On my chassis (126) it's on the passenger side: a hose from the filler neck area to above and beside the rear subframe, with a small black plastic valve and ~ 3" hose extension off that.

Odds are its not pressure you're hearing, but, rather, vacuum release.

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)