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Spongy brakes would most likley be a) air/water in the brake line or caliper body (tap the caliper after bleeding the upper screws then bleed off a little more b) a worn master cylinder c) a failed brake hose that is ballooning. A frozen caliper won't cause this, but will cause other problems such as poor braking, squealing, overheating the hubs and warping the rotors.
Some quick checks for caliper problems
a) After a trip involving some intense braking in the last few minutes (like stopping at highway speed or braking down a long hill), go around the vehicle and check the temerature of the hubs. If a hub is particularly hot then it likely has a dragging piston (a piston with a worn seal that won't retract slightly when brake pressure is removed).
b) Inspect the brake pad linings. If one side is considerably more worn than the other then there is likely a frozen or dragging piston.
To check for frozen caliper pistons, pull the pad retaining clips and pins, stomp on the pedal real good (and release) then go and try to spread the pads away from the rotor. If you can spread them by hand then the piston is moving freely. You can also try spreading them using adjustable jaw pliers by clamping the pad and outside of the caliper body. If the piston moves relatively easy then that's probably okay, but if it won't move or takes a lot of force then you've got a frozen caliper.
To check for a dragging piston, raise the wheel, stomp on the brakes and release. You should be able to turn the wheel relatively easily by hand with only some dragging sounds from the pads. If you spin it real hard and let go it should continue to turn a minimum 1/8-1/4 turn from inertia. If it immediately tries to stop then the piston is dragging.
A frozen piston can be temporarily improved by removing the piston dust cover boot, extending the piston fully without letting it pop out (insert something the same thickness as the metal backing of a disk pad) and cleaning off all the dirt and dried brake fluid using a toothbrush and brake fluid (used clean fluid is okay for this -wipe clean when done). Push the piston back in then push it out again and repeat the process until it runs smooth and clean. This can really only be done for those caliper types using an external lip to retain the boot (ie. not the front 700/900 Lucas brakes). Make sure any damaged dust covers are replaced. Normally a permanent cure will require popping out the piston and replacing the rubber piston seals (getting the piston back in without damaging the new seal requires some skill).
If after cleaning you still have a dragging piston (hot hubs) that won't adequately retract then the seal is likley worn (or the bevelled edge has been installed backward).
All that having been said, this can become a bit of a painful messy job. You normally have to buy replacement seals and dust covers in sets of four. It is often more convenient and cheaper in the long run to simply replace a malfunctioning caliper (it's done on a core exchange basis). You can get these from Volvo or from cheaper aftermarket sources.
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Dave -not to be confused with a real expert, just goofing around at this
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