A programmable electric breast pump system includes a vacuum pump pneumatically coupled to breast cups and a vacuum relief valve bypassing the vacuum pump. A controller receives command input from a vacuum level selector and a vacuum rate selector, and feedback from a pressure sensor measuring vacuum in the breast cups. In response, the controller cycles the relief valve between minimum and maximum vacuum setpoints to create a periodic vacuum pulse in the breast cups. By adjusting the vacuum min / max levels and vacuum rate, a user may change the strength of the pulse after let-down, or synchronize the frequency of the pulse with the natural refractory time of a lactating breast. Related methods for operating a breast pump mimic the suckling cycle of a nursing infant and may include steps for bypassing a vacuum pump with a vacuum relief valve, selecting a vacuum rate and a vacuum level, driving a vacuum pump at the selected vacuum rate, cycling the relief valve according to the vacuum level, and adjusting the vacuum level until the vacuum pulses in phase with a natural refractory time.