Brake Pedal Goes all the Way to the Floor

[2006VolvoNoob]

I have a 2006 Volvo S60 2.5T 137k mi. with a brake pedal that's acting funny. I am trying to learn more about cars and to figure out what's going on with it, so any help with fixing it or diagnosing it is very much appreciated. I did look at other posts on the topic but I'm not sure they had exactly the same problem I'm having.
The brake pedal goes all the way to the floor no matter how much pressure I apply. When I pump the pedal, it only builds up a small amount of pressure for a second or so. The car still comes to a full stop without pumping, even on hard panic stops, but I have to keep the pedal down as far it it will go in order to stop or to keep it stopped. When I push the pedal down, it makes a sound like someone is exhaling.
My brake rotors are a bit worn but I've been told they have some life left in them.
I took it to a Volvo specialist mechanic (non-dealership) who said that it is likely an ABS pump that's gone bad, because I do have a brake position sensor code on my car, but no light comes up for it. He said that in the past he's tried bleeding all the fluid for the same problem, but nothing improved on the other cars with this problem until he replaced the ABS pump. The price he quoted for the part alone was also significantly higher than the price I'm finding for the same part online.
Looking at other posts has led me to believe it could be any number of things. I'm therefore wondering how I can diagnose what the problem is and how to fix it, myself if possible. Is this an issue with the brakes, the master cylinder, the ABS pump, or the fluids?
Thank you for any help.

[chrism]

If the reservoir has plenty of fluid and there are no leaks anywhere then the master cylinder is faulty.
Also, it might be that you are getting a brake pedal position code because the pedal IS going all the way to the floor.

[jonesg]

If the reservoir has plenty of fluid and there are no leaks anywhere then the master cylinder is faulty.
Also, it might be that you are getting a brake pedal position code because the pedal IS going all the way to the floor.

that was my experience with my ford van. Turned out to be a big air pocket trapped in the master cyl.
Mechanic friend told me not to bother bench bleeding it, just attach a straw to a baster and suck it out.

I just looked at my v70, its the same situation, the end of the MC is angled upward, any air pocket stays trapped.

Otherwise I'd look at a rebuilt master cyl, seals on the plunger/piston might be blown.