To investigate generic power optimization problems of hydrostatic transmissions, an Earthmoving Vehicle Power-train Simulator (EVPS) was built at the University of Illinois. A model reference load controller is designed to simulate different load dynamics for this "hardware-in-the-loop" test-bed. A two-stage pressure relief valve serves as an actuator to change the load loop pressure, thus to control the resistance on a hydraulic motor. The actuator dynamics result in a zero in such a load control problem, which limits the performance. The zero can be avoided by using an online simulation method. A Least Square Estimator as an adaptation updates plant parameters in real time to improve the online simulation accuracy. Simulation and experimental results of the load controller for a traction load reference model are presented. Relevant models of the hydraulic drive, the pressure valve and the traction load are also included.