There were several different transmissions mated to the 4-108. Possible problems would depend on which one you have - since they actually work in different ways.
I have a hydraulic transmission mated to a 4-107. It uses hydraulic pressure generated by the drive shaft to operate a clutch. When I blew the heat exchanger (it has its own heat exchanger) and a hole developed, the transmission fluid was rapidly pumped into the cooling water and then out the exhaust. So now the clutch was not held closed and the shaft did not turn. Replaced the heat exchanger, filled with new transmission fluid and have been using ever since (that was about 6 years ago).
A common way to lose fluid is when the seal on the prop shaft develops a leak. This is easy to fix, especially if you have already removed it. Cjeck the bilge for transmission fluid (frequently red).