Years ago I had a customer bring me his 1968 383 C.I. Charger that he had put a set of after market cast aluminum valve covers on with the stock type PVC and twist on oil filler cap with no vent. It would start and idle fine for a short while(1 1/2 to 2 1/2 minutes) and then slow down by several hundred RPM, the motor was sealed up tight enough that the PCV was making a vacuum in the motor after it suck the pressure out of the motor I put a breather cap on it and he was happy after that
A vacuum pump will do the same thing on a leak free motor
The pumping action of the pistons and rods going up and down and ring blow by will cause crankcase pressure, especially at high RPM