Truck Air Brake System Maintenance
The following is a guest post by one of our resident Diesel Experts, Jeff Celentano!
Air brake maintenance is a critical part of a truck, especially during the northern winter months when air lines are prone to freezing. In this post, we will cover the details of proper air brake maintenance, as well as address some myths regarding using alcohol as a form of antifreeze.
Each US State has specific guidelines for operating a vehicle with air brakes, and you should be aware of the guidelines pertaining to your state. However, there are some general steps and guidelines you can follow.
- Make sure the minimum operating pressure for a vehicle airbrake system is no less than 85 PSI for a bus and 100 PSIfor a truck.
Check that it takes no longer than two minutes for air pressure to rise from 85 PSI to 100 PSI at 600 to 900 rpm. This is called the air pressure buildup rate.
- Confirm that the correct cut-out governor pressure for the air compressor is between 120 PSI and 135 PSI. Cut-in pressure is 20 PSI to 25 PSI below cut-out pressure.
Additionally, you should always be on the lookout for water in the air-brake system. This is especially true during the winter months. Water in the air-brake system is typically a byproduct of condensed air, and when it freezes up, it can block air from reaching the brake mechanism. When air is blocked in the system, this can cause the wheel to lock up.
To prevent this, many modern air-brake systems utilize automatic drain valves in each air tank that purge automatically.However, another problem that you may commonly see, which is air couplers. When the rubber seals become worn or chemically dried out, air can escape. While air compressors can typically overcome small leaks, pushing the compressors too hard will ultimately lead to failure and other issues all for an over the counter $2.59 cent part that is often very overlooked.
This leads to another problem -- Placing alcohol into an air-brake system. This can be a serious hazard. Alcohol, especially acetone, will eat rubbers and plastics. What do you think your seals made of? Rubber. The alcohol will eventually eat through the seals and cause a lot of problems for you.
Fortunately, seeing a complete brake failure on a rig is not common. Today’s modern safety systems such as dual air brake systems and anti-lock brakes (ABS) help with brake system failures. If one system will fail, the second system should still work. However, if you are running alcohol in your air brake system, plus going through a bunch of ice and snow, well letsjust say your secondary air system could easily fail as well.
Air Brake Maintenance
Air brake maintenance is critical. Here is a quick checklist of things to check your air brake system for.
- Checking for valve failure in the primary and secondary circuits. Valve failures can compromise the system, so check these by opening the drain valve on the wet tank.
Check for dirt, debris and salt build up on the glad hands.
Inspect any rubber boots and seals for ABS connecters and also inspect for any signs of wear.
Confirm that the push rods and slack adjusters are properly operating, because these are essential to maintaining the adjustment of the brake shoes in relation to the drum.
- Inspect the push rod actuation from the brake chambers, particularly any broken or weak springs.
- Inspect the parking brakes for leaks in each chamber, or damaged components.
Let’s talk a little more about alcohol in truck air brake system. Putting alcohol into the air brake system is a common method that some use to prevent brake line freezing. Methyl alcohol vapor is typically introduced to the air system, and compressed air travels from the compressor to the air tank. The methyl alcohol will help to prevent freeze.
While this has temporary benefits, it also comes with a range of issues. As we mentioned earlier, harsh alcohol like acetone can and will wear out or even eat through rubbers and plastics. While you might be preventing brake freeze, you’ll be introducing your air brake system to a range of other complications down the road.
Alcohol doesn’t really eat acrylics, but it can dissolve the glues and solvents used to hold acrylic pieces together. It’s not unheard of to see acrylic reservoirs falling apart and leaking, due to alcohol dissolving the solvents.
Furthermore, alcohol can also choke the air supplies when used incorrectly. Some air brake systems have an alcohol evaporator for the purpose of putting alcohol into the air system. This is an older technology that still works and will usually have an alcohol container specifically for pouring alcohol into.
Even if you use an alcohol evaporator in your air brake system, it is still absolutely crucial to perform daily air brake system maintenance. This includes tank drainage, for eliminating water and oil. The exception here is unless the system has automatic drain valves but don’t trust those completely. They are mechanical components, and mechanical things fail every day.
For the most part, you should pour approved deicers only into the emergency side of an air system (the red glad hand side). For example, the manufacturer might approve methyl alcohol, but not approve isopropyl or ethylene glycol.
This is because ethylene glycol can swell the o-rings within brake valves and break down the lubricants which protect the internal pistons that control the air flow. The other issue Is that people often don’t know or make a mistake and pour the fluids into the blue glad hand. The fluid will only sit on top systems air brake valves. This could prevent the valve from opening, which is necessary for allowing air to pass.
Alcohol needs a path to travel. If you don’t first drain the air tank, you could pour gallons of alcohol into the front, and it’s not going anywhere. It will just go to the lowest point in the air line and sit there, useless. Remember that it’s not the alcohol itself that thaws frozen components in the air-brake system, but the alcohol vapor.
A little bit will go a long way, such as an eighth of an ounce, rather than half a jug. Furthermore, if you’re considering putting alcohol into your air-brake system, you should actually beasking yourself why you even have moisture in the air-brake system to begin with.
In recap, using alcohol in an air-brake system may be a temporary solution, but it does not address the real problem.
Because it’s the moisture in the air that freezes, and ice is what jams the internal workings of your brake valve, you need to combat air moisture in the first place.
Air dryers are still not, but should be, standard equipment. These need to be maintained again. One of the worst things that can happen with air dryers is progressive damage. By not performing simple checks and regular maintenance, a simple issue can turn expensive very quickly. For example, running your air dryer over a long period of time means that small amounts of carbon and oil contaminates are pushed through the compressor internal components. This includes the piston rings and coating inside of the cartridge. If your cartridge fails, you could find yourself with millions of tiny desiccant balls invading every inch of your trucks air brake system. This is the slow death, and costly repair, of your entire air brake system.
During annual winter maintenance, it’s recommended to replace the air dryer’s desiccant. These cartridges are filled with a water-absorbing material, but not all are created equal. Spend a little extra money for high-quality cartridges, as cheaper materials are prone to breakdown on the job. Its not about the cheapest up-front cost, its about total cost of ownership.
Cheap air dryers will also run hotter, which results in a breakdown of their desiccant which will then feed through the air system. This will end up clogging orifices within the ABS valve and destroy your brakes.
A glance at the air gauge can’t be your only test of air supply. If a line collapses or ice is lodged, air can still get in but will become trapped, which will inflate the pressure, giving you a false reading of your true air supplies.
During pre-trip check or vehicle inspection, it’s easy to perform this test while sitting in the driver’s seat. Pump the brake pedal until you hear the warning buzzer - then watch the needle climb up again. When you hear the air dryer blowing off, how far up does it go?
If it only climbs up to around 105 psi, you don’t have enough air. You should be able to do around seven full brake applications without hearing the buzzer warning, anything less than that is a serious problem.
This is story time. In the early eighties I was working a second job at night to feed my Alcohol Drag Bike addiction. I got a job at a major fleets trailer shop where we refurbished trailers that were very special for breweries and were equipped with electric floors. The things we build so American can get its beer.
A trailer was dropped due to the brakes being locked up on a return trip and the driver said he dragged it to the yard and left it in the drive lane. We had a trailer yard that was about 1/8 of a mile in length. This is January in Upstate NY around 2 am withtemperatures well below zero.
I was trying to retrieve a trailer using the yard horse, but the brakes were frozen solid. I crawled under with a torch and started heating up the air tanks and valves to thaw frozen brake valves. What I didn’t know was that the air tank was full of alcohol. Well, the tank exploded with such force it blew a hole through the floor of the trailer and bent floor bracing.
As for me I lost the hearing in both ears for a few weeks and felt that I cheated death. My friends that knew me back then say I never recovered, despite what the doctors said. If that tank blows the other way towards me, I wouldn’t be here right now. The lesson I learned is that alcohol belongs in beverages and Drag bikes should run on ten percent nitro. I quickly converted mine. This is a terrible practice, and if you’re doing it please stop, I was lucky.
At the end of the day, you should go through a checklist of things to maintain. This includes bleeding the air from your air-brake system, monitoring for water in the system, checking the automatic drain valves, and everything else mentioned. Be safe out there!
Before getting into Heavy trucks in 2001, I worked as an Automotive Mechanic for Cadillac during the day Spent a few years as a trailer mechanic and a Factory Trained Suzuki / Kawasaki Motorcycle Mechanic at night and weekends unless that weekend involved Drag racing my Motorcycle.
Before joining Diesel Laptops, I spent eighteen years working my way up in a Navistar Dealership. Nine of those years were working in back counterparts looking up and suppling diesel technicians with parts to repair trucks. I moved into service as the dealer Warranty writer and filer, Next was Shop Foremen then Service Manager.