It's the same booster Volvo used (ATE), as the ones on Mercedes. ATE is an european part maker so they put these parts in several european cars (I think in VW as well).
If the pump runs a lot, it looks like there is a vacuum leak indeed. May also look where the hose enters the booster (engine bay). See perhaps if you notice black smear on that white tube (need to remove the grey 'accordion' cover first. May also follow all hoses from the switch, to the intake, pump, see if everything is tight. Also have a look at the pedal sensor switch at the booster (the black cylinder attached to the booster inside the engine bay), it has a seal to it.
As for the job, I would try replacing the whole booster on a XC90 - because the seal only job can be very tricky. There should be enough space to remove the booster and you know the whole unit is new. Careful with the tricky switch at the brake pedal - I think I have a few videos with these repairs on S60

 

Thanks Orange, that's a helpful video.
I did order the repair seal, so I hope it is the right size. I noticed in Europe that there is a poly seal available as well. I don't hear any leak when the car is running, but rather there is a "gushing" of air when the brake pedal is depressed. Yes, there is the black smearing on the white tube.
i am replacing the booster with a new ATE unit on my 2006 next week after I get the part. I will then attempt to effect the replacement of the seal on the 2006 booster and then transfer it to my 2004 later; which also needs replacement for the same issue. At least I will have the option and the possibility to split the difference in cost. Hopefully this repair will work and also my replacement with the new unit. Just rubs me the wrong way to have to replace the entire booster when the internals seem fine.

 

great maintenance item... never knew!

 

Yes, I think this would be a good maintenance item to lube the seal and tube with silicone grease. Long interval to be sure; but I wonder if the seal failure is due to the grease eventually drying out.
Actually; I had this same issue on a W210 E-Class Mercedes several years back as well, so at least for me, anecdotally, this seems to be a weak point issue across their line.

 

For the amount of effort to R&R the booster, I'd not bother with the URO seal. Granted, my experience is on the early T6 models, and it was a fairly horrid job to do. Volvo dealership book time is 6 hours for this.

One trick you can do is to simply turn the master cylinder "forwards", bending the hard lines a bit. This will save you a good bit of time.

You want some long-ish extensions (10"?)with a wobble to get around the pedals to get at the four nuts holding the booster to the car.

-Ryan

 

For the amount of effort to R&R the booster, I'd not bother with the URO seal. Granted, my experience is on the early T6 models, and it was a fairly horrid job to do. Volvo dealership book time is 6 hours for this.

One trick you can do is to simply turn the master cylinder "forwards", bending the hard lines a bit. This will save you a good bit of time.

You want some long-ish extensions (10"?)with a wobble to get around the pedals to get at the four nuts holding the booster to the car.

-Ryan

Yes, the R&R on the 2006 will likely inform whether I want to proceed with installing the re-sealed booster on the 2004 or proceed with a new unit; but I don't think it will be a terrible job on the 2.5T.