Hydrostatic drive not working

Hi I have an Agria Diesel mower with hydrostatic drive and hrdraulic lift for the cutter. I have suddenly lost all drive but the cutter lifts and lowers ok. I have disconnected the hydraulic pipes which come out of the top of the hydrostatic unit and there is oil flow from each depending upon reverse or forward as I would expect although there does not seem to be a vast amount of pressure and there also appears to be rather alot of air as well. There does not appear to be a way of bleeding the air out of the unit other than disconnection of these pipes. Please can you suggest if this air could be the problem. I believe that the oil had just been changed but I found it was not filled with enough hydraulic oil. Also should there be another way to bleed the system. The two output pipes do go into a valve unit which I have checked for being clogged etc and cleaned but I doubt that the pressure is high enough to force this valve open. Thanks for your help.