Abstract: A pneumatic brake system for a heavy duty vehicle is provided. The heavy duty vehicle has at least one source of pressurized air for pressurizing the brake system, at least one spring brake chamber including a spring brake assembly, and at least one service brake chamber including a service brake assembly. The brake system includes a first control valve, a second control valve and a work brake system. The first control valve is in communication with the source of pressurized air. The first control valve delivers pressurized air to the spring brake chamber and exhausts pressurized air to atmosphere to activate the spring brake assembly when brake system pressure is reduced to a predetermined low trip brake system pressure. The second control valve is in communication with the source of pressurized air and selectively delivers pressurized air to the service brake chamber for application of the service brake assembly.

Abstract: A system and a method which measure and record brake pressure data representing the brake application and release timing of service brake systems and the brake application timing of parking brake systems. The brake pressure data are used to determine if a specific vehicle or item of motor vehicle equipment meets the minimum performance requirements of FMVSS 121. As a result, compliance with the service brake and parking brake timing with the parameters set forth in FMVSS 121 may be verified, and other brake system diagnostic tests may be performed.