The only way I've really been able to get at the source of those small, annoying leaks has been to slather the top of the keg (including the base of the posts) with a soap and water solution after pressurizing the empty keg and looking for the bubbles. Most often, I find I need to replace all of the gaskets and poppet valves (including the gasket at the top of the dip tubes). With stubborn, but new, lid gaskets I give it a light coating of keg lube. On the ball lock kegs I have, I also have found small leaks when leaving the CO2 attached (like the ball lock doesn't fit down tight enough onto the post, a problem I don't have with the pin lock kegs). On one of the ball lock kegs I have, it seems that the poppet valve spring gets mis-set on the dip tube, making it slightly crooked. I take a small screwdriver to the base of the valve to get the feet set back into the proper spot.