Ive had this problem with the s70 ft and the s40xm. It was the packing in the steer sylinder both times.It the steer unit was unloaded by jacking up the rear of truck you couldnt make it fail the more weight or rear of truck the more it failed.I found by taking 1 hose off at full cut.The low side of that cut and see if it bypasses.Messy way to see but it works.