4.3 Brake Failure, Skids and Combination Stability
Key Takeaways
A jackknife can develop when the prime mover loses traction and the combination folds; trailer swing involves loss of trailer stability.
ABS helps prevent wheel lock but does not remove grip limits or guarantee protection against rollover.
Respond to brake warnings early through the actual vehicle’s emergency procedure; distinguish hydraulic-failure pumping from full-air pressure loss and an ABS stop.
Prevent the emergency where possible
Heavy vehicle emergencies leave little room for improvised technique. Know the vehicle’s brakes, warning devices, auxiliary controls and manufacturer emergency instructions before the trip. Correct inspections, a suitable descent speed and a generous following gap reduce the need for a last-second response. A brake warning is a reason to act while controlled stopping remains possible.
Brake failure can arise from pressure loss, mechanical damage, overheating or other faults. Brake fade is a loss of effectiveness associated with heat; it is not the same as an air reservoir losing pressure. The response depends on the fault and system. Do not assume repeatedly pumping an air-brake pedal will restore braking: each application can consume more stored air.
Respond to a developing brake problem
If a warning or abnormal response appears, reduce speed under control and stop safely using the remaining effective systems and the manufacturer’s procedure. Avoid continuing downhill to 'see whether it improves'. Inform the operator and arrange repair or recovery. If there is a suitable emergency escape ramp during a descent, recognise it early and follow its signs rather than bypass it because using it is inconvenient.
Use the available gear and auxiliary braking appropriately, without exceeding engine limits or inducing a skid. A parking-brake control may act very differently across vehicles, particularly on spring-brake systems. Learn its emergency use from the actual vehicle instructions. Do not apply an unfamiliar control abruptly at highway speed or teach one lever sequence as a universal cure for all brake failures.
| Problem | Important distinction | Safer response principle |
|---|---|---|
| Low-air warning | Stored pressure may be inadequate | Stop under control before the reserve is depleted |
| Brake fade on descent | Heat has reduced friction braking | Use the planned emergency option and avoid further uncontrolled descent |
| Wheel lock on non-ABS brakes | Locked wheels reduce directional control | Relieve the cause and regain rolling traction as trained |
| ABS emergency stop | System modulates pressure to limit lock | Use firm, steady braking as specified; continue steering within grip |
| Trailer instability | Different axles may have lost grip | Avoid abrupt steering or inappropriate trailer-only braking |
Hydraulic pressure loss
Some heavy vehicles use hydraulic brakes rather than a full-air system. The handbook’s hydraulic-failure response includes changing down, pumping the pedal because it may partially restore hydraulic pressure, and using the emergency parking brake. Apply this through the actual vehicle’s trained emergency procedure: select a suitable lower ratio without engine overspeed, keep steering and observation controlled, and use the emergency control in the manner its design permits. Pumping is a possible response to hydraulic failure, not the ordinary technique for an ABS emergency stop or a remedy for depleted full-air reservoirs. Stop in a safe place and arrange repair or recovery even if some pressure returns; temporary recovery is not proof the fault has disappeared.
Jackknife and trailer swing
A jackknife describes a combination folding at an articulation point. Loss of traction at the prime mover’s drive wheels can allow the trailer’s forces to push the vehicle out of line. Excessive braking, an abrupt downshift or strong driveline retarding on a slippery surface can contribute. The event has no universal 'unrecoverable at 20 degrees' threshold or guaranteed one-second duration.
Trailer swing occurs when the trailer loses lateral stability and moves sideways relative to the towing vehicle. Trailer wheel lock, poor grip, uneven braking or excessive demands can contribute. A lightly loaded trailer can be especially vulnerable to inappropriate braking. Applying an independent trailer hand control to straighten every skid is unsafe; it can lock trailer wheels and worsen the swing.
The useful prevention is to slow before curves, brake and steer smoothly, maintain tyres and brakes, and avoid demands beyond available grip. If a skid begins, follow the trained response for the system and remove the input causing loss of traction when appropriate. Do not confuse releasing excessive braking on a locked-wheel non-ABS skid with pumping the pedal during an ABS stop.
ABS and electronic aids
Anti-lock Braking System (ABS) detects impending wheel lock and modulates braking. It can help preserve steering during hard braking, but it cannot create road grip. Firm, steady pedal pressure is generally required during an ABS emergency stop, subject to the vehicle’s instructions. A warning lamp signals a system issue that needs investigation; ordinary braking may remain, but the vehicle must be assessed and used only as permitted.
Stability systems can reduce some loss-of-control risks, but they cannot make a high-centre-of-gravity load safe at any speed. Do not use an electronic system as a reason to enter a bend faster. Loading, route, tyres and driver inputs remain decisive.
Rollover and rearward response
A high or off-centre load increases rollover risk. Liquids, livestock and poorly restrained goods can change their position during a manoeuvre. Multi-trailer combinations may amplify a sudden lateral movement toward the rear. The exact response depends on design, loading and speed; invented fixed acceleration multipliers are not useful operational limits.
Complete ordinary slowing before a curve where possible, choose a suitable gear and steer smoothly. If an animal appears, brake under control rather than make an abrupt swerve into opposing traffic or a soft shoulder. A straight controlled response can avoid replacing one hazard with a rollover or head-on collision.
Breakaway protection automatically applies trailer braking when the system detects the relevant loss of supply or separation. It does not guarantee that a detached trailer remains upright or stationary in every circumstance. Test the system only through the safe stationary procedure for the vehicle, not by deliberately disconnecting on a road gradient.
Key decisions
Know the actual system, act early on warnings, preserve traction and use trained emergency procedures. Prevention and safe stopping options are more reliable than dramatic recovery claims.
Official sources
Official handbook, braking, skids and emergency control; NHVR vehicle safety guidance. Sources checked 10 October 2026.
During an ABS emergency stop, which response is generally appropriate?
Apply firm, steady braking as the vehicle instructions specify and steer within available grip
Pump the pedal to defeat ABS modulation
Use only the trailer hand control
Assume ABS prevents every rollover
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