Trying to install a new t-p vavle but it still keeps leaking.what can I do?

T&P on what?
Water heater? Furnace?

Leaking where?

You want help you have to give some information.

What is house pressure? Could be the PRV valve.

If you are installing a T&P relief valve on a boiler, you are making a BIG mistake. Boiler relief valves are pressure only & normally 30# max.

If it is on a water heater & you have an expansion tank, the tank might be bad.