A fluid pressure control valve and a fluid control system (such as for beverage dispensing) with such a valve has a beverage passage (3), pressure fluctuations in which are detected by a diaphragm (17) to displace a sensing member (15) axially against spring loading (23). Spring (23) reacts on thrust member (22) which is adjustable axially through a cam mechanism (25/26) to vary the biasing force of spring (23). A valve member (20) spring loaded at (21) relative to thrust member (15) normally closes an exhaust port (19) to an air or other gas passage (5) through which gas passes under pressure to be injected into the beverage from passage (3).; An increase in beverage pressure in passage (3) displaces sensing member (15) to increase the spring biasing (21) on the closure member (20) and reduce the proportion of air which can escape to atmosphere from passage (5) through exhaust port (19) to provide an increase in air flow into the beverage. A reduction of pressure in passage (3) reduces the spring biasing (21) to permit a greater proportion of air to escape from passage (5) through exhaust port (9) and thereby reduce the proportion of air which is introduced into the beverage. The control valve can include a temperature sensitive unit responsive to temperature of beverage in passage (3) and which reacts to adjust the force of biasing spring (21) independently of pressure in the passage (3).; Plunger (51) can be depressed manually to close the exhaust port (19) temporarily and thereby maintain maximum air pressure in the passage (5).