The pressure output of the pump is controlled by the tension of the spring that pushes against the rubber diaphragm as the lever arm rides down the back side of the camshaft lobe. It's pretty obvious your "rebuilder" used the wrong tension spring when he rebuilt the pump and doesn't want to correct his mistake. That pump should be producing somewhere around 3-5 PSI and the pressure should stay on the fuel line for several seconds after the engine is shut down if the one-way valve in the output section of the pump is in good shape. There is also a vacuum check valve on the input side of the pump which will hold 10-12 inches of vacuum for a while after shutdown. Other than dismantling the pump and installing the correct spring, or using a pressure regulator, you're sort of stuck with a bad rebuild.

Edit: I just noticed your statement that you've got a vacuum booster pump. It's been quite some time since I've had one of them apart, but I think it's possible the spring for the vacuum chamber end and the one for the fuel diaphragm might have been reversed. Just a thought- - - -the only way to be sure would be to dismantle the pump and check the spring tension on a scale. Make one more try to resolve things with the shop before you open it up yourself. That would void whatever imaginary "warranty" you might have.
Jerry


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