Study guide
2.2 Supply and Service Tanks, Contamination, and Draining
Alberta Air Brake Q Endorsement
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1 Chapter 1: Alberta Q Endorsement Program and Air Brake Fundamentals
1.1 Alberta Q Endorsement Path, Course Requirements, and Knowledge Test Logistics 1.2 Licensing Scope, Farm Vehicle Exceptions, and the Technician Route 1.3 Air Brake System Fundamentals: Compressed Air, Pressure Units, Safety, and Schematic Colours
2 Chapter 2: The Supply Circuit: Compressor, Governor, Tanks, and Air Dryers
2.1 Air Compressors, Drive Systems, and Governor Operation 2.2 Supply and Service Tanks, Contamination, and Draining 2.3 Air Dryers, Air Brake Antifreeze, Safety Valves, and Check Valves
3 Chapter 3: Service Circuits, Control Valves, Gauges, and Low-Air Warnings
3.1 Dual Air Systems: Primary and Secondary Service Circuits 3.2 Brake Chambers, Control Valves, Quick Release Valves, and Relay Valves 3.3 Air Pressure Gauges, Low-Air Warnings, and the Driver's Response
4 Chapter 4: The Park Circuit and Tractor/Trailer Circuits
4.1 Spring Parking Brakes, the Park Control Valve, Compounding, and Caging 4.2 The Tractor Protection System, Trailer Supply Valve, and Glad Hands 4.3 Trailer Service Control, the Trailer Hand Valve, and Air Park Brake Trailers
5 Chapter 5: Air System Failures and Emergency Braking
5.1 Tractor Air System Failures: Supply, Primary, Secondary, and Park Circuits 5.2 Trailer Air System Failures: Service Line, Supply Line, Reversed Lines, and Trailer Tanks
6 Chapter 6: Foundation Brakes and Brake Adjustment
6.1 Foundation Brakes: S-Cam Drum, Disc, and Wedge Brakes 6.2 Brake Stroke, Adjustment Limits, and Stroke Indicators 6.3 Adjusting Manual Slack Adjusters and Maintaining Automatic Slack Adjusters
7 Chapter 7: Braking Fundamentals, Driving Techniques, and Brake Management Systems
7.1 Braking Fundamentals: Heat, Brake Lag, Stopping Distance, and Fade 7.2 Downgrades, Auxiliary Retarders, Bobtailing, and Winter Braking 7.3 Antilock Brakes, Automatic Traction Control, and Stability Control
8 Chapter 8: Vehicle Inspections and the Air Brake Practical Test
8.1 Trip Inspection Law, Major and Minor Defects, and Roadside Enforcement 8.2 Inspection Set-Up and Recognizing Air Brake Defects 8.3 Trip Air Brake Inspection Steps 1–3: Tractor Protection and Park Control Checks 8.4 Trip Air Brake Inspection Steps 4–6: Supply Circuit, Leak Test, Brake Adjustment, and Scoring
2.2 Supply and Service Tanks, Contamination, and Draining
Key Takeaways Explain This Topic AI
The first tank to receive air is the supply tank (also called the wet or purge tank); supply is the formal term, and it captures most of the contamination.
Air tanks are hydrostatically tested to at least five times the compressor cut-out pressure or 500 PSI (3,448 kPa), whichever is greater.
Air tank volume must supply at least 12 full service brake applications for power units and 8 for trailers.
Pull the supply tank lanyard often (pre-trip, en-route inspections, fuelling); if moisture is accumulating, drain the tanks completely at the end of the shift, because tanks do not drain properly until the air pressure is depleted.
The easiest way to identify the supply tank on a truck is to find the safety valve.
Last updated: September 2026
Supply and Service Tanks, Contamination, and Draining
Quick Answer: Compressed air goes first to the supply tank , often called the wet tank because it catches most of the water and oil, and sometimes called the purge tank . From there it passes through one-way check valves into the service tanks for each circuit. Tanks must be hydrostatically tested to at least 5 times the compressor cut-out pressure or 500 PSI (3,448 kPa), whichever is greater . They must hold enough air for at least 12 full service applications on a power unit and 8 on a trailer . Alberta teaches drivers to pull the supply tank drain lanyard often . If moisture is building up, drain the tanks completely at the end of the shift , because a tank does not drain properly until its pressure is gone.
The tanks in a typical tractor
Tank Also called Fed by Feeds
Supply tank Wet tank, purge tank Compressor through the air dryer The service tanks, through one-way check valves
Primary service tank Primary tank Supply tank The primary (green) service circuit, normally the drive axles
Secondary service tank Secondary tank Supply tank The secondary (red) service circuit, normally the steering axle
Trailer tanks Trailer service tank(s) The tractor's trailer supply line The trailer's service and spring brakes
The formal name is supply tank . "Wet tank" is common because it captures the bulk of the contamination. "Purge tank" is often used as well, but in fact most air dryers manage their own purge air, so the supply tank rarely does that job. Some vehicles combine tanks. For example, one driver-side tank may be divided into two compartments, with the supply tank in the front portion.
How to find the supply tank on a real truck: look for the safety valve . The supply tank usually carries the safety valve, the drain lanyard underneath, the incoming line from the air dryer, and the small line running to the governor in the engine bay.
Standards for tank strength and capacity
The course gives two standards worth memorizing:
Strength. Air tanks are hydrostatically tested to handle at least five times the compressor cut-out pressure or 500 PSI (3,448 kPa), whichever is greater .
Capacity. Each service tank is sized for the brake chambers it supplies. The total tank volume must provide at least 12 full service brake applications for power units and 8 full service applications for trailers .
These reserves are why a driver who notices a problem early still has several full applications available. They are also why pumping the brakes wastes the reserve so quickly (see Section 3.3).
Why tanks collect water and oil
Outside air always contains water vapour. Compressing air to about 120 PSI squeezes roughly nine volumes of outside air into one volume, and it heats the air as well. As that hot, humid air cools in the lines and tanks, the water vapour condenses into liquid. It collects at the bottom of the tanks, mixed with any oil carried over from the compressor. The supply tank, as the first and coolest stop, collects most of it.
Contamination causes three kinds of problems:
Freezing. In an Alberta winter, water in tanks, lines, or valves can freeze. Ice can block a valve so that air cannot reach the chambers, or trap air so that brakes stay applied and drag.
Corrosion. Water sitting in steel tanks rusts them from the inside, where you cannot see it.
Sludge. Oil and water mix into a sticky sludge that fouls valves and attacks rubber parts.
Drain valves
- Type How it works Notes
- Manual drain valve A petcock or valve at the bottom of the tank, opened by hand Common on service tanks
Drain lanyard (pull cable) A spring-loaded valve opened by pulling a cable routed to the side of the vehicle Most supply tanks have one; lets you drain without crawling under the truck
Automatic drain valve Opens briefly on its own as pressure cycles; often heated Can freeze or clog, so still confirm it is working
How Alberta teaches draining
The course separates checking for contamination from fully draining a tank:
Pull the supply tank lanyard often during the day: at the pre-trip inspection, during en-route inspections, and when fuelling. A quick pull under pressure does not properly drain the tank, but it shows you whether contamination is accumulating.
If moisture is accumulating, drain the tanks completely at the end of your shift. Tanks do not drain properly until the air pressure has been depleted and the contamination settles and starts to drip out.
If necessary, lanyard pulls can be held open with a tarp strap after the shift so the tank drains as the pressure bleeds away. Close them again before the next trip.
Start with the supply tank , which holds most of the contamination. If you drain a service tank while the supply tank is still charged, air flows from the supply tank through the one-way check valve into the service tank.
Always stand clear of the discharge , and wear eye protection.
On vehicles with an AD-IS air dryer module, daily manual draining is no longer necessary. It is still recommended that the lanyards are pulled regularly to confirm the dryer is working properly (see Section 2.3). If you find a lot of water or oil in a tank, report it: the air dryer is not doing its job, or the compressor is passing oil.