What should be checked first if truck radiator hoses collapse internally as the engine cools?

When truck radiator hoses collapse internally as the engine cools, the first check should focus on the radiator cap. The radiator cap is a crucial component of the cooling system that maintains the proper pressure within the system. If the radiator cap is faulty or not sealing correctly, it can lead to a significant drop in pressure when the engine cools down. This drop in pressure can result in a vacuum effect that causes the hoses to collapse inward.

Ensuring that the radiator cap is functioning properly is vital because it plays a direct role in regulating the cooling system's pressure and preventing the hoses from collapsing. Once the integrity of the radiator cap is verified, if necessary, other components such as the thermostat, water pump, and cooling fan can be checked in subsequent steps. These components could contribute to cooling issues, but the immediate inspection should always start with the radiator cap in cases of hose collapse.