low coolant issues

Hello, Just to clarify -- the coolant will not "evaporate". It's going somewhere. Now, there could be an air pocket in the system from making repairs -- this could account for the coolant level going down after operating the engine. Pressure testing the cooling system will reveal any external leaks. If there are internal engine leaks, they will not show up externally of course. This is possible. Testing is really needed further to find out. Top off your coolant, monitor the level now that the reservoir and cap was changed, and perhaps it will even out from an air pocket.