I, and I think the Outdoorking archives, have only one previous report of a fault similar this and from memory, we didn't ever reach a resolution - we were mainly focused on excessive crankcase pressure and were tending to blame that on either leaky headgasket or leaky rings. However, one thing I didn't think of at the time but seems a plausible cause: are you sure the Positive Crankcase Ventilation valve is not stuck closed or obstructed? If so, the fuel pump might leak before the engine gaskets did. I note you've said you've checked the breather tube and "box", but I'm not sure you've checked the valve.
