The Office of Defects Investigation (ODI) opened this Preliminary Evaluation (PE21018) based on eleven Vehicle Owner Questionnaires (VOQ). These VOQs included seven wheel-end fires on various Kenworth and Peterbilt trucks.Several of the VOQs were submitted by a maintenance facility stating that the fires were due to power spring fractures within brake chambers manufactured by Haldex Commercial Vehicle Systems (Haldex). According to the reports, the fractured spring punctured the brake chamber diaphragm, resulting in a partial loss of compressed air to the park brakes, causing the brakes to drag and overheat. ODI sent an information request letter to Haldex on August 3, 2021. On October 17, 2021, Haldex submitted a complete response. In this investigation, ODI analyzed thousands of warranty claims as well as fleet reports of brake chamber replacements. ODI gathered information from multiple vehicle manufacturers and reviewed legal testimony alleging defects and laboratory reports that raised concerns about spring failures initiated by corrosion. ODI found that spring failures were often detectable as soon as they occurred. In the subject incidents, after detecting the failures, some operators continued driving to a maintenance facility rather than stopping and waiting for a mobile technician to address the issue, increasing the risk of fire. While over twenty entities purchased the subject component from Haldex, the VOQs that ODI received involved only PACCAR vehicles. Haldex completed replacement campaigns for several fleets operating PACCAR vehicles, reducing the risk of further failures. Brake chambers have a limited service life. Industry guidance generally recommends replacement every two to four years depending on operating conditions, with some manufacturers and fleets recommending even shorter intervals. Vehicles operating in salt-belt states—where the majority of VOQs originated—require more frequent replacement than vehicles in less corrosive environments.