Don't go overboard too fast.

any leak, however small, will allow coolant to boil (Pv=NrT) or something to that effect (I'm a banker, not a physics professor, dammit).

Radiator caps go bad (frequently), hoses come loose or develop cracks you can't see...the smallest pinprick will allow water vapor to escape, resulting in boiling and fluid loss.

Mechanic should pressurize system and leave standing for several hours to see if pressure falls...if so you have a leak. I've had this addressed several times.

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RJ
'92 500E
'75 Triumph TR 6