According to Delegated Regulation (EU) 2015/68, the braking performance of agricultural vehicles is determined through road tests. Trailer braking performance is evaluated based on either the braking of the entire combination with coupling force measurement or the trailer's individual braking. In the first case, the combination's braking performance is influenced by driver behaviour and transient processes within the tractor and trailer braking systems (e.g., pressure values and pneumatic lag). The same applies when trailer brakes are engaged using a handbrake valve. To reduce the impact of random factors, an innovative device has been developed that simulates the tractor's pneumatic control system, allowing the trailer to brake independently of the tractor. Along with other measuring devices, this simulator forms the measuring and diagnostic system created by the authors for roadside vehicle testing. The simulator enhances the accuracy and repeatability of measurements. Sample results from road tests of a prototype trailer and tractor are included.