While I'm not disagreeing the Hatt......the chances of bad prefilters are less likely than a leak elsewhere, IMO. If you have another prefilter, test it....or you might have enough play in your supply line to just stick your existing right on the pump. It sounds like your leak happened all of a sudden, not at a change.....and the plastic prefilters don't just all of sudden start leaking....but rusted fuel lines will, or deteriorated rubber.

Point is though - you have a leak before or at the prefilter.......so if you do as I suggested.....simply pop them off (which you are proposing to do anyway -yes, 5/16" is fine) and stick them in a fuel container - and you can pinpoint the problem that way and isolate the front half or back half of the car.

Do what you want though.....but that's where I would start.

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1981 240d - 135k - Arlene