Home Articles Air Brakes – Park Brake Testing

Air Brakes – Park Brake Testing

The video below is how we check the spring brakes during a regular service. The plug that covers the hole at the rear of the brake chamber is a convenient way to do a visual inside the chamber checking for a broken spring or spring misalignment. The primary use for the access hole is to cage the spring when installing a new chamber or releasing the spring brakes manually when required.

I had one of these pots blow apart on me once and let me tell you there is a literally a ton of force behind this spring. It holds a lot of iron from rolling away so you can understand how powerful the spring brakes really are on an air brake system.

The maxi brakes, spring brakes or park brake…. whatever you want to call them is the most important part of the air brake system. It prevents the vehicle from rolling away when unattended…simple logic dictates that they have to be tested daily to keep them operating at 100% efficiency.

The driver is the first person who checks the spring brake. It’s part of the pre-trip inspection to do a “tug test” which is a simple procedure. With the spring brakes ON and the transmission in DRIVE the throttle gets a quick hit with the foot to see how the spring brakes hold the vehicle in place.

If there is no forward progress the spring brakes are good….simple. I’ve talked to a lot of drivers who say they do a tug test and the bus moves so they think there spring brakes are weak. Most of the time when I ask them how they did the tug test it’s usually the wrong method. When you load up the engine in drive and hold the RPM at 1200 to 1500 the vehicle will definitely move ahead.

The spring brakes are not strong enough to keep the vehicle stationary under that much of a torque load. It’s not healthy for the driveline and puts a strain on the transmission as well. The best way to double check the spring brakes is to physically inspect them for a broken spring by removing the rear plug at the brake chamber. Then you can look inside the chamber and do a visual check. If the spring inside the brake chamber is broken or out of alignment it must be replaced.

In our fleet we change them every 5 years as part of our PM program. Since we started doing this there have been no premature failures. When a spring brake fails it usually punctures the brake chamber diaphragm and causes an air leak and of course the bus is out of service. A simple check at the inspection hole every 5,000 km prevents mishaps down the road. It makes perfect sense and saves on extra service calls.

So what do you think? Is this how you do your spring brake inspections? Leave a comment below I’m interested on reading your experiences and feedback.