I've never taken a water pump apart so I'm not sure but I do know that wear will allow one to turn but not work efficiently.

If you want to be sure that you are losing coolant then a quick and easy test is to compare the amount of coolant required to fill the system with the specified capacity (assuming that your system is standard). Once you've done this then you will know for sure that any significant drop is loss, not insufficient filling. It sounds like you're sure that you filled it completely though.

You might like to check along every hose for discolouration from tiny holes. When the system is hot and under pressure it may be losing coolant in a way that isn't immediatly obvious. Every hose end should also be checked to see if there's any seepage.

I'm not familiar with the detailed ES specification but does it have a viscous coupling fan? If so you need to check that its working properly as it may be slipping. Normally this will result in the car having normal temperature at speed (because of airflow) but overheating at lower speeds. Its possible to check - if you're careful - by trying to impede the rotation of the fan while the engine is hot (with a long implement held against the fan along the direction of rotation). With a faulty fan it can be possible to stop the fan entirely this way.

See Joaquin's post about testing the water temperature. http://www.brickboard.com/RWD/?id=658660

I don't know of any specifications that you can compare with but as you have a 122S it might be interesting to see if the ES runs much hotter then it.

Aidan
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1967 131, 1969 131