I guess I used a double negative but what I meant to say is that the lapped valves won't last. If you get the valves done at a good shop they will probably cut the seats instead of grinding which leaves a sharper edge to seal better and a less likely to be affected by carbon build up but also often leaves a slight burr or ridge where the cutter stops causing even more of a valve leak than what you are talking about, enough to see light through but not enough that the engine won't start. If the rest of the cylinders are as low as you are predicting that is a problem. I think the only way you can get a pressure build up in the intake is if there are some serious valve timing issues or the exhaust is plugged. Even with leaking valves there would still be a full stroke of valve open suction for every stroke with bit of pressure leaked by. I don't know what you have for a cccv or how you have the intake is connected to the crankcase but it might help to check/disconnect that for further clues.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)