Bad Master Cylinder or Air?

Just wanted to give an update on this. I bought a vacuum bleeder and finally got around to bleeding the rear brakes today. I did both the passenger and driver side. I pulled the fluid from the front reservoir to about 1/2 way twice for each side. Had a good steady stream of fluid and didn't notice any air even when I first started. Pedal still felt the same. I tried the items AJ/FormS suggested for the hydraulic portion and the pedal never was able to get a hard pedal. The hoses up front are new (doesn't mean they are good, as we've all bought new items that ended up being junk..) When bleeding the front the pedal goes to the floor so I believe it's a rear brake issue. I did replace the rubber line to the rear end housing 2 years ago when I first got the brakes working as the old one was clogged. Found that issue since I couldn't get any fluid to the rear wheel cylinders. I'm 99.9% sure it's a master cylinder issue so I've order another one to see for sure and than will have the one on the car warranty if that is indeed the case. With the virus stuff going on it's hard to find parts at the moment. Hopefully the new master cylinder will be here this week and I can get it cleaned up and painted, bench bleed and installed. Will report back later.