Power steering problem 2000 Isuzu NPR

First, there obviously is a leak that caused the fluid to be low. That has to be located and repaired. When it was low, the pump sucked in air which is circulating in the fluid. Since air can be compressed, it continues circulating but it expands when you stop the engine. That will push the fluid out of the reservoir.

That may continue to occur after the repair. Each time you stop the engine, the microscopic air bubbles in the reservoir will float up and out of the fluid. Don't fill the fluid to its maximum level. Just keep it high enough to prevent the pump from drawing air. Eventually all of the air will work its way out and you can fill the reservoir.