Bought new master cylinder. When bench bleeding front part bleeds fine, rear part will only bleed by gravity no fluid will come out when depressing plunger. Is this a bad m/c?

 

Maybe but even the bad ones will make pressure, they may leak, but they'll make pressure.

You probably have an air lock in the cylinder. It may be a poor feed hole. You can take it apart in about 30 seconds to make sure the pistons were properly assembled and do a quick clean out with some brake cleaner.

 

Should be taken apart anyways..... I take all of them apart.... most of them are already rusty on the inside from sitting on the shelf too long.