I have just rebuilt my lift system on my 9N. I installed a new control valve, safty valve, new gaskets new o'ring on lift piston, new valves and valve guides in the chambers I did not replace the pump pistons because they looked to be in good shape. I can get it to lift with a draw bar and about 250lbs but when I hook up my box blade it will not lift except when I get off the tractor and lift up on the blade. It will hold the blade up for at least 4 days now with out leaking down. I used Multi G 134 oil. Does anyone have any suggestions?