The business end of your basic air system consists of an air compressor, a governor to control the load and unload cycles of the compressor, three air reservoirs, and an air dryer. There’s about a mile of hose, a handful of valves, and a few other parts in there as well, but most of the problems you’re likely to experience will occur with, or because of, the compressor, governor, or the dryer.

Oily Compressors

Oil in the air lines? It’s not unusual, because normal operation of the air compressor sends a bit of lube oil past the piston rings, according to the Technology and Maintenance Council (TMC) of the American Trucking Associations (ATA). The oil gets mixed with water vapor from the intake air to form an oily emulsion. You can see it spitting out of drain valves on the air tanks.

Of course, there is such a thing as ‘excessive’ oil consumption by the compressor. You’ll see increasing amounts of oil as valves spit out the emulsion and that may set you to worrying. Is the compressor wearing out? Maybe, maybe not. TMC’s Recommended Practice 630 says there may be other causes:

* Restricted air intake – check the air filter canister and inlet piping for blockages, kinks, excessive bends, and collapsed hoses. Inlet lines should be of at least 5/8-in. inside diameter for highway trucks. High-output applications (using compressors with a capacity of 30 cu ft per minute or larger) need larger lines; check the manufacturer’s specs.

* Poorly filtered air – dirty air impedes performance, so check the filter element’s condition. Also, be sure the compressor is not connected in any way to the engine’s exhaust-gas-recirculation (EGR) valve if you have a current-model diesel.

* Excessive cycling – the major cause of premature compressor failure, it can be caused by leaks in the air lines. Another less likely cause is a compressor of insufficient capacity for the job (a tractor built for tandem/tandem work but reassigned to Super-Bs on air suspensions, for example, may have too small a compressor to cope with the added volume required). Or it might indicate a brake stroke problem: too much air drawn for each application.

* Knock or rattle – a noisy compressor could be a sign of a restricted air-discharge line, which is a simple enough fix, or it could indicate a mechanical problem such as worn drive-gear components, carbon formation on top of pistons/bottom of cylinder head, or a loose mounting.

* Slow air buildup – if it takes an abnormally long time to build system pressure, check for restrictions in the intake of discharge lines, watching for other symptoms of restriction. Check for system leakage, inadequate compressor size for vehicle application, air-dryer purge valve stuck open, or a defective unloader mechanism and/or governor.

* Will not unload – a compressor that won’t stop pumping is problematic too. The safety valves on the air tanks should release before 150 psi. If not, shut the engine down immediately. There could be a restriction in the unloader line providing a false pressure reading to the governor. Check the line and/or replace the governor.
Air Dryers

Draining air tanks was once part of a driver’s daily routine. Valves at the bottom of a truck’s air reservoirs were opened each morning to let accumulated water spit out. This reduced the amount of moisture that found its way into more sensitive parts in the air system. If you open an air-tank valve and water spits out, it’s time to check the air dryer because it’s probably not doing its job.

Typically, a removable desiccant cartridge will last from two to three years. They’re easy to replace, and not terribly expensive. A slobbery compressor could reduce that lifespan considerably. Oil contamination can reduce the effectiveness of the desiccant beads, allowing more moisture to enter the reservoirs. Check for oily discharge from the air tanks. Slightly oily discharge from the air dryer purge valve is normal.

The discharge, or purge, event is triggered by the unloading signal from the governor. If that line becomes blocked it may affect the air discharge cycles. Contaminants can become lodged in the purge valve, causing a constant leak. The purge valve can be easily replaced as well. And there’s a thermostat-controlled heating element in the purge valve that can wear out over time. Check the bottom of the air dryer body for warmth in colder weather.

Sludge Removal

If you’ve suffered from that slobbery compressor, you’ll know what a mess all that oil can make. Over time, it can contaminate the rest of truck’s air system. Come winter, all that black sludge can thicken up and wreak havoc with valves and can even freeze in an air line causing a blockage. Cleaning the oil out is a messy and technically exacting process, so if you’re not sure what you’re doing, take the truck in for service.
The TMC’s Recommended Practice 617 lists nine steps for removing contaminants from tractor air-brake systems. You’ll have to disconnect and/or disassemble air lines, valves and other components and blow out these components with clean air from the shop compressor, usually in the opposite direction of the system’s normal air flow (in other words, backward). Inspect all parts and replace them if necessary. For this, it’s important that you follow instructions and guidelines from each component’s manufacturer.

The Recommended Practice says to proceed in this order:
1. Clean the line between the truck’s compressor and the air dryer.
2. Service and clean the dryer, including replacing its desiccant and filter.
3. Clean out the wet tank. To do it right, the shop compressor’s regulating valve should be set at 5 to 8 psi and air run through the tank and out its petcock for half an hour.
4. Clean out the governor regulator line, or any other line supplied by the wet tank.
5. Clean the one-way check valves protecting the two service tanks.
6. Blow out the lines between service and wet tanks.
7. Disconnect the supply line between the service reservoir and the foot valve and blow it out. If no oil is found in this line, you’re done cleaning. If oil is found, blow out all other lines from the service reservoir.
8. If oil is found in the service line, inspect the bottom of the foot valve by removing the lower housing. If there’s oil inside, all other valves (foot, dash, tractor protection, relay and two-way checks) must be disassembled, cleaned or replaced.
9. With everything back together, do a system check for proper operation.

In replacing valves and air lines, TMC has long preached that it’s vital for new parts to be of the exact same specifications. Each replacement valve should be of the same make and model as the old one, so that each one’s crack pressure (the point at which the valve opens) is the same. Each new air line should be of the same diameter and fittings should be of the same type (connectors, unions, elbows and tees) as the old so the same volume of air is carried.

If not, the performance of the air brake system will change. This will probably throw off balance among brakes on all axles and perhaps between brakes on the same axle, and may adversely affect brake timing between tractor and trailer.

Severely contaminated air systems are rare, relatively easy to prevent. A little vigilance on a daily basis should alert you to a problem in the making. Try to catch it before you’ve got a mess on your hands.