Emergency Braking and Backup Systems
What happens automatically
If system pressure falls far enough, the spring brakes apply by themselves, typically beginning somewhere around 20 to 45 psi, and the low-pressure warning sounds by about 60 psi. On a combination vehicle the tractor protection valve closes at low pressure, trapping the tractor's remaining air so it can still brake, while the trailer's emergency relay valve applies the trailer spring brakes. If a trailer breaks away, its supply line separates and its spring brakes set automatically. This is why the trailer is never parked on air alone.
Driver actions when service brakes fail
Downshift to use engine compression, use a retarder if it is safe on the surface, move toward the shoulder and apply the parking brake control gradually rather than slamming it. Look for an escape ramp or a safe runoff. Use the trailer hand valve only if you must, and remember it applies trailer brakes alone, which can jackknife a combination.
Controlled versus stab braking
Controlled braking means holding a firm, steady application without locking the wheels. Stab braking, used when you need maximum effort, means a full application until the wheels almost stop rolling, then a complete release, then reapply. Never fan the brakes in short pulses, which wastes air and does not build full pressure.
Traps and a rule of thumb
Assuming spring brakes can be modulated like service brakes - they cannot; they are for parking and emergency stopping. Driving with a spring brake dragging will destroy the chamber or start a fire. Older vehicles may have a front brake limiting valve; it belongs in the normal position for everyday driving. Rule: when the warning sounds, get off the road before the springs set the brakes for you.
Faults and Safety Procedures
Recognising a fault early
Watch for a warning buzzer or light, a gauge that will not reach cut-out, slow build-up, a leak you can hear at a glad hand or chamber, moisture or oil in the tanks, an air dryer that never purges, and brakes that pull, drag or feel spongy. Oil in the reservoirs usually means the compressor is passing oil; heavy moisture means the dryer is not working and lines may freeze in winter. A dragging brake shows up as a hot drum, a pull to one side or a wheel that smells burnt at a stop.
What a driver must do
Stop as soon as it is safe, set the parking brake, and investigate. A leak does not fix itself and gets worse with every application; on a long descent, repeated use can outrun the compressor and drop you into automatic spring brake application in moving traffic. Defects must be reported and the vehicle must not be operated until repaired. Never use an alcohol evaporator or a shot of air-line antifreeze as a substitute for repairing a failed dryer.
Traps
Treating a small leak as acceptable. Ignoring an out-of-adjustment slack adjuster because the brakes still seem to work. Believing the vehicle is fine because it passed the pretrip yesterday. Assuming a low-pressure warning is an electrical glitch. Thinking you can drive to the shop with a known major leak; call for repair instead.
Rule of thumb with a worked example
If the warning sounds and the gauge is falling while you drive, treat the remaining pressure as seconds, not minutes: signal, move right, stop. In a check where pressure drops 5 psi per minute with the brakes released, the vehicle exceeds a typical 2 to 3 psi limit and must be placed out of service until repaired.