Sustained System Pressure Retention is one of the Hall Marks of the whole
head gasket nightmare.

BUT,You may be "Looking For Trouble".
The pressures are caused by Heat,let the engine (AND System ) cool off
[Minimum 12 hours in this heat] and then squeeze the Upper Radiator hose.
'Firm to Hard hose MAY indicate the need for further SCIENTIFIC diagnosis.

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'84 300SD sold
124.128