I have a case mx 135 with no steering power,was hooking on a trailer when it went dead,when foot brake is pressed the steering will work but under load,will do the same if a spool levers is pulled,the lift arms are fine,had a mechanic looking at it and we where sure it was the priority valve,we put a brand new one on costing 700 quid and there is no change,we are a bit lost,any ideas are very wellcome,has this happened too these tractors before.