What does a car pressure cap do?

Answer 1

The most common use of a pressure cap is on the cooling system.

This is to allow :

• the coolant to get to pressures high enough to raise the boiling point to a useful level,
• create enough static pressure that some of the flexible rubber hoses do not collapse while the engine operates.
• keep that pressure below a level that will cause failure
• on what are older cars now, allow the cooling system to draw coolant from the overflow system

another location are the fuel cap on most modern cars and trucks.