The basics

If you're reading this you probably already know at least a little something about brake air compressors. So we won't bore you with the simple stuff. What you really need to know is that the compressor is built of two main sections, the block and cylinder head. The block holds the crank and piston. The cylinder head holds the valves and unloader. If you're having problems with the compressor, here's some simple fixes to common issues.

Air Compressor Won't build Air Pressure or Air Compressor Slow To Build Air Pressure

This is the #1 complaint when it comes to air compressors. There a few things that can cause this. They are: compressor drive, piston rings, air governor, unloader, valves and the head gasket. Here how to quickly check all these parts.

1. If the compressor drive failed, in most cases the compressor won't put out any air at all. It is possible that the drive catches intermittently, but that is rare. If the piston rings / cylinder wall are worn out, depending on the severity, the compressor either won't pump or will pump slowly. The easiest ways to test both the drive and rings at once are to disconnect the discharge line and push a compression tester equipped with a rubber tip into the outlet fitting. (Note: Do not thread a tester onto the discharge line as it could extreme pressure and damage.) If the pump is capable of building at least 120psi of pressure this means the drive and rings are fine.
2. If the air compressor doesn't build at least 120psi, remove the air governor and test again. If it builds air with the governor removed, verify that the air dryer is not sending a signal to the governor to unload the compressor. If no air is coming from the air dryer signal line, change the governor. If there is air present, repair the dryer.
3. If the air compressor still won't build air pressure with the governor removed, the next step is to disassemble the unloader. Inspect the parts for damage and correct operation. If a problem is found, replace the damaged pieces and re-test with the compression gauge.
4. If no problem is found, remove the parts necessary to inspect the cylinder head valves and head gasket. Repair or replace any damaged parts and use the compression gauge to test again.
5. If all these checks are completed and no problems are found either the drive is bad or the most economical thing to do is change the compressor. Extensive troubleshooting may or may not reveal the cause, but factoring time and labor rates it is very easy to exceed the cost of replacing the compressor.

If at any time during these tests the compressor is found to be producing at least 120 psi the compressor should be considered fine. The problem of not building or slow building air pressure will be found somewhere in the air system. The most common issues are a blocked discharge line or a malfunctioning air dryer. Visually inspect the discharge line for blockage or kinks. Use a shop air compressor to blow through the line to insure it's clear of obstruction. Replace the line if any problem is found. The easiest way to test the air dryer is to bypass it. If the system builds air with the dryer bypassed, rebuild or replace the dryer.

Air Compressor Cycles Constantly

This problem usually points back to the air dryer, air governor and sensing line, or the air compressor unloader.
1. The easiest way to troubleshoot this issue is to remove the air governor sensing line at the governor and use regulated shop air to cycle the compressor. If supplying and removing the shop air cycles the compressor normally, it indicates the air compressor unloader and governor are working normally. If not, inspect and replace the unloader parts or governor as needed.
2. Check the air governor sensing line and fittings for leaks. Leaks in this line will let the pressure bleed off and cause the compressor to cycle. If a leak is found fix or replace the line.
3. If no problems are found, check the air dryer for correct operation. Rebuild or replace it if any problems are found.

Air Compressor Leaking Coolant Externally

If the air compressor is leaking coolant externally, it's probably either a head gasket or a fitting. In some rare cases the leak can be contributed to a cracked head. To find the leak a can of break clean, mirror and a good light is all you should need. Many times a slow leak will show as a white stain left by the evaporating coolant. Larger leaks will have a clean area where normal road grime has been washed away. If you get really stumped, buy a bottle of coolant dye and use a black light to find the source. Head gasket repair kits are available for most air compressors as are replacement fittings or the o-rings for the fittings. You need some mechanical skills to swap out these parts, but it's usually a straightforward fix.

Air Compressor Leaking Coolant Internally

Most times an internal air compressor coolant leak is the result of a failed head gasket. In some cases the air compressor may have a cracked head or develop a hole in the cylinder wall. A large problem is that since the engine and the air compressor share the same coolant, many times a mechanic will go after the engine as the source of the coolant loss. The truck owner can help prevent this by regularly draining the air tanks and servicing the air dryer. This way the truck owner has a better chance of noticing excess water/coolant in the tanks or dryer and can alert the mechanic. If the compressor is found to be at fault you now have the option of trying to find the source of the leak or just changing out the compressor. If it's a high hour / mileage compressor, it's usually the most cost effective to change it out.

Air Compressor Leaking Oil Externally

External leaks on the air compressor are usually going to be coming from the oil pan, head gasket, crank seals or mounting gaskets. Look for areas that dripping or are washed clean by leaking the oil to find the source. If it's a small leak, oil dye and a black light are great tools to locate it. The oil pan and mounting gaskets are simple fixes. Many times the pan is just a flat plate sealed by RTV silicon sealant. It may however require removal of the compressor to re-seal it. The head gasket is also not too difficult of a fix and gasket kits are available for most compressors. If the crank seals are found to be at fault, most times the easiest fix is to replace the compressor.

Air Compressor Leaking Oil Internally

When the air compressor is leaking oil internally one of the first noticeable symptoms is that the air dryer is passing oil. It's normal for the air dryer to collect some oil but if the dryer is maintained regularly it's easy to spot what would be considered excessive. Another possible symptom is that the air system builds air slowly. This can be caused by the oil being passed by the air compressor burns in the discharge line and eventually blocks it off. If you find that you're having these problems, it's usually the most economical to change the compressor. The problem possibly could the result of a failed head gasket. However, in many cases it is due to oil passing by the piston rings.


These checks and tests are meant to be a quick and simple way to test a brake air compressor and determine its functionality. There are always exceptions and rare occurrences but these should be able to diagnose most issues. Highway and Heavy Parts has an extensive selection of compressors for nearly all applications. If you find that you need to replace your compressor, Please call us at 844-215-3406.