I recently had work done on my Volvo 2008 V50. The front axles were replaced, new brake pads in front, and new calipers and pads in the rear. When I got the car back the brake pedal would go to the floor… turned out the master had been compromised when the mechanics were bleeding the system. That was replaced (at no charge) as well.
All seems well except that now when I brake suddenly, which happens a lot in So Cal traffic, I feel a thump under the floor. I don’t hear anything but can feel thump or a knock right under my feet (driver’s side). This only happens when braking quickly. Braking at a slow speed does not produce the thump.
Do you hear a thump when switching from forward to reverse with the brakes applied?
Make sure there’s nothing in the trunck or under the seats that has worked loose. A spare tire or jack that has worked loose in the truck can cause this sensation for example. I’ve had it happen to me with a golf ball in the trunk or under the seat.
If it isn’t that, you need a shop to put it on the lift for a visual inspection. Replacing the axels usually requires various bushings and ball joints be disconnected to get the old ones out, so maybe something didn’t get tightened up completely in the process of installing the new replacement axels.
Zapping an older brake MC when performing a manual bleeding operation is a pretty common problem reported here. Good that you quickly discovered it and had a new MC installed.