Don't replace the expansion valve unless the high and low side pressures are telling you that it is bad. If it loses mercury and closes, the high side will be low and the low side will be lower or even into vacuum. If it is too open, you will have a high low side and a low high side because you don't have enough restriction between low and high side.
The heater valve on the 124 cars is not rebuildable like the 123 and 126 cars. It is, however, much more reliable. Look at Steve Brothertons DIY article to see how to test. You can get to it by clicking on DIY at the top of this page.