<p>An improved high pressure diaphragm pump comprising an improved pumping unit including a pumping chamber, a transfer chamber, a reciprocating piston means defining one end of the transfer chamber, a diaphragm defining the other end of the transfer chamber bias means urging the diaphragm toward the piston, and a stop for limiting the movement of the diaphragm toward the piston; and an improved pressure control and unloader valve incorporating a floating check valve which is responsive to a gradual restriction in the discharge line and is also responsive to the shock wave formed as a result of a sudden shut-off of the discharge line.</p>
