5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits

Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits

Quick Answer: Each tractor failure has its own signs and its own braking result. Supply circuit failure: the gauges do not recover after falling below cut-in, and the brakes work only on the stored air. Primary failure: the steering axle and trailer brake. On many trucks an inversion (spring brake) valve also applies the drive axles through their springs. Secondary failure: the steering axle brakes do not apply, which makes it the only tractor failure that disables a service circuit's brakes. Park circuit failure: both service circuits lose air, and the spring brakes gradually apply around 55–60 PSI. In every case, use a steady brake application and stop safely.

How to think about failures

The Alberta course pairs each failure with two questions: what does the failure look like, and what happens when you then make a service application? Answer those two questions for each circuit and you can handle any failure question on the test. Three facts from earlier chapters drive every answer:

1. Check valves protect each service tank from a failure upstream (Section 2.3).
2. The park circuit and the trailer supply draw from the higher-pressure service tank (Section 3.1).
3. The steering axle is on the secondary circuit, and the drive axles are on the primary circuit.

1. Supply circuit failure

What fails: anything from a plugged or collapsed line to a ruptured supply tank. Air from the compressor no longer reaches the service tanks.

How you recognize it: the check valves protect the service tanks, so nothing may seem wrong at first. The warning sign is that the gauges do not replenish when pressure falls below the governor cut-in point. Every application lowers the pressure, and it does not come back.

What still works: all of the brakes work properly, but only on the air already stored in the service tanks. That reserve is at least the 12 full applications that power-unit tanks are sized for, less whatever you have used.

What to do: do not keep driving and hoping. Plan a controlled stop while you still have plenty of pressure, use steady applications rather than repeated ones, and get the fault repaired.

2. Primary circuit failure

What fails: a line, chamber, or tank in the primary (green) circuit, which normally feeds the drive axles.

How you recognize it: the primary gauge drops, and the low-air warning comes on when it falls below 60 PSI.

Spring brakes: they stay released, because the park circuit now draws its air from the secondary tank.

On a service application: the steering axle and trailer brakes apply. The drive axles have no service air. Many trucks, however, have a tractor spring brake valve, sometimes called an inversion valve, which is drawn to the right of the foot valve in the course schematics. The secondary service signal controls it, and it lets a controlled amount of air out of the drive-axle spring brakes. The springs then apply the drive-axle foundation brakes in proportion to your pedal application, and they release again when you let off. More air is used to re-release the springs each time, so the reserve still runs down.

What to do: a steady brake application to a safe stop. Do not pump.

3. Secondary circuit failure

What fails: a line, chamber, or tank in the secondary (red) circuit, which normally feeds the steering axle.

How you recognize it: the secondary gauge drops and the low-air warning comes on.

Spring brakes: they stay released, on air from the primary tank.

On a service application: the drive axles and trailer brake normally, but the steering axle brakes do not apply. The course points out that this is the only tractor air system failure that stops a service circuit's brakes from working during a service application. In a primary failure the drive axles can still be braked through the inversion valve, but nothing replaces the steering-axle service brakes.

What to do: allow more stopping distance, make a steady application, and stop safely.

4. Park circuit failure

What fails: a line or chamber in the orange park circuit. The park circuit is fed from both service tanks, through the double check valve that selects the higher one, so a leak there draws air out of both service circuits.

How you recognize it: both gauges fall and the low-air warning comes on.

What happens next: as pressure falls, the spring brakes begin to apply. They start to take hold at about 55–60 PSI and are firmly applied by about 45–50 PSI.

What to do: the course's advice is to apply a steady service application and expect the spring brakes to begin to gradually apply as pressure drops to about 55–60 PSI.

Will the wheels lock? Students often ask this. A spring park application is only about half as strong as a full service application. Whether the wheels lock depends on the vehicle's weight, how sharply the brakes apply (a shock application or a gradual one), and the available traction. Once the spring brakes apply, you have no control of the vehicle if it starts to slide. ABS operates only on the service circuits and will not help.

Summary table

General rules for any air loss

• Do not pump the brakes. Each release exhausts chamber air, so pumping drains the reserve quickly.
• Use steady, controlled applications, and downshift to use engine braking where it is safe.
• Stop in a safe place and secure the vehicle before the spring brakes take over by themselves.
• Do not drive on. An activated low-air warning or an inoperative service or parking brake is a major defect.