No brake pressure on front circuit after master cylinder replacement

[LightlyToasted]

Replaced MC with OE (TRW). Brakes feel and function fine now.

Was difficult to bleed the system. I bench bled the MC before installing it, then:

1. pressure bled
2. pressure bled + cycled DSC
3. manual 2-person bled
4. manual 2-person bled + cycled DSC
5. manual 2-person bled with pressure bottle hooked up
6. repeated above steps

Front circuit was really hard to bleed. Brakes were feeling awful until step 5. For some reason that blasted out a huge amount of air that no step before it did. This leads me to suspect that even though I bench bled the MC the act of installing it reintroduced air into the MC. Scotty Kilmer on YT advocates "burping" the MC by applying brake pressure and briefly cracking open the line at the MC which I guess provides any air the shortest route to escape. I suspect any air at the highest point in the system may tend to remain there despite bleeding efforts.

So first new MC had clearly failed, 2nd new MC was probably OK but the system wasn't bled properly, and 3rd new OE MC worked when system was thoroughly bled. I disassembled the 2nd new MC and couldn't find anything wrong with the front circuit seal. Oh well.

What a nightmare.