My wife's S70 (N/A, 2000 year model) has blown its rear main seal. It sounds like the flame-trap is blocked and the excess crankcase pressure blew the seal. My local indie-Volvo repair shop said I'd have to replace the PCV unit on the block which would require removing the inlet manifold (cost including labour $450). I checked various on-line parts suppliers and VLV World has a flame trap replacement kit for 1993-2000 models for about $14 which consists of the rubber parts and the plastic trap. Would replacing this help? Anybody know where I can get the unit on the block?
Has anybody got advise on how to replace the rear main seal? I've never tried this on a FWD car. They were talking about detaching the transmission and dropping the sub-frame.

Your mechanic is correct, the oil separator box is under the intake manifold. It takes about 5 hours to replace at home for a NA engine. This includes all of the breather tubing that goes to that box.
The "flame trap" probably isn't your problem as much as the small tube that goes to it is clogged.

The rear main seal (RMS), is between the engine and transmission. Not a fun job, as the left axle and tranny need to be separated from the rear of the engine to replace the $30 part.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

99 and newer do not have a flame trap. Yes, the manifold has to come off to clean or replace the oil separator box. To remove the transmission, the axles have to come out, the subframe lowered or removed, the engine and trans tilted down on the left side to clear the body, then the trans can be unbolted. An experienced tech in a good shop can do it in 3/4 hours, but expect to pay 10 hours labor.

Thank guys, I appreciate the responses. So the oil separator box can be cleaned. Does anybody have the protocol for doing this? Do you take it off and flush with a solvent like carb cleaner?

The labor is fairly intensive and the cost of a new box is not that much, spend the extra money and just hang a new box and be done with it.

Mark