Try breaking the fitting upstream and measure as close to the pump as you can. No sense fooling w/ the pump if the low reading is from a tubing leak. If you see the needle on the gage pulsing with the engine (1/2 rpm), the outlet check valve on the pump is probably bad or gunked (easy to clean) and they don't come w/ a new pump. The "check-valves & seals" rebuild kit for your 1-tube pump cost ~$110. A new pump cost ~$350. If you do rebuild, change the roller's bearings if they might be bad (posts here w/ PN's).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans