[Sirrotica]

Defective new master cylinder, or a used one, that has the rear most seal able to suck air where the pushrod joins the spool valve. You may or may not see brake fluid around this area. The bore of the master cylinder can admit air if it's pitted as the spool moves over the pitted portion of the bore.

Air can come into the system if the rear most seal on the spool is defective. Sometimes they will leak fluid, but not always. Depends on how the seals are damaged.

I've seen this problem over the years, but it's not real common.

Not saying this is your problem, but it can be a possibility.