2017 merc sprinter air in brakes

Try a pressure bleeder check autodata for know n fixes other than that cicerly merc ring them in Blackburn

Seen a lot of these master cylinders changed though poor diagnoses where there’s nothing wrong with them .
Pressing the brake pedal under a constant pressure will see the pedal travel virtually to the floor,but not touch the floor ,the movement of the brake pedal is due to the design of the system .
When you say brakes are poor ,are u referring to this pedal travel or the overall effectiveness of the brakes during driving and stopping ?

Do you know if it’s got brake assist ?

Also it’s not lost any brake fluid and everything seems dry

If have ABS  doubt it

No load valve

You may not have any fluid loss but if there is any air trapped on the abs valve block you will not clear it without opening the valves.

If you don't have the diagnostic gear to do this, cheat.
Test drive somewhere where you can make the abs kick in and open these valves, you don't need to go so fast at all as long as you can lock wheels up. Then try bleeding as normal again.

I've done this a few times with two wheels on a grass verge and then turn round and do other side.