During gear retraction, there was a loss of hydraulic pressure after the nose gear came up and was locked, but while the main gear was in-transient. The pressure raised to 300 psi when the auxiliary hydraulic pump was turned on, but dropped rapidly back to zero. The landing gear handle was placed in the down position and all gear indicators then showed in-transient indications. Attempts were made to lower the gear with emergency air, then with a hand pump, but both methods failed. After dumping fuel, an approach was made for landing. During touchdown, the nose gear locked to the down position while the main gear partially retracted into the wheel wells. The nose gear held the nose off of the runway while the tail dragged. An examination revealed that the hydraulic tank was empty. However, no evidence was found of a ruptured line or hydraulic spill. After the plane was raised, the solenoid operated air valve would release air into the system, but the bogies failed to rotate to a landing position.