Motorplex Diagnoses Complex Brake System Issue on Commercial Truck

Written by the Motorplex Team
With Fleet Service Shops located in Auburn and Spokane, Washington

When a commercial truck loses braking power, it’s a serious problem. A brake warning light, unusual noises, or changes in pedal performance can point to a serious mechanical and safety issue that needs immediate attention.

Recently, a customer contacted Motorplex after their truck was brought in from Seattle with a brake system concern. The driver reported that the brake booster system warning appeared on the dashboard, followed by a loud noise and a loss of braking power.

We got the truck in the shop and got to work.

Finding the Root Cause Behind a Brake Warning

During the initial road test, the technicians confirmed that the brakes were operating normally. However, they noticed the brake booster warning light and alarm appeared when the vehicle required additional vacuum assistance.

Things weren’t exactly straightforward, so the Motorplex team began a detailed diagnostic process. We inspected the brake system for common warning signs, including uneven brake wear, fluid leaks, vacuum system problems, and issues with the electrical system that could impact brake components.

The brake fluid reservoir was above the minimum level, and the brake components showed no visible signs of failure. The next step was identifying why the auxiliary vacuum system was not operating correctly.

Advanced Diagnostics to Identify the Failed Component

The technicians tested the complete vacuum pump circuit, including wiring, relays, and fuses. Everything checked out correctly.

They then tested vacuum pressure throughout the system. The belt-driven vacuum pump was producing vacuum, but the auxiliary vacuum pump was not supplying vacuum to the brake booster.

The challenge was that the auxiliary pump was located behind the dashboard, requiring the Motorplex team to carefully remove interior panels, HVAC ducting, and dash components to access it.

Once they reached the pump, the failure became clear. The auxiliary vacuum pump was extremely hot and was drawing power but failing to operate correctly. Additional testing confirmed that the pump was internally failing and needed to be replaced.

Sometimes, When it Rains It Pours

After installing the new auxiliary vacuum pump, Motorplex confirmed that the system was functioning properly. The team also inspected other areas of the truck and addressed additional concerns found during the repair process.

During the inspection, technicians identified:

• • Damaged driver-side splash guard
  • A worn lift gate retaining plate
  • Inoperative marker lights caused by damaged wiring and failed bullet lights
  • A collapsed lower radiator hose requiring further attention

The lighting issue was traced to damaged wiring near the connector before the box. After repairing the harness and replacing failed lights, all marker lights were restored to proper operation. This truck was getting a complete workover, so it would be efficient, safe, and perform its best.