I haven't seen this on the forum, and it is weird. Pump pressure/suction is fine from skimmer (about 20psi) until we put the hose in to vac. The hose fills quickly with water, suction on the hose is good, then the pump pressure drops to zero. Have run water, small items, etc. through hose to see if it's clogged - all clear. Any ideas why the pressure drops to nothing when we try to vacuum? It only happens then - otherwise, pump pressure is good.

Inground concrete pool - 30k gallons.