Brake booster

A brake booster including a valve plunger slidably fitted in a valve body and forming part of a valve mechanism, a locking portion of a key member which is inserted into a bore formed within the valve body from a radially outer location for engagement with the valve plunger to prevent the withdrawal of the valve plunger from the vile body, an arcuate portion of the key member extending integrally from the locking portion and surrounding substantially one-half the perimeter of the valve body, and a retainer surrounding substantially one-half the perimeter of the valve body at a location opposite to the arcuate portion and having opposite ends which are connected to the arcuate portion to prevent the withdrawal of the key member from the bore;

• characterized in that engaging recesses are formed at the opposite ends of the arcuate portion of the key member on the inner side while engaging pawls are formed at the opposite ends of the retainer on the outer side for engaging with the engaging recesses under the resilience of the retainer to prevent the key member and the retainer from being separated from each other in the radially outward direction of the valve body, the axially opposite ends of the retainer being held between a pair of stepped end faced formed on the valve body or between a stepped end face formed on the valve body and the key member, thereby preventing the disengagement between the engaging recesses and the engaging pawls as a result of an axial displacement, the outer peripheral surface of the valve body including a large diameter portion, a medium diameter portion and a reduced diameter portion sequentially formed as viewed from the front to the rear as viewed in the direction of operation thereof, the medium diameter portion being formed with a first and a second engaging hole at opposing locations and adjacent to a first end face which is defined between the medium and the large diameter portion,
• a power piston being connected to the valve body and including a shank portion in which a fitting opening is formed through which the medium diameter portion of the valve body extends, the outer portion of the fitting opening being disposed for abutment against the first end face, a first and a second engaging projection being formed at opposing locations around the inner peripheral surface of the fitting opening and extending radially inward beyond the outer peripheral surface of the medium diameter portion for engagement with the respective engaging holes,
• the medium diameter portion of the valve body having an axial length which permits the second engaging projection to be located over the second engaging hole while maintaining the first engaging projection rearward of a second end face defined between the medium and the reduced diameter portion with the power piston skewed,
• at least one of the outer portion of the fitting opening in the power piston and the outer peripheral surface of the valve body being formed with a relief which permits the axis of the power piston to be disposed eccentric with respect to the axis of the valve body, the relief being effective to permit the power piston to be disposed eccentrically in a direction to permit the second engaging projection to be inserted deep into the second engaging hole while the power piston is skewed, such eccentricity permitting the first engaging projection to move forwardly beyond the second end face to be engaged with the first engaging hole,
• the respective engaging projections being disposed in contact with the rear end face of the respective engaging holes to block a retracting movement of the power piston with respect to the valve body.