Help 96 bonneville no heat under 20 degrees

Help , 96 bonneville with newer thermostate gets no heat under 20 but ok over that except sometimes at a light i get almost nothing . My water pump is leaking but i keep it topped of if that helps . Any advice would help , its real cold up here in buffalo ! thanks!

You need to replace the water pump, then get the air out of the cooling system. An air pocket is finding it's way to the heater core, and that is preventing you from getting heat. Coolant is denser at cold temps than at warmer temps, so since the heater core is one of the highest points in the cooling system, that's why the air ends up in the heater core when it's cold out, and essentially "vapor locks" and prevents coolant form circulating through the heater core.