Finding the root cause of: an intermittent air leak

In this series of troubleshooting postmortems, technicians will explain how they fixed a common issue on commercial vehicles.

The challenge

Arriving at the TravelCenters of America facility in De Forest, Wisconsin, a Freightliner ServicePoint location, a customer reported an intermittent air leak when applying the foot brake pedal on a Freightliner Cascadia. In the shop, a check of the tractor did not find any leaks on the emergency or service sides of the air brake system.

“I called the customer to discuss the issue,” explained Scott Davidson, ASE Master Technician at the facility. “They said the leak happens when driving and the brake pedal is pressed lightly. The customer also reported that the trailer brakes felt like they were dragging, and that the air pressure gauge was reading over 110 PSI.”

The diagnosis

Offering to ride along to see the problem firsthand, Davidson asked the customer to hook up their trailer, after which he checked the emergency and service sides of the system again to verify that there were still no leaks. Before heading out on the road, the problem did not occur during a couple laps around the facility’s parking lot, either.

On the approach to the interstate ramp, the air leak became obvious, presenting itself from inside the dash. When the customer released the service brake, the air leak stopped.

“We traveled one exit and circled back to the shop,” Davidson said. “Each time the service brakes were applied, I listened and found that the air leak was coming from the center section of the dash.

“I asked the customer to pull over and set the tractor emergency brakes only and when I got out I noticed that the brake lights were on, so I verified that the service brake pedal and hand brake were not applied,” Davidson added. “I then removed the blue trailer service line, got a small burst of air, and then checked for a back feed from the service gladhand. After that, the brake lights had gone out, so I reconnected the line and we continued back to the shop with the leak repeating itself with each service brake application.”

The solution

Back in the shop, after removing the center section of the dash, Davidson quickly located the air leak coming from the exhaust port of a valve connected to the fifth wheel release button.

“Having been to many air brake classes over the years, I remembered being told that if air is leaking from the exhaust port to check the delivery lines for a back feed,” Davidson explained. “In this situation, the air was coming back up the delivery line so I traced the line out of the cab, then moved back to check the tractor protection valve due to the air trapped in the trailer service line during the ride along.

“What I found was that there were two double check valves in the system,” Davidson continued. “I located some fittings and plumbed in a pressure gauge to each valve and tested them, which revealed that one of the valves had a check ball that was not seating completely in one direction.”

After calling the local Freightliner dealer and confirming they had a replacement valve in their inventory, Davidson related the repair need to the customer for approval. With the new part installed, another test in the shop and a test drive to verify that there were no leaks, the satisfied customer was back on the road.

The Technician