Brake pedal feel issue
 

Hi everyone. My 2013 FR-S with 60k miles (solo spec coupe autocross/time trials car) is exhibiting an unexpected issue with brake pedal feel. First application of the pedal while driving seems to have a bit more travel than the second application immediately thereafter. My initial thought is brake master cylinder, but I can't feel any sinking of the pedal with constant pressure on it. This issue seems to have arisen after the racing season ended and when the car gets parked for a week or so without being driven, left outside but under a carport. No apparent buildup of rust on the rotors. I am interested in whether anyone has experienced this before. Thank you.

When was the last brake fluid change\bleed?

Middle of last season Stoptech 660.

i would agree with your diagnosis given the pedal feel, however i would try flushing the lines first to be hopeful for some air in one of the lines before going through the ordeal of changing the brake booster.

How worn are the front and rear brake pads?

Shouldn't I just replace the master cylinder and leave the booster in the car? I don't think it's exhibiting any symptoms of a bad booster.

They're OK. Half worn at most.

Buy a brake fluid moisture tester from amazon for £5. Place into reservoir, if it flashes orange to red, you need to change it out. What you’re experiencing is similar to me. What is happening is the heat from braking is actually boiling the “water” inside the brake fluid. Causing the brake line to expand. If you leave the brake fluid in there, it will corrode your brake lines, soften your pedal dramatically and therefore decrease stopping power. I have racing fluid and SS braided lines with stock callipers, EBC Grooved and EBC yellowstuff pads. You may not need the racing fluid as it is extremely expensive for a complete change, but I wouldn’t advise buying DOT 3/4/5.1. Somewhere in between. If you’re not sure how to bleed the system in the correct order and what steps to take, or any questions at all, PM me.

Hope this helps!!!