Variable displacement pump

A variable displacement pump comprising:

• a housing;
• a cam ring which has a circular shape, and which is swingably received within the housing;
• a pump unit which is received within the cam ring,
• which is arranged to be drivingly rotated by a drive shaft, and whose a capacity is varied in accordance with a swing position of the cam ring;
• a first pressure control chamber which is defined between an inner circumference surface of the housing and an outer circumference surface of the cam ring, and which is arranged to urge the cam ring in a first swing direction;
• a second pressure control chamber which is defined between the inner circumference surface of the housing and the outer circumference surface of the cam ring to confront the first pressure control chamber, which is arranged to urge the cam ring in a second swing direction, and which has a pressure receiving surface which is smaller than a pressure receiving surface of the first pressure control chamber;
• a spring arranged to urge the cam ring in the second swing direction;
• a hydraulic pressure supply valve arranged to be opened by a predetermined hydraulic pressure, and thereby to introduce a control hydraulic pressure to the first control chamber;
• a connection passage which is formed in one of the housing and the cam ring, and which is arranged to connect the first pressure control chamber and the second pressure control chamber;
• and
• a relief circuit which is arranged to connect the second pressure control chamber and a low pressure side, to be opened or closed in accordance with a swing position of the cam ring, and to be closed when the cam ring is swung by a predetermined amount in the first direction.