arcturus;
Force applied by hydraulics must always be considered as a trade-off between force necessary to do the work, and actuation distance (the product/pressure being a constant)...so given the force required to apply the brakes as a constant and coming from YOU, I would say that increasing the ID of the Wheel Cylinders can be done modestly (ie 7/8" to 1", which Volvo also did on the 122), and this will decrease the effort necessary to depress the pedal at the cost of needing to depress it just a bit more...and that little bit of pedal difference you can probably get used to...
HOWEVER...when changing the ID of the rear Brake Cyls without changing the front cyls, the relative front to rear brake force "Balance" has also been changed...if Volvo did this from the factory, you can figure it was a tried and safe combination, but if this combination was not available from the factory, careful roadtests would be highly advised (if you can't get to a brake test stand, use my maximum-braking-on-a-slightly-sanded-otherwise-even-surface-test)...you wouldn't want to make these changes, and in your first maximum braking, have the vehicle swapping ends uncontrollably...
Reading "octogenarian", was a bit of a surprise to me...most of your age-peers have long ago "gone soft", and now drive cars where they don't need to work so hard, let alone climb under the dashboard to repair them, or re-engineer the brakes! You've got a few years on me, but knowing this now, I'm quite impressed by your interest and exploits in things vintage Volvo, and I am most happy to have helped you in the past, and will be honored to continue to try to help going forward!
Finally, of course, the brake balance is not changed by keeping the wheel cyl, and balance the way they are, and simply plumbing in an assist booster (and I wouldn't hold "going soft" against you if you were to do that!).
Cheers
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