You cant check level when hot, must be checked when cold. Do not over fill reservoir. Im pretty sure he is losing something as you have to because the pressure cap would let fluid out at 120 degrees. Again there is a piece missing because maybe too close to the problem. At 120 the water being full/hot(expanded) would allow opening of the cap and siphoning back and fourth into the overflow in the fender. The reservoir is actually and expansion tank. The hose from it goes into the passenger fenderwell where it has the overflow tank.

Again a piece of the puzzle is missing. The system seems complicated but its not really. Fairly easy and straight forward.

Coolant is in the system held at atmosperic pressure from the cap, pumped by the water pump through the engine, as it reaches thermostat temp that open and allows water into the radiator, it cools it 15 degrees in its process and already cooled water goes into the block to circulate. Thats it for the coolant. For the radiator to do it job it has 3 ways. Aux fans/fan, vicous fan, and air passing while moving. The aux fan will come on if/when a/c is turned on with acceptable freon pressure, or the temp sensor for the fan reaches 105. If he says the viscous fan is good and his fan/fans operate full time after 105 then his radiator is bad, thermostat, or a pressure leak because if the water pump was bad he wouldnt cool off while moving because water would not circulate at all. A thermostat stuck shut or open will cause his issue, or a pressure leak allowing the water to get hot and boil. The radiator could be full of scale if its a few years old and he didnt fill it with distilled water, but I doubt it. He replaced the radiator that was bad but the expansion tank is plastic also why didnt it get replaced? it was now the weakest and most sensible thing to go out. Mine leaked for a long time before i noticed a white line along the edge that was dried scale from a leak that seamed small but when it got hot it sprayed while driving at temp and rpms, then at idle it wouldnt do it. When I would shut the car off the fluid would appear full, because the overflow would suck fluid back it from the cooling off.


Your results may vary but im just saying.

Dont forget to check your transmission fluid level also, too low could add extra heat from the transmission cooler lines.