Compounding is a problem with braking systems for heavy-duty vehicles that can occur when a driver applies the service brakes and the emergency brake is already engaged. If the combined service and emergency forces on the brakes are too great, damage can occur to the brakes. This situation occurs when the emergency air is exhausted, which activates the emergency brake, and at the same time the service air is pressurized which would activate the service brake and apply excessive force in the absence of an anti-compounding feature. Anti-compounding devices are used so that when the service brake is attempted to be applied while the emergency brake is on, the service air is exhausted so the service brakes become inactive to prevent the combined forces from damaging the brake system.
The Midland N-30162 series tractor protection valve is a prior art anti-compounding device that employs a one way check valve disk. When the emergency air cavity is pressurized thus preventing the emergency brake from engaging, the check valve disk seals off the service air and prevents it from flowing into the emergency air cavity. When the emergency air cavity is exhausted thus activating the emergency brake, the check valve disk allows the service air to bleed into the emergency air cavity preventing the service air from becoming pressurized and the service brake from being applied in addition to the emergency brake. A disadvantage of this device is that for proper functioning of the check disk valve, the size of the hole between the emergency air cavity and the service air cavity must be tightly controlled. Otherwise, if too much service air can bleed to the emergency cavity when the emergency spring brake is on, it may undesirably cycle the piston in the valve allowing service air to flow through thereby defeating the anti-compounding feature.
U.S. Pat. No. 5,172,958 to Sell discloses an anti-compounding feature for an air brake control valve system that uses a slide valve/piston which exhausts the service brake line to the atmosphere whenever the emergency supply air has been interrupted due to the application of the tractor's spring brakes. The Sell control valve housing also performs other functions, such as controlling pressure to the spring brakes and filling and protecting the pressure in the air reservoir tank. Disadvantageously, these extra functions can make the Sell control valve more expensive to manufacture and can create a higher risk of part failure.
U.S. Pat. No. 4,407,548 to Graham discloses the use of a safety valve including a spring to eliminate undesirable compounding.
What is desired, therefore, is an anti-compounding device for brakes that does not leak service control air into the emergency air cavity when the emergency brake is activated, that does not require a check valve fitted to a hole having a strict size tolerance, and that is reliable and uses a fewer number of parts than previous anti-compounding devices.