I confirmed that 1998 V70 has 3 ABS circuits:
1 circuit for left front caliper
1 circuit for right front caliper
1 circuit for both rear calipers

The master cylinder primary section serves the front calipers and the secondary section serves the rear calipers. Fluid loss in either front brake circuit would leave only the rear brakes functioning.

Starting in 1999, V70s have 4 ABS brake circuits with the master cylinder sections connected in dual diagonal configuration. A fluid loss in one brake circuit would leave the other brake circuit (one front caliper and one rear caliper) still functioning.

Initially I thought the lack of brake force on my rear calipers was not related to the calipers because it seemed unlikely that both calipers would seize at the same time. Connecting a brake line pressure gauge in place of the left rear caliper bleeder indicated that max brake line pressure was OK - in the 1000 to 1500 psi range with the ABS fuses removed.

A closer look at the rear calipers showed that the outboard pad on both calipers was rusted solid to the caliper body.

After replacing both calipers with rebuilt units made by Centric, the rear brakes are now working fine.

Mike