Air Brake vs Hydraulic Brake System Checks and After-Departure Inspection
Key Takeaways
- An air-brake system must build to the governor cut-out range (around 825–930 kPa on most heavy rigid trucks) with the engine at fast idle, and the low-pressure warning must activate before pressure drops to around 380–520 kPa.
- A hydraulic-brake system is checked by holding the pedal firm for 30 seconds — a pedal that slowly sinks indicates an internal leak in the master cylinder or a faulty seal, and the vehicle must not be driven.
- The parking (spring) brake on an air-braked vehicle must hold the stationary truck on its own; the service brake is tested with a brief low-speed stop after departure.
- After departure, stop within a few hundred metres and re-check: secure load, tyre and brake-drum temperature by touch, air system leaks by watching the gauge with the engine off, and any new fluid underneath.
Last updated: July 2026
Why two brake systems need two check routines
A heavy rigid truck is fitted with one of two braking systems. Each system stores and applies braking force differently, so the daily check routine is different. The Victorian Heavy Rigid test asks you to know which checks belong to which system and what reading or behaviour is acceptable.
| Check item | Air brake system | Hydraulic brake system |
|---|---|---|
| Energy storage | Compressed air in reservoirs at 825–930 kPa (typical cut-out) | Incompressible brake fluid in master cylinder and lines |
| Build-up check (engine on) | Build pressure to governor cut-out with engine at fast idle; gauge must reach and hold the cut-out range | No build-up test; system is pre-charged |
| Low-pressure warning | Buzzer and/or light must activate before pressure falls to around 380–520 kPa; must not be disabled | No low-pressure warning device; fluid-level warning light (if fitted) must extinguish after start |
| Pedal hold test | Apply service brake several times to drain reservoirs and confirm warning activates; then rebuild | Hold pedal firm for 30 seconds; pedal must not creep toward the floor (a sinking pedal = internal master-cylinder leak) |
| Fluid / air inspection | Drain a small amount from each reservoir to check for water and oil contamination | Check fluid level at reservoir between MIN and MAX; inspect for leaks at callipers, wheel cylinders, master cylinder |
| Parking brake | Spring brakes apply automatically on air loss; must hold the vehicle on the parked gradient | Mechanical park brake (hand or foot applied); must hold on the parked gradient |
| Hose / pipe inspection | Air lines and fittings must be secure, not chafed, cracked, kinked, or leaking; glad-hand seals intact (if towing) | Hydraulic hoses must not swell, crack, chafe, or leak; no fluid at fittings |
| Brake fade risk | Air delay through long lines (longer on combinations); retarder usually fitted | Hydraulic application is near-instant on a rigid; less air-delay risk but fluid boil can cause fade on long descents |
Air brake pre-departure checks (step by step)
- Drain the reservoirs. With the engine off and the parking brake applied, briefly open each reservoir drain valve to eject water and oil that have condensed overnight. Close the valves. If more than a trace of water or oil comes out, report it — the air dryer or compressor may be passing oil.
- Start and build pressure. With the engine at fast idle, watch the dash gauge. Pressure should rise to the governor cut-out range (commonly 825–930 kPa; confirm against the vehicle placard). The compressor should stop loading at cut-out.
- Test the low-pressure warning. With the engine off, fan the service brake pedal to drain the reservoirs. The low-pressure warning buzzer and light must activate before the gauge falls to the warning threshold — around 380–520 kPa depending on the vehicle. The warning must not be bypassed or muted.
- Test the spring (parking) brake. With the park-brake control out and the system at full pressure, place the transmission in gear and gently attempt to move. The spring brake must hold the truck. Then push the park-brake control in (release) and confirm the truck can move normally.
- Test the service brake. At very low speed in a safe area, apply the service brake firmly. The pedal should feel firm with normal travel, the truck should pull up straight, and there should be no unusual noise, pulling, or delayed response.
- Check the ABS warning. Turn the ignition on; the ABS warning lamp should illuminate, then extinguish once road speed exceeds about 7 km/h. A lamp that stays on indicates an ABS fault — the service brake still works but ABS is disabled and the defect must be reported.
Hydraulic brake pre-departure checks (step by step)
- Check fluid level. With the engine off, confirm the brake-fluid reservoir is between MIN and MAX. Use the correct DOT-rated fluid specified by the manufacturer. A low level with no visible external leak may indicate worn friction material — have it investigated before topping up.
- Inspect for leaks. Look at the master cylinder, along each hydraulic hose, and at each wheel cylinder or calliper. Any wetness at a fitting or a soft, swollen hose is a defect.
- Pedal hold test. With the engine off, pump the pedal until it is firm. Then press and hold firmly for 30 seconds. The pedal must not creep or sink toward the floor. A sinking pedal indicates an internal master-cylinder seal failure — the vehicle must not be driven.
- Assisted pedal feel. Start the engine; the pedal should drop slightly as the vacuum or hydraulic servo assist engages, then feel firm. A pedal that feels hard with the engine on (loss of assist) or that feels spongy (air in the fluid) is a defect.
- Parking brake. Apply the mechanical park brake. With the transmission in gear, gently attempt to move. The park brake must hold. Release it and confirm the brake-warning lamp extinguishes.
- Service brake. At low speed in a safe area, apply the service brake firmly. The truck must stop in a straight line without pulling, unusual noise, or excessive pedal travel.
After-departure inspection
The first few hundred metres of a trip are when an overnight defect most often shows up — a load settles, a brake lining beds in unevenly, or a slow air leak becomes audible once the compressor stops loading. The Victorian test expects a driver to make a short after-departure stop to confirm the truck is still roadworthy. The stop is typically made at the first safe pull-off point within 1–5 km of departure.
After-departure checks
- Visual load check. Walk around the load. Confirm restraint equipment (chains, straps, turnbuckles, twist locks) is still tight and correctly seated; the load has not shifted, settled, or leaked; and the tailgate, doors, and curtains are secure.
- Tyre temperature by touch. Carefully place the back of your hand near (not on) each wheel after the first few kilometres. All wheels on the same axle should be at a similar temperature. A noticeably hotter wheel indicates a dragging brake, a tight wheel bearing, or under-inflation — investigate before continuing. A cold wheel on an axle where the others are warm can indicate a brake that is not working at all.
- Brake drum / disc temperature. Hotter-than-expected drums or discs on one side point to uneven brake adjustment or a dragging brake. This is a common early sign of an air-system leak at a chamber or a stuck slack adjuster.
- Air system leak check (air-braked vehicles). With the engine off and the parking brake applied, watch the dash air-pressure gauge for one minute. Pressure must not drop by more than the manufacturer-specified amount (commonly around 20 kPa per minute for a single straight truck; confirm against the vehicle placard). A faster drop indicates a leak that must be found and repaired.
- Hydraulic system leak check. Look under the truck for any new fluid. Re-check the master-cylinder reservoir level. A drop in level with no visible external leak may indicate an internal master-cylinder fault.
- Underbody and load area. Re-scan for new fluid pools, a shifted load, loose equipment, or anything hanging underneath the vehicle.
- Mirrors and lights. A quick re-check that mirrors are still adjusted, lights are still working (reflection on the road behind, or a helper), and the windscreen is not smeared.
If anything found at the after-departure stop is a safety defect — a leaking air chamber, a shifted load, a brake drum hot enough to smell hot brake — the driver must stop and arrange repair or re-restraint before continuing. Continuing with a known defect is a Primary Duty breach under HVNL s26C.
Test Your Knowledge
You are doing the air-brake pre-departure check. With the engine off you fan the service brake to drain the reservoirs. At what point must the low-pressure warning buzzer and light activate?
A
B
C
D
Test Your Knowledge
On a hydraulic-braked rigid truck you hold the brake pedal firm with the engine off. After 30 seconds the pedal has slowly moved closer to the floor. What does this indicate and what must you do?
A
B
C
D
Test Your Knowledge
At your after-departure stop you feel the steer-axle wheels and find the left wheel noticeably hotter than the right. What is the most likely cause and the correct action?
A
B
C
D