A disposable pump for a collapsible reservoir has an upper chamber, a lower chamber and a seal base in between. The upper and lower chambers have flexible sections or bellows that expand or contract the chambers, and the seal base has a rigid rod that selectively forms a seal on an opening in the seal base between the upper and lower chambers. One of the two chambers has a nozzle and the other chamber has a top cap that leads in many embodiments to a collapsible bag reservoir. In at least one embodiment a solenoid overcomes the biasing force of a compression spring and lifts the seal base off the rigid rod. This opens the seal and assisting pushing out potentially very viscous liquids into the chamber that includes the nozzle and the liquid ultimately dispenses from the pump through the nozzle.