A spring parking brake can exert a force of up to 2,000 pounds, and a Type 30 service brake chamber can exert a force of 3,000 pounds or more. If both forces were applied at the same time (which is called compounding the service and parking brakes), the combined force could damage parts of the braking system. To prevent this, most commercial vehicles employ an anti-compounding valve. One input port of this valve is connected to the service brake valve (foot valve), and another input port is connected to the parking brake valve (yellow button). Suppose you have set the parking brakes by pulling out the yellow button, causing that valve to send air to the anti-compounding valve. Then if you press the brake pedal, causing the foot valve to also send air to the anti-compounding valve, the anti-compounding valve will partially release the parking brakes by the same amount of force that the foot valve has applied.