Eventually this process will stop when the pressure throughout equalizes. . Sucrose is loaded into sieve tubes by active transport. Water potential decreases and water enters sieve tubes by osmosis, turgor pressure. Turgor pressure pushes solutes to sink, were it is needed while water moving in and out of the sieve tubes. Removal of sucrose at sink increases water potential causing water to move out of the sieve tube at the sink.
Solutes move to sink cells and water goes back to xylem.
In this model, there is both passive and active transport. Osmosis, water movement into and pushing solutes down the tube. Loading of sieve tubes from the cell walls requires energy which is derived indirectly by the proton gradient. Diffusion of proton back into the sieve tube, through ATP-ase, is coupled to a carrier and powers the transport of sugar into the sieve tube. It may vary in different species.