RE: 5900 series and Futaba pcm
Any revision v1.04 early release servo may not function with a wye driven from a Futaba PCM RX it was discovered subsequent to release. Futaba PCM RX's are not able to provide adequate sinking current to drive multiples. Once Hitec realized this was an issue they updated a component in the servos to remedy this shortcoming.
Return all of your v1.04 servos to Hitec Service for the free update.