You dont even know if your gauge is accurate, Add water to a COLD boiler
of 70f while bleeding air out of the top floor while boiler is running
till water comes out the radiator. Your issue is not enough Altitude or
Pressure. Dont mess with anything else. Then see how it works and
monitor pressure cold and hot, it may only take a Lb or 2 of water to
get heat to the top. After a few hours bleed radiators again. It may
take a few tries over a few days.