No pressure in the gas tank anymore???

[sptt]

I switched out my fuel accumulator last month ('91 300E) and car started better for a about three weeks then started doing it's good 'ol long start cranks again (cold or hot). I have recently noticed that right after I drive, if I open the gas tank cap, there is no "phewww" sound air pressure release anymore. I have noticed it with an almost empty tank, half tank, and full tank...zero pressure. Is it possible my fuel pumps are leaking even though I can hear the buzzzz sound when turning the key? Anything else I should check? Just ordered a new fuel filter for the heck of it but didn't think that would have anything to do with it.