The crankcase ventilation on a PV is very passive. There is a vented cap, but the bulk of the crankcase breathing is done via a dump tube mounted to the side of the block, just forward of the distributor. It is a curved pipe that goes up, curves over, then goes down near the crossmember, bends backward slightly, and just opens to the air. There's a slight preservative effect on the nearby body parts. (heh).

Perhaps some insects built a nest in the tube and blocked it? Felt seals will only work at all (and even then not really that well compared to modern expectations) if there is no pressure in the block at all.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)