rear brake master cylinder

Hi Folks
had an issue with my rear brake not working so started investigating and obviously the problems with a 40 year old bike started appearing i.e the master cylinder screws completely seized i have drilled these out but my question is can i test my master cylinder is doing what it should without putting it back in, it would be nice to test it without bolting it all back in as i would like to check a component at a time to identify the core issue
as always many thanks for any help
Paulb

It needs to be sealed and pressurized. In this case, also physically stable to manually pressurize - there is no pump like in most hydraulic systems. In a way it's like a simple hydraulic jack. So, after assembling and attaching the break pedal, I'd put it in a vise. The pressure line would need to be blocked either at the master cylinder, or end of the hose. Any hydraulic hose supplier will have such plugs. (I used to carry a bunch of different sizes and combinations for any scenario when working with hydraulics). Then bleed it as normally and see if it holds pressure. So, the two key points are to have it solid in a vise and plug the fluid line.