A spring brake actuator for braking a wheel of a vehicle, the spring brake actuator comprising: - an axially elongated housing having a parking brake chamber and a service brake chamber, - a main compression spring in the parking brake chamber, wherein the main compression spring is axially movable into and between a compressed position and an extended position, - a flexible diaphragm in the parking brake chamber, and - a pressure plate located between the main compression spring and the flexible diaphragm, wherein the pressure plate has a hub portion and a radial flange that extends from the hub portion to a radially outer diameter, wherein the flexible diaphragm comprises a contoured portion that inhibits radial shifting of the pressure plate with respect to the flexible diaphragm, at least when the compression spring is in the extended position, wherein the contoured portion has a uniform thickness, wherein the flexible diaphragm comprises a radial center portion that axially abuts the pressure plate and an intermediate portion located radially between a radially outer diameter of the flexible diaphragm and the radial center portion, and wherein the contoured portion is located along the intermediate portion, wherein the contoured portion defines a pocket in which the radially outer diameter of the pressure plate is disposed, at least when the main compression spring is in the extended position, wherein the pocket abuts and wraps around an entirety of the radially outer diameter, at least when the main compression spring is in an extended position. - an axially elongated housing having a parking brake chamber and a service brake chamber, - a main compression spring in the parking brake chamber, wherein the main compression spring is axially movable into and between a compressed position and an extended position, - a flexible diaphragm in the parking brake chamber, and - a pressure plate located between the main compression spring and the flexible diaphragm, wherein the pressure plate has a hub portion and a radial flange that extends from the hub portion to a radially outer diameter, wherein the flexible diaphragm comprises a contoured portion that inhibits radial shifting of the pressure plate with respect to the flexible diaphragm, at least when the compression spring is in the extended position, wherein the contoured portion has a uniform thickness, wherein the flexible diaphragm comprises a radial center portion that axially abuts the pressure plate and an intermediate portion located radially between a radially outer diameter of the flexible diaphragm and the radial center portion, and wherein the contoured portion is located along the intermediate portion, wherein the contoured portion defines a pocket in which the radially outer diameter of the pressure plate is disposed, at least when the main compression spring is in the extended position, wherein the pocket abuts and wraps around an entirety of the radially outer diameter, at least when the main compression spring is in an extended position. wherein the flexible diaphragm comprises a contoured portion that inhibits radial shifting of the pressure plate with respect to the flexible diaphragm, at least when the compression spring is in the extended position, wherein the contoured portion has a uniform thickness, wherein the flexible diaphragm comprises a radial center portion that axially abuts the pressure plate and an intermediate portion located radially between a radially outer diameter of the flexible diaphragm and the radial center portion, and wherein the contoured portion is located along the intermediate portion, wherein the contoured portion defines a pocket in which the radially outer diameter of the pressure plate is disposed, at least when the main compression spring is in the extended position, wherein the pocket abuts and wraps around an entirety of the radially outer diameter, at least when the main compression spring is in an extended position.