A pneumatic actuator for use with a pump for dispensing molten solder on a surface, such as an integrated circuit substrate. The pump has a reservoir for containing molten solder, a nozzle for dispensing molten solder on the substrate, a diaphragm pump with a pump cavity situated in fluid communication with the reservoir and with a flow intake portion of the nozzle, and an adjustable throttle for controlling the flow of molten solder from the reservoir to the diaphragm pump. The throttle defines an annular throttle gap between the reservoir and the pump cavity to vary the effective throttle area. A spring-loaded diaphragm for adjusting the throttle defines in part a pneumatic chamber. A solenoid valve is situated in a compressed gas feed passage extending to the pneumatic chamber. The spring-loaded diaphragm is effective to control the throttle opening with a controlled delay following activation of the solenoid valve so that optimum throttle resistance can be achieved at the beginning of a solder-dispensing pulse, while providing optimum throttle resistance during the remainder of the pulse such that instantaneous cutoff of the flow through the nozzle is achieved without effecting a reverse flow of air through the nozzle into the pump chamber at the end of the solder dispensing pulse.