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engaged by the spring metal diaphragm in the cap. The pressure cap consists of two major valves which
prevent radiator tanks and hoses from collapsing due to partial vacuum which would be created if air cannot
enter during the cool down of the engine and cooling system.
1. The spring pressure relief valve, which closes off the lower sealing seat in the filler neck. This valve allows
pressure to build up to a specified level, protecting the cooling system from damage due to over-pressur-
ization. When exceeding the designated range of the pressure valve spring, it permits excess pressure to
escape through the overflow tube (providing the cap is functional).
2. The vacuum relief valve which, in the open system, allows air to enter as the coolant cools.
THE RESERVOIR OR CLOSED SYSTEM
The following disadvantages of the open system brought on the development of the reservoir or closed
system.
1. Since the coolant expands every time the temperature rises, these open cooling systems are designed
to operate with less than full radiator tanks. The excessive coolant is expelled through the overflow tube
until proper operating level is achieved. This leads to less coolant in the system left to do its job. This and the
fact that there is air in the system reduces the efficiency of the cooling system and can create overheating
problems.
2. Cars today tend to have smaller engine compartments and smaller hoods, therefore, the open system
which are generally larger (to allow for coolant expansion) needed to be replaced with the closed cooling
system which tend to be smaller.
The closed system consists of a special type of pressure cap and a reservoir tank. The pressure cap has an
additional sealing seat directly beneath the cap, enabling the cap to sit on the upper sealing seat of the filler
neck. The reservoir is connected by the overflow hose from the filler neck, the hose extending below the
normal liquid level. In the closed system, contrary to the open system, the system is filled completely with
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