With no leaks in the AC system or faults with the test equipment (manifold gauge set, vac pump), the gauge needle should be centered over the brass rivet= 30 in Hg on the gauge in less than 2 minutes of running the vac pump. If it was me, I'd be dying to find out what is going on and be trying different tests to determine the cause of the low vacuum reading.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked