To establish the master cylinder is involved you have to release a bleeder at least. If the rear wheels then turn substantially easier then you know it involves the master cylinder.

So far it seems you have really done nothing. Actually it sounds more like a pair of old unused calipers that have some cylinder drag. Yet until you release the pressure in the brake system if any there is no way to absolutly know.

For all practical purposes the calipers keep the pads very close to the rotors. They do not back off to actually allow a really visable space even with all new components. There are no return springs. The turning rotor not being absolutly true wobbles a tiny amount loosening them to some extent. Sometimes the friction drag on the caliper cylinder sealing rubber may contribute a little as well. So the only sane test is rotating the wheels. Very slight drag may even still be possible on a good system. But not much if any.

It is unwise to leave brake systems sitting a long time between uses in most climates. Also not replacing the brake fluid on ocassion is costly. The water contamination in the fluid over time does corrosion damage. Especially to the pistons in the calipers. Then the rust inhibits free movement by hanging up on the rubber caliper seals. The pads must also move along their guides well. The guides can develop rust as well. They should have a periodic touch of lubricant. Again a maintenance thing.

I fully realise the majority of cars out there have very old brake fluid that really should have been changed out several times at least but never where. The majority of cars do not recieve adaquate maintenance.

All manufactures want brake fluid flushed and replaced periodically. Since you can still turn the wheels by hand it is unlikley the master cylinder is involved. Old tired calipers are a very common thing. Loosen a bleeder to separate as suggested.