I have always felt that my wife's XC70 had poor braking. We bought the car 4 Years ago. It stops, but just not as good as I would like. This became more obvious when i picked up a 04 V70 which stops on a dime.

Today I replaced both the inner and outer Tie rod ends, and took a look at the pads while I was down there. I am surprised by how little wear the front pads have. They have to be a few yrs old, but look new. When i replaced the pads a few years ago i remember that the calipers looked good and had no issues moving the pistons back into the housing.

What about the proportioning Valve? Any way to test?

There is no proportioning valve, the ABS takes care of that function. Flush the brake fluid first. You could take the pads off and use brake cleaner on them as well as the rotors. Or swap pads, many owners complained of the soft OEM version of excessive brake dust and swapped to ceramic or harder metallic pads. Pads are cheap.

Check the rear pads, they sometimes wear faster than the front. But always use OEM in the rear.
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My back feels better when I sit in a Volvo seat

Bled the pass side front no problem.
Opened the drivers side bleed valve and nothing came out. Took a lot of pushing to get what looked like a plug of goop, then a steady stream of fluid. Braking is much improved. I can actually get the ABS to kick in if I stomp on them. I think I will flush some more this weekend.

What kind of pads are in the XC70? Perhaps you might try a different brand to see if the braking improves. A full brake fluid swap is never a bad idea either...

Good luck!