Compressor discharge line: carbon, oil and slow air build-up

The discharge line between the compressor and the air dryer (pipe or hose) does more than carry air: it cools the air on the way. A line narrowed by carbon, or kinked, makes a truck slow to build air, and heavy carbon and oil inside it point back to the compressor. Bendix puts a figure on it: more than 1/16 inch (about 1.6 mm) of carbon and the line is replaced.

What does the discharge line do?

Bendix, part of the Knorr-Bremse group, defines the line’s job as cooling in its compressor troubleshooting guide: the mixture of air, water vapour and oil vapour cools here on its way to the dryer. At a normal inlet temperature the dryer removes most of the oil. Above that, the oil, as vapour, “is able to pass through the air dryer and into the air system.”

That is why length and bore matter. The same guide: “Larger diameter discharge lines and/or longer discharge line lengths can help reduce the temperature.”

Slow to build air: is the line to blame?

It can be, but in Bendix’s order of checks leaks come before the line. By its measure the system is normal if the gauge climbs from 85 to 100 psi (about 5.9 to 6.9 bar) in 40 seconds or less at full governed speed. The UK heavy goods vehicle inspection manual uses another yardstick: a fully depleted system should normally reach minimum effective working pressure within 3 minutes, or 6 minutes on type-approved vehicles designed to draw a trailer.

If the time is too long, the order is: leakage, a compressor too small for the vehicle, the unloader mechanism, the compressor head gasket, the discharge line, the inlet side. The compressor comes last:

“Replace the compressor only after making certain that none of the preceding conditions, 9.0 (a) through 9.0 (h), exist.”

Bendix, Advanced Troubleshooting Guide for Air Brake Compressors (BW1971), January 2014

A restricted line shows in two other ways: the compressor’s safety valve releases air, or air leaks at the head gasket. The guide says the second may indicate a downstream restriction, such as a freeze-up or carbon blockage, or a defective safety valve. The blockage is then found first, the compressor replaced second.

The line may also be leaking: Meritor WABCO’s air dryer manual lists a leak in this line among the causes of no pressure build-up.

Oil at the air dryer: is the compressor finished?

Not on that evidence alone. In the Bendix guide, oil at the dryer purge belongs to a normally working system: “Air dryers remove water and oil from the air brake charging system.” Some oil from the tanks of a maintained vehicle is normal too. The amount is judged by draining the tanks into a graduated cup, not by eye.

A Bendix technical note says the first sign of excessive oil passing is usually a valve failure. It suggests two checks: for oil in the water drained from the tanks, and inside the discharge line.

“If it’s especially dirty or fouled with carbon buildup or evidence of oil, it’s a sign that the problem is at the source, from the compressor itself.”

Bendix Tech Tips, 24 June 2020

Even then, look first for what is overworking the compressor: the guide names air leakage as the number one reason a compressor has to pump too much air, runs too hot and passes oil vapour.

What do manufacturers specify for length, bore and slope?

The three documents we opened ask for the same things: enough bore, enough length to cool the air, and an unbroken fall towards the dryer. Their figures differ.

All three were written for the North American market; the metric equivalents are our conversions, except Meritor WABCO’s. Where the vehicle manufacturer gives different dimensions, those apply.

Bendix gives the reason for the slope: “The discharge line must maintain a constant slope down from the compressor to the air dryer inlet fitting to avoid low points where ice may form and block the flow.”

Length is a balance. If the dryer inlet still runs hot, Bendix suggests the next bore up and a longer line. But moisture in a long line can freeze: Bendix notes that cold climates may need a shorter line or insulation, and Meritor WABCO wants a line over 10 ft (3.0 m) insulated.

When is the line replaced, and can it be cleaned?

Bendix: “If discharge line is restricted or more than 1/16" carbon build up is found, replace the discharge line.” A kinked or flattened line gets the same remedy. The guide has no cleaning step, and we found none in the other documents we opened.

The new line must match the old one: Bendix requires replacement hose, tubing and fittings of equivalent size, type and strength to the original equipment. Our conclusion: a general-purpose air hose with the right thread is not a substitute.

The UK manual treats the pipe as part of the brake system: “The air pipe between the compressor and the reservoir is to be taken as a brake pipe.” It accepts a sleeved repair to a metal air pipe that looks sound, but not one brazed in place.

In what order should a mechanic check?

1. Time the build-up. The measure is the 40 seconds above.
2. Test for leaks. Wait one minute at cut-out pressure, then watch the gauges for two minutes with the brakes off and two with them applied. A drop of more than 4 psi (about 0.3 bar; 2 psi more per additional trailer) is a leak. Find it with soapy water.
3. Follow the line from end to end. Any flattening, kinks or chafing? Does it fall all the way to the dryer?
4. Take the line off and look inside. Over 1/16 inch of carbon, replace it. Heavy carbon and oil point to the compressor.
5. Measure the temperature. In Bendix’s test, a compressor discharge fitting under 360°F (about 182°C) with a dryer inlet over 200°F suggests a restriction in the line. A discharge fitting over 360°F means a hot-running compressor; check the coolant lines and the discharge line.
6. Check the inlet side. A restricted or sagging inlet hose, an air filter due for service. Then the compressor.