2.6 petrol manual saloon
I noticed while road testing daughter's 2.6 following misfire repair that it needs more pedal pressure to stop than the other 5 Omegas. It has just passed MOT, I could bring in the ABS on a dry road with a good heave, but it requires more force than it should. Vacuum line looks OK. What might cause this?
The only way it could be a friction problem is if you tried new brake pads to fix the misfire. I doubt that is the case
You've moved/dislodged/damaged a vacuum pipe or fitting as part of fixing the misfire, and the servo isn't as effective as it is designed to be. That does require a pretty bad leak, but is the first, second and third place to check. If you don't find it that way, have a decent lunch and go over the same stuff again.
This is rule one for diagnosing a new fault after fixing a previous one: go over(or better yet, get someone else to go over) everything that you might have been near as part of the job.