Have you thought about adding a low coolant sensor circuit on your cars?

Some of the rectangular reservoirs have accommodations for inserting a Volvo float switch while others do not. Adding an after market float switch on those did not look like a possibility so I converted those to the cylindrical reservoir and bracket from the 7 series cars which makes it easy to add an aftermarket float switch and install a warning light on the trim panel to the right of the ignition switch.

Three times on ours and my son's car that has saved us from a probable overheating situation. Twice for a leaking heater hose and another on a water pump shaft seal.

Randy