Last week I had to bleed my XT rear brake three times to clear all the air from it.

This week my reverb is slow down and glacial up and a bleed is needed.

Where does it all come from?

From in the fluid and through leaky seals.

sounds like a seal somewhere in the system is not sealing properly. When you bleed them are you pumping them to squeeze the bubbles out to double check?

The reverb might not just need a bleed, check the air pressure, seat clamp torque & if the hose is kinked anywhere.

hmmm. its a stealth so a kink might be possible. Its never been that quick to return even when freshly bled.

Shortened hose as I think it was kinking in the frame, decent bleed and seems to be back to normal.