The rigs are typically rotten in the brake department, just like everywhere else!
Once ya start in on the brake system, you must do the whole deal and follow through every plumbing path for now, let's consider that the master cylinder is functional and not rotted internally since you have pressure to at least one wheel cylinder.
So verify there is no blockage in the plumbing run down to the "tee" (splitter) mounted on the driver side lower inside fender. Then verify flow back to each rear wheel cylinder. Do the same on the front. It's common to find rot/blockage at the transition on the fronts from the soft hoses to the short steel lines that go to the wheel cylinders. Also verify flow at the splitter on the rear axle.
If all the plumbing path checks out, then no doubt the wheel cylinders are rotten inside and should be serviced. But in order to do that, ya gotta know which brake set is installed front and rear. The "standard" brake set for s80 was a 9" front and 11" rear. The "option" brake set was a 10" front and 12" rear, those were available only with the inclusion of a d44 rear axle, there was only one steering axle available for the s80 4x4 throughout the production run, that was a d27.
If the entire wet side of the system checks out (including wheel cylinders), have you verified that the brake shoes are not badly glazed/overheated?
The 9"/10" brake set is barely adequate to stop a stock s80 with oem size tires. Conversely, the 10"/12" brakes set is very powerful (the front 10" is a lockheed design) and can handle tires up to 33" very adequately if functioning correctly.
