Sounds like you have air trapped in the system, which is getting compressed then pushing the fluid back into the reservoir. Or a seep on a connection that is allowing air to be drawn in.
Is the fluid new & clean, do you have to top up when you bleed.
Carry out the initial bleed with the front off the ground & without the engine running, turn to full left lock if level drops there is air in system- top up fluid, turn smoothly & slowly to full right lock - recheck & top up. turn slowly & smoothly left to right several times then check fluid level which should not be dropping by this time.
Ensure fluid has no air bubbles or foam in it. recheck all connections - ensure rack is not pumping fluid into gaiters. start engine & turn slowly from lock to lock - do not hold steering on full lock.
Re-establish fluid level , be careful not to overfill because it expands when hot.