According to EU Regulation 2015/68 on the approval of agricultural vehicles, high-speed trailers of categories R3 and R4 must meet the brake force distribution requirements between the axles. Five metaheuristic algorithms (MIDACO, Cuckoo Search, Firefly Algorithm, Simulated Annealing, and Harmony Search) implemented in open-source MATLAB code were used to optimize the linear brake force distribution found in trailers with braking force regulators. The optimization results obtained for a two-axle and a three-axle trailer with two variants of tandem axles (bogie and two leaf springs) are very close to the results obtained by the Quasi-Monte Carlo method. Although metaheuristic methods do not always guarantee that an exact optimum solution will be found, they can be successfully applied to the selection of brake force distribution in the initial stages of brake system design. They have the advantage of flexibility, simplicity, less mathematical complexity, and avoidance of local optima. The brake force distribution optimization results can be used to establish the parameters of the wheel braking mechanisms of agricultural trailer axles.