Chances are you have a bad water control valve. The repair is simple. The guts can be replaced by removing the four small screws on the top (with the engine cold preferably).
When you remove it if the diaphram is broken it is the problem for sure. Unlike the 124 body the 126 body doesn't use the pump much (at least not in Florida). There is enough water flow without the pump when the car isn't idling.
Bosch Master, ASE Master, L1
33 years MB technician