A pneumatic brake actuator (10) comprising a spring brake actuator (14) with a spring chamber (58) and a spring brake pressure chamber (56). A service brake actuator (12) is coupled with the spring brake actuator (14) and comprises a service brake pressure chamber (38). A valve body (106) of the pneumatic brake actuator defines a channel (142) between the spring chamber and the service brake pressure chamber. A seal (136) is positioned within the channel in the valve body. The seal and the valve body enclose a valve chamber (208) that is not in fluid communication with the spring chamber (58) and the service brake pressure chamber (38). The seal (136) is moveable between an open position, in which fluid can flow through the channel between the spring chamber and the service brake pressure chamber, and a closed position, in which fluid is blocked from flowing through the channel. The seal is biased to the open position and comprises an opening (188), through which fluid flows when the seal is in the open position. The opening extends through the seal in a direction that is aligned with a direction of movement of the seal.