Abstract: A single-piece piston in a dual diaphragm brake booster provides mounting for both diaphragms and support and guide sections for the primary and secondary piston bearings while eliminating the threaded assembly of prior two-piece pistons, or other interlocking arrangements, to make a piston assembly out of a plurality of pieces. The secondary support plate and the secondary diaphragm slip over one end of the piston without requiring piston parts to be interlocked afterward. The diaphragm seals on the outside of the piston. The primary support plate and primary diaphragm are similarly installed. The booster chamber divider and bearing are installed between the two support plate and diaphragm sub-assemblies. A reaction chamber containing reaction means is positioned radially inwardly of one guide and support section of the piston located between the two diaphragm and mounting plate means. A control valve chamber is similarly positioned radially inwardly of the other support and guide section of the piston.