2008 Mercedes Benz ML350 Power steering

After 8 hours of rest and for approximately 15 minutes the power steering works normally when I park the vehicle, however after that is almost impossible to operate for parking, the wheel gets extremely hard either way, front or back. If I'm driving the wheel presents no problems whatsoever in turning, but the problem starts once I stop and try to park. I changed the reservoir a year ago, because it had the same problem, once changed the problem disappeared for six months, then it returned. I've found that by stepping on the break and accelerating at the same time, the power steering returns to normal, but just during that operation. What's wrong and how can it be repaired once and for all, thanks.