Leak proof fittings are a special purpose fitting for use in those applications where a positive seal is essential. A special synthetic rubber seal, conforming wth ASTM D2000 and SAE J200 specifications, is used to provide an effective seal against light or heavy viscosity lubricants and will withstand a back pressure up to 5,000 psig (345 bars). Not suitable as a check valve against high, sustained back pressure. Check and spring are designed to permit easy flow of lubricants.