[bwmac]

If it is the style of cap to O-ring seal that I am thinking of, where the O-ring is to be squeezed into a slot or grove. Then your choice is to file the cap or housing where ever the o-ring sits so that there is more squash. Now you can use a fatter o-ring which is my first choice, but if the housing is getting pitted then it should be filed smooth. caution, filing is not easy to get flat. I mark the surface lightly with a small punch making little equal dents all the way around. then file till they are equally removed. Thats how i do it. a good sealer at the auto parts store is called aviation gasket sealer and i think its by Permatex

http://www.permatex.com/products/Automotive/automotive_gasketing/gasket_sealants/auto_Permatex_Aviation_Form-A-Gasket_No_3_Sealant_a.htm