Ok..You installed new thermostat and water pump.
The reason your expansion tank fluctuates is cooling system heats up , causing expansion, as pressure rises to set point cap bleeds off to tank. On cool down contraction returns to system.
If your new thermostat and water pump and coolant are operating more effectively you could be seeing less expansion therefore less cap bleed off to expansion tank.
The tank expansion is secondary to how is the water temperature operating , maintaining desired temperature? In normal conditions properly configured engine cooling system should not have much fluctuating tank level, except in high extremes.
So I went to the landfill today and it was packed.... left to go do some other errands and went back. Still packed. Probably 30-40 minutes of waiting in line idling.
Truck temps kept slowly climbing....reached 225* until I could get moving then it did it's thing and stayed around 200*.
Trans temp got near 190*... dropped to about 180* in the 15 minute drive after.....
Checked my reservoir and it's still at cold mark....
Something doesn't seem right. I don't remember ever having to deal with rising temps like this.... shouldn't the 195* thermostat have kept it lower?? Why am I not able to keep things cool enough? Obviously the transmission is suffering because of this lack of cooling too?..