Strange braking behavior - need to "double-brake" to get full braking power
I am new to this forum and don't do my own work, but I own a 2004 325 XiT with 50k miles on it that has been taken care of meticulously.
I am experiencing a strange braking behavior from my car. When I first press down on the pedal, it sinks in just lightly lower than normal and the braking is there but not OEM-strong. When I release the brake pedal and immediately apply the brakes again, the pedal sinks in less and the full braking power comes back. The difference is amount of pedal sink is very slight, but noticeable to me and the difference in braking power is easily noticeable. I had all 4 brakes done earlier this year (new rotors, new pads) and the service records show that the brake fluid was replaced and the lines were properly bled. When the brakes were new, there was good braking power and this behavior was not noticeable. However, as the brakes wore in a bit, this behavior became noticeable. I have had 3 different shops (including a dealer) look at my car and they all told me there doesn't seem to be anything wrong (rotors, pads, calipers). I am now turning my suspicion to the master cylinder, but really have no clue.
Has anyone had similar problems? How could something like this be investigated or solved? Is it even a problem to be concerned about?