A couple of other possibilities?
1. A cracked plunger
2. A damaged plunger retainer o-ring
Either condition will allow high pressure water to flow between the plunger and its rod. The water will then blow past the lip of the crosshead oil seal, into the crankcase.
You?ll likely see milky oil spewing from the vent hole on the oil cap if these conditions exist.
Cracked plungers are not always easy to detect, occasionally requiring removal to inspect. Loosen the plunger retainer 4 to 5 turns, and then push the plunger towards the crankcase. Once the plunger pops free of the retainer, remove the retainer and inspect it?s o-ring and metal gasket. Since it?s apart, replace the o-ring. Pull the plunger off the rod and inspect. If it comes out in several pieces you?ve found the crack. If the retainer o-ring has been leaking, the plunger rod will likely be quite corroded. Clean it off with a green Scotchbrite pad before reassembly.
Hope this is useful.