fine.
Now for some more pictures
In this series of 5 you can see the first tear down steps, first the rear cover comes off, in order to take it off you need a special socket for the yolke nut, and the nut itself must be replaced.
The rear cover has quite a few parts behind it, the speed cable comes in through it, the governor which is transmission specific, so if you ever take it out, make sure you don't break it, and put it back in to the same transmission, these are not interchangable. You can also see the parking ratchet, if you ever wonder what makes that click when you put your car into park and it rolls a bit, then clicks and stops, that big dirty lever does it when it clicks into the sprocket on the output shaft.
Finally, you can see what's behind all the pretty parts hiding behind the rear cover... nothing really, the output shaft goes into a seperate compartment, so you are done with this part
And now the governor itself, the cover is real easy to take off, no pressure on it what so ever, just don't break it
With the governor and all the parts behind the rear cover out, you can take the entire planet assembly out, but it will have some challanges, so it's better to do what the manual says.
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