Depending on how much blow by you have and the efficacy of your crankcase ventilation system, the crankcase can be under considerable pressure. I used to have an old Mercury that had so much blow by that it would actually push oil out the dipstick tube.

I do question the wisdom of pressure testing, since this could lead to failure of a marginal seal that could have continued adequately for months or years of continued service. If it isn't leaking under the use the operator puts it to, then it isn't broke, and doesn't need fixing.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.