hot water heat pressure on 2# should be 12#?

Repaired a pipe on my H/Water heating system.
Boiler type Weil-MclaLain. Worked well for 30 years. Now have air in lines. Draine about 2 quarts with air hising from all the lines but when boil is heated, temp up to 180 degreens, the pressure doesn't go up more than 2#. Suppose to be a 12#. Is is possible that the air still trapped in the line could be keeping the pressue reducing valve from allowing water to enter? Could I add water manually by lifting the handle on it until it reaches 12#. or should I drain more air out?