How do you know that it is a low pressure problem ? if it is more likely that the OH stat has tripped, your boiler can be installed open vent or sealed system, but the boiler wont have a pressure guage or filling assembly actually within the boiler pipe group, if it is indeed a sealed system then the filling loop will be external, usually near the HW cylinder but not always, post some pics of what you have and we will be able to advise you better
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