Helping a friend with their s40.
He told me the master cylinder was bad. So we replaced that.

I was looking at the car and the rear rotors were shot along with most of the pads. So we replaced all of them.

Once I better understood the problem:
Car takes tremendous amount of pedal pressure to stop first two time.

I replaced the check valve (which also has the hose and a pressure sensor.)

I tested the vacuum pump (which passed with flying colors) Same problem. I even triggered the pump on and still had the same problem.

So the only thing left was the brake booster. We swapped that out... no change!

At this point I have no clue what could be wrong. The hoses are in good shape. The caliper pistons push out and back in easily enough...

Any thoughts are greatly appreciated!