Brake Pedal Slowly Sinking When Braking

[Peezen]

I have question for the community as to what my next step should be. I will explain what I have done so far and why.

One day coming out from work my brake pedal was super hard when braking, but only when the X5 was really slow. Braking was normal at higher speeds and then when close to stop the pedal would get super hard and I would essentially be manually stoping the vehicle.

After researching it seemed like it was a brake booster issue, either from the pump, check valve, or the booster itself.

I was about to embark on changing the alternator bracket gasket anyways, so I added this task to the list. Since I had just recently replaced the vacuum pump I crossed that off the list as a problem decided to go with the check valve first.

I disconnected the driver side wheel speed sensor, and did the alternator bracket job (convoluted job for sure) then I replaced the front brake pads while putting the front axles back together and made sure to start compressing the pistons with the MC cap of. All seemed good.

I then moved to the booster job where I replaced the check valve and tested the breaks, but that didn’t fix the issue. I figured the brake booster then must be the issue and went about replacing it.I took the MC off and move it out of the way without disconnecting the brake lines and replaced the booster with a Cardone reman. When it replaced the MC and trested the brakes the hard pedal was gone. Except now what I was getting was a normal feeling boosted brake that would slowly sink to the floor if I kept the pressure on.

I now have the brake trifecta on for the front driver side wheel speed sensor, and a brake pedal that slowly sinks when pressure is consistently applied. The brakes work it the pedal continues to sink.

I can get a new wheel speed sensor since maybe it was damaged, but what would cause the new braking issue?

I didn’t open the brake system at all, but could air have found its way in while I was moving the reservoir and MC out of the way of the booster? Could the internal seals have blown when I compressed the caliper pistons?

The fact that there seems to be boost for the brakes would indicate that the boost side is now repaired correct?

I just want to know what to do next. I do have the Scwabben Foxwell BMW specific reader and it does have a bleed service that I could run to see if air was introduced, or I could have a shop do it, or I could source a new or used MC and install it and then do the bleed at home or shop.

Any advice with my situation would be greatly appreciated. Love that it doesn’t leak oil anymore, not stoked that now the brakes don’t work.