Air Brake Relay – How It Works

Jump to:
1. What the Air Brake Relay Valve Does
2. How It Works – Step by Step
3. Key Components Involved
4. Common Misconceptions
5. Why This Matters

Anchor 1

An air brake relay is used in air braking systems to reduce brake lag on large and commercial vehicles. It allows the brakes to be applied and released more quickly by using a local supply of compressed air close to the brake chambers.

Instead of supplying air directly from the driver’s control valve to distant brake chambers, the relay uses a small signal pressure to control a nearby supply of compressed air. This allows the brakes to respond more quickly, particularly on larger vehicles where air travel distance would otherwise cause delay.

This page explains why air brake relays are needed, where they are positioned in the system, and how their internal operation speeds up brake response.

How it Works - Step by Step

Anchor 2

Signal from the driver
When the driver applies the foot brake, a signal pressure is sent from the foot control valve to the relay.
Relay activation
The signal pressure acts on the top of the relay piston, causing it to move and seal the exhaust port.
Brake application
Continued movement of the piston opens the inlet valve, allowing compressed air from the relay’s own reservoir to flow directly to the brake chambers and apply the brakes.
Pressure holding
As pressure builds in the brake chambers, it acts on the underside of the piston. When pressure above and below the piston equalises, the inlet valve closes and the relay holds the applied pressure.
Brake release
When the driver reduces or removes brake input, the signal pressure decreases. Higher pressure beneath the piston opens the exhaust valve, allowing air from the brake chambers to exhaust through the relay.
Rapid response
Because the main exhaust occurs at the relay rather than the control valve, brake release happens more quickly, reducing lag.

Throughout operation, the relay rapidly switches between three states: on, hold, and off.

Key Components Involved

Anchor 3

Air Brake Relay
Uses a signal pressure to control a local supply of compressed air.
Supply port
Receives air from the reservoir.
Delivery port(s)
Sends air to the brake chambers; multiple delivery ports may be used when mounted on an axle.
Exhaust port
Vents air to atmosphere during brake release.
Piston/Diaphragm
Responds to pressure differences to open or close the inlet and exhaust paths.

Common Misconceptions

Anchor 4

Relays do not increase braking force
They improve response time, not braking power.
They do not replace control valves
The driver’s control valves still determine when braking occurs.
Relays are not limited to service brakes
They can also be used in secondary and parking brake circuits.

Why This Matters

Anchor 5

Understanding how air brake relays work explains how large vehicles achieve responsive braking despite long air lines and high air volumes. It also clarifies how brake lag is managed safely in both service and parking brake systems.

This knowledge supports a broader understanding of air brake system design and operation.

Quick Reference

System: Braking systems
Component: Air Brake Relay Valve
Primary Function: Reduce Brake Lag
Operating States: On, Hold, Off