first highlight = your engine is overcooling - no thermostat valve, excellent way of killing your engine very quickly
second highlight = your system is either air locked or there is a leak somewhere, boil cannot occur in the cooling system as it runs under pressure.
third highlight = either your mechanic is stupid, or you are not fully understanding what he was doing.
when you say you pressed the pipes on hot engine - were they hard? They should be hard like tires as the system is regulated at 13 psi pressure