2010 Lexus IS250 w/ 150k miles. My brake pedal keeps getting soft. If I bleed the brakes, the pedal is firm for a week, and then it starts getting soft again. There are no visible leaks and the fluid level is not dropping. I’ve replaced the booster check valve and vacuum hose.
Did you use a bidirectional scan tool to properly bleed the ABS?
No
There's a good possibility that's the problem.
if that were the case, I think the pedal would be soft from the get go
Was there any work done on the car when this started, or was it out of the blue?
I think air is slowly creeping in somewhere. Master cylinder seals, brake lines and connections, or caliper piston seals or bleed screws, etc.
It was out of the blue for the most part. I’ve replaced the pads and rotors all around and I flushed the fluid several months ago because it was very dark.
This is the second time I’ve bled them and both times the pedal only felt firm for a week.