To ensure a system drains properly after a hydro test, what must you always do?

To ensure a system drains properly after a hydro test, opening the high point vent is crucial because it allows air to escape from the system as water drains out. When the system is filled with water for a hydrostatic test, any trapped air, especially at higher points in the piping or tank, can create a vacuum or impede the flow of water exiting the system. By venting the high points, you ensure that the entire system can drain freely and completely, preventing any leftover water that might cause issues such as corrosion or contamination.

The other options, while relevant in different contexts of system maintenance and operation, do not directly facilitate proper drainage after a hydro test. Closing the low point drain would prevent drainage, re-checking all connections is typically a pre-operational step, and reducing system pressure may not contribute to ensuring complete drainage if air pockets remain trapped.