or three equilibrium operating pressures in the embodiment illustrated in fig2 a resilient seal 112 is used to seal one end of control chamber 104 and resilient seal 52 seals the other as well of one side of control chamber 76 the other side of which is sealed by a resilient seal 116 as will be apparent to those of skill in the art the use of such seals is not required but such seals can provide a manufacturing cost advantage in that relatively expensive machining steps which would otherwise be required to ensure adequate sealing of control chambers 76 and 104 can be avoided fig3 shows a pump 200 which is another embodiment of the present invention wherein similar components to those of pump 20 of fig1 are indicated with like reference numerals unlike pump 20 pump 200 employs a sliding control ring 204 instead of a pivoting control ring as shown control ring 204 includes reaction surface 60 and a biasing spring 64 acts between pump housing 24 and reaction surface 60 to bias control ring 204 to the maximum eccentricity maximum displacement position control ring 204 further includes two reaction surfaces 68 and 72 which serve as a moveable portion of control chambers 76 and 80 respectively as illustrated control ring 204 is sealed with resilient seals 212 as mentioned above the use of such seals is not required but such seals can provide a manufacturing cost advantage in that relatively expensive machining steps which would otherwise be required to ensure adequate sealing of control ring 204 with respect to control chambers 76 and 80 etc can be avoided in operation one or both of.