A frozen brake line can take a rig out of service for hours, maybe days depending on a department’s maintenance capabilities. Frozen lines and other problems are preventable, but first, we need to know more about air brake requirements.
According to NFPA 1901: Standard for Automotive Fire Apparatus (Section 12.3.1), air-actuated braking systems must include the following:
- An automatic moisture ejector
- An air dryer
- A pressure protection valve to prevent all air-operated accessories from drawing air from the air brake system when the air systems pressure drops below 80 psi
- A quick build-up section in the air reservoir system arranged so that if the apparatus has a completely discharged air system, it can move within 60 seconds of startup
The standard explains: “The quick buildup system shall provide enough air pressure so that the apparatus has no brake drag and is able to stop under the intended operating conditions following the 60-second buildup time. For a chassis that cannot be equipped with a quick buildup air brake system, an onboard automatic electric compressor or a fire station compressed air shoreline hookup shall be permitted in order to maintain full operating air pressure while the vehicle is not running.”
All air brake systems use air pressure to apply the brakes when you step on the pedal. The air is pressurized by the air compressor on the truck’s engine and then stored in a series of pressure tanks on the truck. An air governor on the compressor regulates the pressure. Most, if not all systems, work on 120 psi of pressure.
Most systems have an air dryer, which removes moisture from the air to inhibit corrosion and keep the system from freezing in the winter. The air dryer has a cartridge that should be maintained according to the manufacturer’s recommendations.
The pressurized air passes through the air dryer and into the tanks. From the tanks, it goes to two different systems.
air brakes: two-component system
Air brakes for heavy trucks, including fire apparatus, are a two-component system. The first component releases the parking brake and holds some reserve air to stop you if there is a leak. The parking brake on most trucks has springs in the air chambers on the rear axle or axles that apply the parking brakes.
Pushing the parking brake knob in supplies air to those air chambers and pushes against the springs to release the parking brakes. If the pressure in the air system pulls below 60 psi, the parking brake knob will pop out and set the parking brakes. This is a safety feature so the truck will stop, because if the pressure falls too low, the service brakes will not work.
The other system component stops the vehicle when you push on the brake pedal. When you push on the brake pedal it goes through the pedal valve, which modulates the airflow to the brake chambers. The brake chambers in turn push on the slack adjusters, which keep the brakes in adjustment.
There are different types of slack adjusters: manual and automatic. Manual slack adjusters are just as they sound. You must manually adjust them to keep the brakes in adjustment. Many truck mechanics say that this is the preferred style.
The automatic slack adjusters automatically keep the brakes in adjustment. These work well if you keep them greased. The problem is that most people don’t keep and they seize up. If you have automatics, the best grease to use is white lithium grease.
The slack adjusters turn a shaft that turns the S-cams, which push the brake shoes out into the drums. The S-cam shafts have bushings that should be checked every time you change the shoes. They also require grease.
air brakes: Replacement parts
There are such things as air disc brakes, but they are uncommon. Most brake shoes have an indicator built into the end of the pads that tell when they need to be replaced.
When you replace the brake shoes get hardware kits that include the springs, pins and bushings for the brakes, and replace the drums. This is a good idea because the drums will have a wear ridge and could also have heat cracks.
Here are some steps to take before heading out on the road:
- Make sure the minimum operating pressure for a vehicle air-brake system is no less than 100 psi for a truck.
- Check that it takes no longer than 2 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.
Winter weather practices
Watch for water in the air-brake system, a byproduct of the condensed air. Air-brake lines don’t like water, especially in colder climates where ice can block air from reaching the brake mechanism and cause the wheel to lock.
To prevent this problem, many modern systems have automatic drain valves installed in each air tank. Check these valves according to the manufacturer’s recommendations on a regular basis to ensure they are functioning properly, especially during extreme cold conditions.
The air dryer is worth its weight in gold. In colder climates, water can freeze in the valves and cause serious problems with the operation of your air-brake system.
Air dryer maintenance is imperative. If you haven’t done so before winter arrived, now is the perfect time to have your air dryer filter serviced according to the manufacturer’s recommendations.
I posted a question on the Emergency Vehicle Technician TechTalk forum asking if air brake system anti-freeze was necessary or if the air dryer was sufficient. I received several replies from EVTs and they were all consistent in their message: Take good care of the air dryer and the air dryer will take care of you.
Driving tips
Stopping distances on slippery surfaces are from three to 12 times loner than on dry roads. Under ideal conditions, the safe following distance rule is 1 second for every 10 feet of vehicle length, so adjust your distance according to the existing conditions.
A recommended way to stop safely without Antilock Brake System brakes is to use a rapid light pumping of the brakes. By pumping the brakes, you can maintain steering control. Apply the brakes for an instant and release them. Repeat this action until you come to a complete stop.
ABS automatically pumps the brakes if the vehicle’s wheels begin to lock up. This allows the vehicle operator to maintain effective steering control and reduces the risk of skidding. The brake pedal will pulsate, but this is normal.
With air brakes, be careful to avoid reducing the air pressure to a low level. The air pressure required to lock wheels on ice can be as little as 10 psi, so a great deal of pumping can be done with a gentle touch on the brake pedal.
Winter driving with air brakes can best be summarized as follows: Keep the system dry and keep the pressure up.
This article, originally published in 2014, has been updated.
