My burnham series 2 (model B)gas boiler won't build pressure. Installed a new thermostat and now the pressure won't build up to circulate the water. Any ideas?

Is this steam or hot water?

Hot water

Hot water Boiler pressure is caused first by the water pressure in the system then as the water heats by expansion. That is why your system has an expansion tank.

Check the systems water pressure on the indicator and add water by the auto fill valve if necessary.
Single story 12 psi is a normal setting.