leaking shower taps

I have a older set of shower taps that are leaking.
It is a waltec # 40. I replaced the washers on both taps and it still leaks. The taps are about 20 years old. I am just wondering is there some adjustment inside the taps. When I take out the stem and look inside it looks like a allen screw in there. I don't really want to get into the mess of opening the wall and replacing the taps.

Not familiar with that particular valve...What you are seeing is probably the valve seat. This may be replaceable (thus the allen key hole). Is that surface smooth and flat? If not, then it needs to be replaced, as the washer needs to press against that and make the seal. My unprofessional opinion. NOte, that is probably not an allen hex hole - it is probably a tapered hex hole. They make valve seat tools to remove them.

What you are looking at when you look into the valve are the seats. You need a seat removeal tool,you then remove the seats and go to a place that has a good assortment of them(seats) and match them. Then install the new ones. You want the tool that has individual removeal bars and operates with a ratchet wrench. They make an L shaped removeal tool but I don't recomend it's use.