What does "pressure" have to do with how well a crankshaft seal works? If there's any pressure at all in the crankcase where the seal rides, there's something dramatically wrong with the engine. Rear seals, either rope or lip type, leak because the crankshaft is bouncing up and down on excessive clearance, and the seal can't keep up with the movement of the shaft.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!