I am wanting to rebuild the rear shock as the oil seal head is shot. shaft looks good. Nitrogen resevoir has some kind of metal cap at the bottom covering the valve. Does this pop off, do I I drill it? I have seen seal heads advertised on ebay 30 - 35 bucks - any good or is oem available? From what I can gather 7.5w oil is what is generally used in these. Any advice appreciated!