This was our first setup, and it was a complete pain in my ass, but it worked. The syringe “pump” was literally a printer stepper motor, a skateboard bearing, a nylon gear, a screw, a nut pressed inside a laser pointer optics housing and some epoxy (thanks hackaday). I drove the stepper with a controller off e-bay (this was before I knew anything about microcontrollers) and to top it off, a jury-rigged ATX power supply that nearly caught fire when I left it on too long.
This was our new setup with (most) the kinks worked out. The pressure you see in the diagram is of the vacuum that held back the metal against hydrostatic pressure. The pulse pressure is 0-80 psi. We are using an off the shelf dispenser designed for solder paste, which is essentially a regulator + aspirator vacuum + millisecond range solenoid valve.