Son's 2004 S60 was stuck in the snow last night. I think there was much tire spinning going on to get unstuck. Afterwards he said the brake pedal is very soft, going almost to the floor. I think the brakes work but the pedal is super soft. He was almost home when stuck so don't have much experience with exactly how well they work.

Anyway, any idea what is wrong?

People are so feeble minded sometimes. When the wheels start to spin on ice and snow, giving the engine more gas does NOT help as the traction is already lost.

In your son's case, a good rest should have the brakes back to normal. He probably boiled the brake fluid and caused air bubbles in the fluid next to the calipers. Time to do a fluid flush.

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My name is Klaus and I am a V♂lv♂holic

I'm not familiar with a 2004 (my daughter had a 2007 S60 2.5T), but if your son's car had some electronic anti-spin, or traction, aids that weren't turned off?...

What I'm thinking is that perhaps, while your son was frantically spinning his wheels trying to unstuck himself, the car's was trying to stop the wheelspin by applying brakes -- the calipers might have gotten really hot, and/or the pads might have been worn down ... something like that?

Maybe he should have invested in a set of real snow tires?