🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Air Brakes: Systems, Service, and Compliance › Lesson 2
TRN-DIES20-03 · Lesson 2 · 18 clock hours · demo

Air Supply System: Compressors, Governors, Dryers, and Tanks

Objective. Students will describe the function of the compressor, governor, air dryer, and wet/dry tanks, and demonstrate how to drain tanks and test governor cut-in and cut-out pressures.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

How the Air Supply System Works

Heavy trucks equipped with air brakes depend on a continuous, clean, and dry supply of compressed air. Four major components work together to build, regulate, and store that air: the compressor, the governor, the air dryer, and the reservoir tanks. Understanding each component—and knowing how to test and maintain them—is foundational to air brake service and to the ASE T4 (Brakes) credential.

The Air Compressor

The compressor is engine-driven, typically by a gear on the accessory drive or by a V-belt. Most trucks use a single-cylinder, reciprocating piston compressor, though two-cylinder units appear on larger vehicles. On the compression stroke, the piston moves upward, pressure closes the inlet valve, and air is forced through the discharge valve into the system. On the intake stroke, the piston moves downward, the discharge valve closes, and fresh air is drawn in through the inlet valve.

When the system reaches a set pressure, the governor signals the compressor to unload. Most modern compressors unload by holding the inlet valves open with pressurized air from the governor, so the piston moves air back and forth without compressing it. This protects the compressor from over-pressurizing the system and reduces engine load. During the bench demonstration, locate the unloading port—it is the small fitting on the valve plate cover where governor signal pressure arrives.

The Governor

The governor is the pressure-sensing switch of the system. It monitors system pressure and controls when the compressor loads (builds pressure) and unloads (stops building pressure).

A governor that allows cut-out pressure to exceed specification can overstress hoses, fittings, and valves. A governor that cuts in too late leaves insufficient reserve pressure for brake applications. Both conditions are out-of-spec and require correction before the vehicle returns to service.

The Air Dryer

Compressors pull in ambient air, which always contains water vapor. As compressed air cools in the lines and tanks, that vapor condenses into liquid water. Water promotes corrosion inside tanks and valves, freezes in cold weather, and degrades brake valve performance.

The air dryer is plumbed immediately downstream of the compressor. It uses a desiccant cartridge (a bed of moisture-absorbing beads) to strip water vapor from the compressed air before it enters the reservoirs. A purge valve at the bottom of the dryer opens each time the governor signals cut-out, blowing accumulated moisture out of the dryer body. Some dryers also have a heater to prevent freeze-up in cold climates.

Inspect the dryer cartridge per the manufacturer's service interval. A saturated cartridge allows wet air to pass through, and you will find excessive moisture in the wet tank—the first reservoir downstream.

Wet Tank and Dry Tanks

The wet tank (supply reservoir) is the first tank to receive air from the dryer. It acts as a primary moisture trap and surge dampener. Any residual water that makes it past the dryer collects here.

Dry tanks (service reservoirs) are downstream of the wet tank and supply the front and rear brake circuits, the park/emergency circuit, and accessories. Because the dryer removes most moisture before the wet tank, dry tanks should contain very little water under normal conditions.

Every reservoir has a drain valve at its lowest point. Federal Motor Carrier Safety Administration (FMCSA) regulations and most fleet policies require daily manual draining when automatic drain valves are not installed. Draining removes accumulated water and oil contamination before it can damage system components.

Safety Requirements for This Lesson

PPE: Wear safety glasses at all times during air brake work. Gloves protect hands from sharp edges on disassembled components. Never stand directly in front of a drain valve opening—discharge can carry debris at high velocity.

Depressurize before disassembly: Before removing any fitting, valve, or tank plug, verify system pressure is at zero on a calibrated gauge. Follow your shop's lockout/tagout procedure if the vehicle must be rendered inoperative during service.

Chock the wheels: On the training trailer, always chock wheels before working around brake components, even with the park brake applied.

Gauge accuracy: Use only a calibrated test gauge in the specified range (0–200 psi for most air brake work). A gauge that reads incorrectly will produce false pass results on governor tests.

Draining the Tanks: Procedure Overview

1. Build system pressure to governor cut-out (confirmed on the dash gauge).

2. Shut down the engine.

3. Put on safety glasses and gloves.

4. Locate each reservoir drain valve—wet tank first, then rear dry tank, then front dry tank, then any auxiliary reservoirs.

5. Open each drain valve fully. Note the color and volume of discharge: clear/dry is acceptable; milky, oily, or a large volume of water indicates a maintenance problem.

6. Record findings on the shop form before closing valves.

7. Close all drain valves and rebuild system pressure; verify no air loss at the drain valves when closed.

Governor Pressure Test: Procedure Overview

1. Connect the calibrated 0–200 psi test gauge to the designated governor test port (or a downstream Schrader fitting if the instructor specifies).

2. Start the engine and observe the gauge as the compressor builds pressure. Note the pressure at which the gauge stops rising—this is cut-out.

3. Make repeated moderate brake applications to draw down system pressure while watching the gauge. Note the pressure at which the gauge begins rising again—this is cut-in.

4. Record both readings on the shop form and compare to the specification range for the training vehicle.

5. Flag any reading outside specification and document the finding.

Mastering these fundamentals prepares you for FMCSA pre-trip inspection requirements, ASE T4 exam content, and the day-to-day responsibilities of a commercial vehicle technician.

Key terms10

CompressorEngine-driven pump that compresses atmospheric air and delivers it to the air brake system reservoirs.
GovernorPressure-sensing device that signals the compressor to load (cut-in) or unload (cut-out) to maintain system pressure within a specified range.
Cut-out pressureThe system pressure at which the governor unloads the compressor and stops air delivery to the reservoirs.
Cut-in pressureThe system pressure at which the governor reloads the compressor so it resumes building air pressure.
Air dryerA desiccant-based component mounted downstream of the compressor that removes water vapor from compressed air before it enters the reservoirs.
Wet tankThe first reservoir downstream of the air dryer; serves as a primary moisture trap and pressure surge dampener.
Dry tankA service reservoir downstream of the wet tank that stores clean, dry air for the brake circuits and accessories.
Unloading mechanismA feature of the compressor—typically pressurized inlet valves—that allows the compressor to cycle without building pressure when the governor signals cut-out.
Drain valveA manual or automatic valve at the lowest point of each reservoir used to expel accumulated water and contaminants.
Pressure differential (governor band)The difference in psi between cut-out and cut-in pressures; indicates the operating range the governor maintains in the system.

Worked examples4

1. A student records a governor cut-out of 127 psi and a cut-in of 103 psi. The specification sheet states cut-out must be 120–130 psi and cut-in must be 100–110 psi. Are these readings within specification?
  1. Check cut-out: specification is 120–130 psi. Recorded value is 127 psi. 120 ≤ 127 ≤ 130, so cut-out is within spec.
  2. Check cut-in: specification is 100–110 psi. Recorded value is 103 psi. 100 ≤ 103 ≤ 110, so cut-in is within spec.
  3. Calculate the pressure differential: 127 − 103 = 24 psi. This is the governor band for reference and documentation.
Answer: Both cut-out (127 psi) and cut-in (103 psi) are within specification. No corrective action is required. The governor band is 24 psi.
2. A student records a governor cut-out of 134 psi and a cut-in of 112 psi. The specification is cut-out 120–130 psi, cut-in 100–110 psi. What is the finding, and what should the student document?
  1. Check cut-out: specification is 120–130 psi. Recorded value is 134 psi. 134 > 130, so cut-out is OUT OF SPEC (high by 4 psi).
  2. Check cut-in: specification is 100–110 psi. Recorded value is 112 psi. 112 > 110, so cut-in is OUT OF SPEC (high by 2 psi).
  3. Calculate pressure differential: 134 − 112 = 22 psi. Document differential for reference.
  4. On the shop form, flag both readings as out of specification, note the direction of deviation (both high), and mark the vehicle as requiring governor adjustment or replacement before return to service.
Answer: Both readings are out of specification on the high side. Cut-out exceeds the limit by 4 psi; cut-in exceeds the limit by 2 psi. The student must flag these results and the vehicle requires correction before it is returned to service.
3. The lesson check requires the student's recorded governor pressures to be within ±2 psi of the accepted range limits to pass. A student records cut-out at 131 psi and cut-in at 109 psi against a spec of cut-out 120–130 psi, cut-in 100–110 psi. Does this student pass the lesson check?
  1. Evaluate cut-out reading: 131 psi is 1 psi above the 130 psi upper limit. The lesson check allows ±2 psi, meaning the acceptable measurement window extends to 132 psi on the high end. 131 ≤ 132, so cut-out passes the lesson check tolerance.
  2. Evaluate cut-in reading: 109 psi is within the 100–110 psi specification. No tolerance adjustment needed; 109 is inside the spec. Cut-in passes.
  3. Note on the shop form that while the student passes the lesson check, the actual cut-out reading of 131 psi is outside the vehicle specification and should be flagged as a real-world finding requiring follow-up.
Answer: The student passes the lesson check (both readings fall within ±2 psi of the specification limits), but must note that cut-out at 131 psi is technically out of specification for the vehicle and would require governor service in actual practice.
4. After draining the wet tank, a student observes approximately 8 fluid ounces of milky, rust-colored water discharge. The dry tanks produce only a small amount of clear mist. What conclusions can the student record on the shop form?
  1. Evaluate wet tank finding: milky, rust-colored water suggests the air dryer is not adequately removing moisture and that internal corrosion is occurring in the wet tank. Volume of 8 oz is significant.
  2. Evaluate dry tank finding: small clear mist is normal; it indicates the dryer is passing mostly dry air to the service reservoirs, which is the expected result.
  3. Conclusion for shop form: wet tank shows evidence of excessive moisture contamination and internal corrosion. Air dryer cartridge should be inspected and likely replaced. Wet tank interior should be inspected for corrosion damage.
Answer: The student records that the wet tank has excessive moisture and corrosion contamination, indicating a likely failed or saturated air dryer cartridge. Dry tanks are within normal limits. Recommend air dryer service and wet tank inspection.
In class: Instructor-led bench demonstration of a disassembled single-cylinder compressor: students identify pistons, valves, and unloading mechanism, then sketch and label parts. · Hands-on tank drain procedure on a training trailer: each student drains all reservoirs, observes moisture content, and records findings on a shop form. · Governor pressure test using a calibrated gauge at the test port; students record cut-in and cut-out readings and compare to the specification range, flagging any out-of-spec result.
Materials: Training tractor-trailer, shop air supply, calibrated 0-200 psi test gauge, disassembled compressor bench unit, air dryer cutaway, tank drain valves, PPE (safety glasses, gloves), shop forms
Check: Student correctly drains tanks and records governor pressures within ±2 psi of accepted range; instructor observation checklist, 100% steps completed

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

ASE T4 Brakes – Air Supply System
1. What is the primary purpose of the governor in an air brake system?
ASE T4 Brakes – Air Supply System
2. A technician connects a calibrated gauge to the governor test port and observes that the compressor stops building pressure at 135 psi. The specification for that vehicle is 120–130 psi cut-out. What is the correct conclusion?
ASE T4 Brakes – Preventive Maintenance
3. Which reservoir should be drained first during a routine tank drain procedure, and why?
ASE T4 Brakes – Air Supply System
4. During a bench demonstration, the instructor points to a small port on the compressor valve plate cover. This port is connected to the governor signal line. What function does this port serve?
ASE T4 Brakes – Air Dryer Service
5. A student drains the wet tank and observes a large discharge of milky, oily water. The dry tanks produce only a light mist. What is the most likely cause of the wet tank finding?
Safety – Lockout/Tagout and PPE
6. Before opening any reservoir drain valve, a technician should FIRST:
ASE T4 Brakes – Governor Testing
7. A governor test shows cut-out at 125 psi and cut-in at 98 psi. The specification is cut-out 120–130 psi, cut-in 100–110 psi. Which statement is correct?
ASE T4 Brakes – Air Dryer Operation
8. Which of the following BEST describes the function of the air dryer's purge valve?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →