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TM 9-8000
closed, sealing the system.  The larger of these two
valves is the pressure valve and the smaller is the
vacuum valve. The pressure valve acts as a safety valve
that will vent any pressure over the rated maximum
through the overflow pipe. The vacuum valve allows air
to enter the system as the engine cools down. This is to
prevent atmospheric pressure from collapsing the hoses.
9-11. Expansion Tank (Fig. 9-11). Some vehicles use
an expansion tank in their cooling systems.
expansion tank is mounted in series with the upper
radiator hose. It is used to supply extra room for coolant
expansion and generally takes the place of the upper
radiator tank.
The radiator pressure cap and the
overflow line also are mounted on the expansion tank.
9-12. Closed Cooling System (Fig. 9-12).
a.  Purpose.  The purpose of a closed cooling
system is twofold.  First, the system is designed to
maintain a completely full radiator at all times. This will
increase the efficiency of the system by allowing a
maximum amount of coolant in the system during all
operating conditions.  Second, during an overheating
condition, the closed cooling system prevents coolant
loss through the overflow line by collecting it in the
recovery tank.
b.  Operation.  As the temperature of the cooling
system rises, the pressure also will rise. This will open
Figure 9-7. Bellows Type Thermostat
the pressure valve in the pressure cap, causing coolant
to exit through the overflow tube, thus venting excess
9-10. Radiator Pressure Cap.
pressure.  An open-type cooling system will empty
coolant onto the road, causing a low coolant level after
a.  Purpose.  Modern automotive vehicles use
the temperature returns to normal.  When a closed
pressurized cooling systems that allow a certain amount
cooling system is used, the overflow line is connected to
of pressure to develop within the system as its
the bottom of a coolant recovery tank to catch and hold
temperature goes up. The increase in pressure will raise
any expelled coolant. As the temperature of the coolant
the boiling point of the coolant proportionally, helping to
drops, the corresponding drop in pressure causes
prevent boilover.
atmospheric pressure to push the coolant in the recovery
tank back into the cooling system through the open
b.  Operation (Fig. 9-10).  The cap contains two


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