The International Cessna^® 170 Association

Does anyone have any tricks to stop an oil leak from the O-300 oil pressure relief valve cap? No matter how many times I change the AN900-14 gasket, the new gasket always seems to leak. I've even tried applying Loctite 592 Teflon PST Pipe Sealant IAW TCM SIL 99-2B, but even that doesn't seem to help.

I would check the nut or cap for a crack as well as the accessory case. After thatI might also try two crush washers though doing so will slightly lower the relief blow off pressure. I wouldn't be worried about the lowered pressure but if you found it worked the pressure could be restored with a washer the thickness of second crush washer placed in the cap under the spring. Of course two crush washers and the washer under the spring are not specified anywhere.

It sounds as if there may be a crack that the plug is opening-up when torqued....or there is a rough seat where the AN900 gasket sits.
If neither is the case, you might try a neat bead of Permatex Oil-resistant RTV gasket-maker ("Ultra-Black" at NAPA.)