toilet siphon has pressure beam of water
Under normal circumstances, the water source should react on full/non-full condition. In your case, I'd try to disassemble the unit to make sure there is no rust/salt/etc preventing it to stop the water beam.

Then, about the beam itself. It depends on a water pressure and effective gate size, however it affects the fulfillment speed only. Once floating bobber is high enough, it pushes a lever that intends to shut the water gate.