Based on our experience with two very old Bryden Boy Heads, I suggest a complete rebuild if you can obtain the parts, including the lip-seal for the pump. Then, do routine mainenance on the unit at least annually but never ever allowing chlorine bleach into the system. The lips on Joker Valves deteriorate very quickly when exposed to chlorine bleach.
Also, FWIW we keep squire bottles filled with a mixture of water and "SaltAway" in our heads and, once the bowls have been pumped clear--10 pumps each please-we add a shot of the SaltAway mixture to the bowls and give one or two more pumps. The SaltAway keeps the lines clear and seems to protect the Joker Valve lips. Our heads have been truble free since we began this process quite a few years ago.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."