What is a ratio valve in air brake systems primarily used for?

The primary function of a ratio valve in air brake systems is to limit air pressure to the front axle. This is crucial in maintaining balanced braking performance across the different axles of a vehicle, particularly in situations where the vehicle may carry varying loads. By regulating the amount of air pressure that reaches the front axle, the ratio valve helps prevent the front brakes from locking up, which could lead to loss of control during braking.

In air brake systems, it is vital to ensure that the front and rear brakes engage appropriately, especially under load conditions. If too much pressure is applied to the front axle, it can result in premature wear or failure of the components and diminished stability. Therefore, the ratio valve is essential in managing this pressure to promote safe and effective braking dynamics.

The focus on pressure limitation rather than amplification or enhancement of brake fluid levels addresses the core requirement of balancing brake force and improving driving safety. This function of the ratio valve is critical in heavy-duty applications like buses, where safety and effective braking performance are paramount.