Abstract The research described herein was carried out by the mechanical engineering staff at the mechanical laboratory of Miguel Hernández University (UMH) in Elche, Spain. This study compares the braking of a vehicle on two different Ministry of Transport (MOT) brake testers. We have analyzed mathematical formulas about braking on MOT brake testers, and we indicate which factors could have a theoretical influence in braking on a rolling road. If the tire pressure is varied, false results might be obtained from the brake tester, meaning that if tires are inflated at a low pressure, the vehicle will not pass the MOT brake test even if the brakes are in good condition. Conversely, if the brakes are in bad condition but the tire pressure is higher than that recommended by the manufacturer, a false passing rating might be produced. This article attempts to show that the MOT brake testing equipment is often wrong and inexact, and it provides data and graphs to prove that tire pressure can be a determining factor when assessing the condition of brakes. We also study the influence of the wheel base between rollers in brake and slip data measurements. A car is placed on a bank of rollers with the handbrake released; the rollers then move at a constant speed of 5 km/h, and the brake pedal is depressed until 100 % slippage is obtained. A velocity of 5 km/h is sufficient to obtain 100 % slippage. The pressure in the hydraulic brake circuit on the right front wheel of the vehicle is also measured with a hydraulic sensor in the hydraulic pipe of the right front wheel. We can obtain the slippage by measuring the angular velocity of the wheel and the roller. The angular velocity is determined by two encoders, the first fitted on the brake roller tester and the second on the wheel of the vehicle. At UMH we have one MOT brake roller tester with a 450 mm distance between each roller where we have measured brake and slip. We also compare the results with those from another MOT station at which the roller tester has only 410 mm between the rollers and analyze the difference. This article demonstrates that the MOT brake and slide data results depend on the distance between rollers, the tire type, and the pneumatic pressure. Data and graphs are provided to substantiate the findings.