It could be a vac leak at one of the lines or fittings. The easy way is to take a known good pump from another car and install it in your car, if that solves the problem then you know its the pump.
BILL

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J. William Tarbrake
NY State Dealer 7014303DLU
Registered repair shop R7014303
82 300SD modified for waste vegatable oil
92 300SE 270K miles perfect
79 450SEL (2 tone custom paint for special occations) SOLD
80 450SL
79 450SL 69K Miles FOR SALE
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94 CARVER 32 Voyager(SOLD)
67 Chris Craft 40' Aft Cabin(twin ford 427 side oilers)