The pilot advised Sydney Flight Service, that on departure from Parkes, his aircraft had experienced a hydraulic failure, which resulted in landing gear retraction problems. The pilot then requested a diversion to Sydney instead of his intended destination of Bathurst, advising that the landing gear would be lowered using the alternate system. The aircraft subsequently made a normal approach into Sydney for an uneventful landing. Investigation by company maintenance personnel could not fault the landing gear system, and put the problem down to air in the system. This aircraft was fitted with antiskid which is known to cause problems with hydraulic fluid loss. The operator was informed of other operator experience in this regard. Safety Action Statement As a result of this occurrence, the Bureau of Air Safety Investigation is currently analysing a possible safety deficiency. The deficiency identified involves aircraft fitted with antiskid braking systems. This braking option suffers from static internal leakage, which may result in the loss of hydraulic system contents and subsequent loss of all hydraulic services. Any recommendation issued as a result of this analysis will be published in the Bureau's Quarterly Safety Deficiency Report.