3.1 Dual Air Systems: Primary and Secondary Service Circuits
Key Takeaways
- Each axle group has an independent service brake circuit; on a truck or tractor the primary (green) circuit normally serves the drive axles and the secondary (red) circuit the steering axle.
- One-way check valves isolate each service tank from the supply tank, so a failure upstream or in one circuit does not drain the other.
- The dual foot valve has a primary and a secondary section; if primary air is lost, pushing the pedal further mechanically operates the secondary section.
- Trailer service tanks and the park circuit both receive air from whichever tractor service tank has the higher pressure.
- A secondary circuit failure is the one tractor failure in which a service circuit's brakes do not apply: the steering axle brakes will not work.
Dual Air Systems: Primary and Secondary Service Circuits
Quick Answer: Modern air-brake power units use a dual air system: two independent service circuits, each with its own service tank, controlled by one dual foot valve. In the Alberta course the primary circuit (green) normally brakes the drive axle(s) and the secondary circuit (red) normally brakes the steering axle. One-way check valves keep each tank isolated. Trailer service tanks and the park circuit are supplied from whichever tractor service tank has the higher pressure, so losing one circuit does not immediately apply the spring brakes or starve the trailer.
Why split the system?
With a single service circuit, one ruptured hose or diaphragm could vent all the stored air and leave the vehicle with no service brakes. Splitting the service brakes into two independent circuits means that one failure leaves the other circuit working. The driver then has service braking to make a controlled stop. The Alberta course takes this idea further: every axle group has its own independent service brake circuit. On trailers, some manufacturers fit one circuit per axle, and others fit one circuit for the whole axle group.
The standard tractor layout
| Circuit | Schematic colour | Normally brakes | Tank |
|---|---|---|---|
| Supply | Blue | Nothing directly; it feeds the service tanks | Supply (wet) tank |
| Primary service | Green | Drive axle(s) | Primary tank |
| Secondary service | Red | Steering axle | Secondary tank |
| Park | Orange | Spring brakes on the drive axles (and trailer, through the trailer supply) | Fed from the higher-pressure service tank |
The course shows a truck whose secondary tank is sized for two Type 20 steering-axle chambers and whose primary tank supplies four Type 30 chambers on the tandem drive axles. That is typical: the tanks are sized for the chambers they feed.
How the dual foot valve works
The driver has one brake pedal, but the foot valve (brake valve) contains two sections, one for each circuit:
- Primary section. Pedal movement opens the primary section directly, which sends primary tank air as a control signal toward the drive-axle relay valve.
- Secondary section. Normally, air pressure from the primary section operates the secondary section, so both circuits apply together.
- Back-up. If primary air is lost, pushing the pedal further makes mechanical contact and opens the secondary section, so the steering axle brakes still apply.
The foot valve can be mounted inside the firewall, outside the firewall, or under the floor. On a tractor, it also sends a service signal to the trailer through the tractor protection valve (Section 4.2).
Higher-pressure selection: trailers and the park circuit
Two supplies are protected by drawing from the higher of the two service tanks:
- Trailer supply. The air used to charge the trailer tanks comes from the higher of the two tractor service tanks.
- Park circuit. The air that holds the spring parking brakes released comes from the higher of the primary and secondary tanks.
This arrangement keeps the spring brakes released and the trailer charged when one circuit fails, which gives the driver time to make a controlled stop instead of having the spring brakes apply at once.
Straight truck versus Super-B
- Straight truck. Two service circuits: primary (drive axles) and secondary (steering axle). The foot valve connects to the primary service circuit through a control line, and a relay valve near the drive axles delivers tank air to the chambers.
- Super-B train. Four axle groups with independent service circuits: primary (drive), secondary (steering), and tank-mounted relay valves for each of the two trailers. Control (pilot) lines connect the service circuits so that one pedal application reaches every axle group.
What the driver notices when one circuit fails
Chapter 5 covers failures in detail. The pattern to remember is:
| Failure | Gauge sign | What still brakes on a service application |
|---|---|---|
| Primary circuit | Primary gauge drops; low-air warning | Steering axle and trailer brakes. Many trucks also have a spring brake (inversion) valve that applies the drive axles through the park springs in a controlled way |
| Secondary circuit | Secondary gauge drops; low-air warning | Drive axles and trailer; the steering axle brakes do not apply |
In both cases the spring brakes stay released on the air from the good circuit. Use a steady brake application to stop, and do not pump the pedal: pumping wastes the air in the circuit that still works.
On a straight truck with a standard dual air system, which service circuit normally brakes the steering axle?
In a dual foot valve, what lets the secondary (steering axle) brakes still apply if the primary circuit has lost all its air?
Where does a tractor get the air it uses to charge the trailer's tanks?