During the operation of vehicles, the properties of liquids and consumables deteriorate, which may affect the level of safety of technical systems and road traffic safety. The results of testing brake fluid aging as a function of boiling point and percentage of water content were presented on a selected group of passenger cars. The tested vehicles were varied in terms of age and kilometer mileage. The methodology for testing the quality of brake fluid based on glycol selected passenger cars remaining in service has been presented. In this study was shown a significant decrease in the brake fluids quality during the two years of operation in vehicles. The determination of boiling point was more efficient than the determination of water content. Despite the lack of full vehicle operational information, the obtained test results allow for exciting observations.