Master cylinder problems 99 Suburban, HydroBoost and EVO.

[Pinger]

Bit more of an update. The sinking pedal seems to have gone. Between the previous efforts to expel air from the ABS unit I replaced the rear flexible hose (the fronts were done a few months ago).
I now have a pedal I can trust - even if it does feel a bit spongy still. Maybe the old flexible was part of the problem, maybe another bout of bleeding made the difference. Whatever it was I now have a pedal I'm no longer petrified of just disappearing to the floor - which is where I was when I started this thread.
If there's anything else I do that brings improvements I'll post further but meantimes, thanks to all who contributed. Your help is greatly appreciated.

 

[smokymtn65]

How long ago? How much do you know about the circumstances of the migrations?
Although I was born in Aberdeen, my father's side of the family are from Loch Lomond (west coast) apparently. Believable as our surname is uncommon up here but commonplace (and rooted in) Loch Lomond. The move north was some 200 years ago.

Traced back to Robert the Bruce.

 

[Schurkey]

My maternal side goes back to Scotland. We've traced the Family Tree to Bruce the Robert. He wasn't nearly as famous.