Lesson 2/Learning Event 2
If the pressure is too low, another test is necessary to tell which unit is
at fault, if the relief valve and pump are separate assemblies. This is
done by blocking the oil flow to the relief valve. Don't let the pressure
build above 750 PSI when you make the pressure test with the relief valve
blocked. If this test raises the pressure, the relief valve is defective.
If the pressure is still too low, the pump is probably defective.
If replacing the pump does not raise the pressure, the trouble is internal
leakage in the control valve or power cylinder.
If the pressure was too high with the relief valve connected properly in the
system, the relief valve is at fault. On trucks with gasoline engines, the
relief valve is replaced separately, but the entire pump must be replaced on
trucks with multifuel and Mack diesel engines.
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