Pneumatic brake servo with an improved valve

A pneumatic booster for braking assistance, comprising:

• an envelope presenting an axis of symmetry;
• at least one movable partition structure for sealingly separating said envelope into at least a first chamber connected permanently to a first source of fluid having a low pressure and at least a second chamber connected selectively to said first chamber or to a second source of fluid having a high pressure;
• a three way valve means having a plunger sliding in a bore of a piston and solid with a control rod, said plunger being positioned in said bore to communicate said first source of fluid between said first chamber and said second chamber or communication said second source of fluid between the second chamber and said second source of high pressure;
• and
• support structure disposed in said piston, said support structure comprising two annular seals which cooperate with a cylindrical surface of said plunger and a median part for guiding sliding of the plunger, each said annular seal having an angled edge with a vertex in a rest condition which is disposed at a distance from said axis of symmetry which is slightly less than a radius of said cylindrical surface of said plunger.