An improved diaphragm-type pneumatic brake actuator includes a flange case, a cover cooperable with the flange case, a flexible diaphragm extending between the flange case and the cover forming a lower pneumatic chamber and an upper pneumatic chamber on opposed sides of the diaphragm. A piston assembly is disposed in the cover for moving a spring between compressed and decompressed positions. A spring guide is disposed between the cover and the piston assembly in the upper chamber. The spring guide prevents direct contact of the spring with the cover to prevent formation of a corrosion cell.