For the brakes calipers I have, originally the mustang hydroboost units used a 15/16 master. There is a difference in the size of the Hydro unit the mustang used, but it's not a great % if I loosely recall the specs. The mustang brake system seems to be designed around lower fluid volumes (smaller surface areas of MC and caliper pistons) and higher line pressures. So I'm hopeful that I'm not really exceeding anything that calipers cannot handle already. Yes, I do not want to blow seals or have an unsafe over pressure condition either. I'm not even 100% sure the MC I have is 1.25" but the astrovan unit for sure was. The switch fom 1.25" to a 1" is about a 57% increase in possible line pressure. That said, before I had the hydro I had a vacuum assist with 15/16, 1-1/32" and 1-1/16" Master cylinders and I would call those borderline unsafe which is why I went to the hassel of hydroboosting it.

In general driving maximum line pressure is never reached. In emergency braking When teh wheels stop spinning the foot comes off the brakes naturally as that happens so not likely to exceed any limits and certainly not sustain them. I feel safer operating with that limit than not having enough in a more likely scenario where I lose engine power for any number of reasons, my lousy home brew electronics being one very likey reason.