Toyotal Camry gear problem

My toyota v6 camry 1992 model, the gear always stopped working after three days I serviced it, remove and clean the filter and changed the trans oil it will stopped working again, I also noticed that tran soil sometimes leak from the top of the gear, the mechanic said that the gear is too dirty and we will need to remove the gear and loose it for thorough cleaning. But I am afraid of loosing the whole gear, please advice on what to do.