Volvo suggests flushing brake fluid every two years or sooner if you
have severe brake service. I used Blue ATE DOT4 from FCP and a quart
does it with plenty left over. It's not to hard and you bail out the resevoir
with a turkey baster to start off.

That idea of just cracking the brake lines at the M/C is not so good.
You need to get some short lengths of brake lines with fittings
and then use plastic tubing back to the resevoir. Then when you pump the M/C
you might find that air is being introduced at one or more of the four
lines. I had three tries by the M/C rebuider in NYC which is a heads up
to buy an OEM M/C and avoid so-called rebuits like the plague.
Bench bleeding is not so hard if the fittings aren't stuck.
Soak those fittings with mystery oil or PBspray for several days before
starting dissembly.

There's another problem with old master cylinders that the deep part of the
bore tends to corrode and suddenly if the piston goes too far, then the seals can get damaged. I dont think honing the bore is enough, if there is deep pitting, they probably need to resleeve the bore, which might be tough
with a 4-port M/C - get new master cyliders if needed.

Bill