sounds like the center seal. your attempt to pressurize one side by itself, or the other, ends up with no resultant air pressure. yet when both sides are sealed pressure then builds, so the leak can only be from the center seal. if the left and right seals leaked then you would have leakage when both chambers are filled with compressed air. am i reading your description properly?

and if it is the center seal the only way replace it is to split the crank? which i understand is best left for the professional with the experience and proper tools. and center seals are hard to come by?

or buy a new crank.

how long will a center seal last just from aging? mine is 20 years old and i believe i have the original in mine due to low mileage and while it is ok now, i have to wonder just how long before the effects of time cause it to fail.
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MC21R9N track
MC18R2J track
CBR954
CR250