I have an old RepCal shower assembly in my bathroom and the washer in the pull/diverter has apparently worn out since I am unable to get any water to the shower head. My question is this: in order to change the washer do I need to proceed from behind the valves( my bath trap access door is too small so I would have to cut a larger hole in the plaster wall)or can the repair be done by taking off the spout in other words are the mechanisms behind the wall or behind the spout?
I assume the diverter is in the spout since you say it is a push/pull operation. In that case, it is not repairable, but you can change the spout. Just unscrew it and take it to a plumbing supply outlet. There are several different connection variations so having the old one for a reference will help get the proper new one.