A brake system and method includes a test mode for determining a capacity of the brake system provided on a machine having a hydrostatic drive. A hydrostatic drive target differential pressure range is determined to simulate a test torque. A machine ground speed limit, hydrostatic drive actual differential pressure, and actual ground speed of the machine are determined. An output pressure of the hydrostatic pump is modulated to drive the hydrostatic drive actual differential pressure toward the hydrostatic drive target differential pressure range. A first test fault condition is determined when the actual ground speed exceeds the machine ground speed limit. A second test fault condition is determined when the hydrostatic drive actual differential pressure is outside the hydrostatic drive target differential pressure range.