[BikePump]

As most of you know Carmine had some significant brake problems on Friday at DE that were mostly resolved on Saturday and didnt become apparent at all on Sunday.

The issue goes like this:
-Brakes have been flushed bled and tested - nice firm pedal no issues.
-20 min into the session the brake pedal isnt necessarily as hard as it was initially but no degradation in performance and the pedal travel has only increased by 1-2cm (think this is a brake cooling issue - will explain more later in the thread)
-all of a sudden (normally going into the brake zone at 10 for some reason) the pedal goes down almost to the floor. pumping will get me braking so i can stop the car but now the pedal is super long almost to the floor and unsafe and so i end the session. the brakes are unusable at this point
-check brakes in the pit - no bubbles when i bled and the fluid is the reservoir is correct level.
-when i head out for the next session - it all starts over again. this repeated 5 times on Friday.
-overnight i flush and bleed the fluid again
-saturday the major issue doesnt reoccur. strangely enough this is the exact same sequence of events and days that occurred at the August DE - including that on sat-sun there was no problem.

I have talked to many of you up and down the paddock searching for answers with nothing definitive. I am running PFC pads and slotted rotors and the rotors are one month old and the pads have 5 DE days on them. The prevailing theory is that there is an issue with the master cylinder - maybe an internal seal leaking and letting fluid blow past.

Today I was preparing to order a new master cylinder, seals , and booster - when something in the Pelican diagram struck me. There is a vacuum hose from the engine to the brake booster. It terminates on the driver side wheel well wall in the trunk and then there is a short hose from the surface mounted plate to the brake booster. One of the things I have noticed going into Turn 10 braking zone is that there does appear to but a subtle inflection in the brake pedal right when i get onto the pedal - feels almost as if the quick release of the throttle has "pulled" down the brake pedal a minor amount. It happens but more subtly into 3 but not into 5 or 1.

Could there be a vacuum issue at play here? your opinions are valued.

ps - sunday showed me that the small amount of fade that I am experiencing is due to overheating. Session 2 I was out in traffic the entire time and experienced the fade - while in Session 3 when I was driving in clean air the entire time I had zero fade - and I was driving quicker! going to have to address this over the winter with next ducting up front and a caliper rebuild

[N0tt0N]

Do you have ABS on Carmine?

[BikePump]

yes Carmine has ABS

[VA951]

Reading this I was thinking master cylinder as well but your mention of vacuum reminded me of an issue with Boxsters I heard about. Something along the lines of leaves clogging the body drains and water getting into the brake booster vacuum area and creating issues.
May be worth a look.

[HoodPin]

Make sure you don't have a high pressure leak somewhere. Had a situation once where the hose fitting on a caliper was failing. The Motive didn't provide enough pressure to get fluid leaking at the hose, but firm pedal pressure did. Any evidence of "wet" around any of the brake lines under the car?

[BikePump]

no evidence of any fluid leaks Tony.

[BlackTalon]

1. Not that it likely is the issue, but a small amount of pedal movement before the brakes engage usually means you should check the wheel bearings to see if one (or more) is loose.

2. Do you really name your cars?

[BikePump]

David - there are no bearing issues they have recently been replaced.

Pat named the car...

[Roo]

Hmmmm....sure sounds like a master cylinder spool seal failure. With a well operating brake system, pushing the brake pedal pushes a spool inside the master cylinder bore which forces fluid from the reservoir through the lines into the brake calipers moving the pistons againt the brake pad backing plate.
The spool has several seals which separate the two halves of the brake system and force fluid into the lines. When the seals fail, pushing the pedal doesn't transfer as much fluid (or pressure) through the lines to the calipers resulting in reduced braking. Pumping the pedal may help as you're resetting the spool with fluid from the reservoir.
If you lost the vacuum boost on an otherwise good brake system, you would still have a functioning brake system. However, the pedal effort would be much higher (and the pedal wouldn't go to the floor).

[Trak Ratt]

Originally Posted by BikePump

David - there are no bearing issues they have recently been replaced.

Not really "proof" and a lot less expensive than an MC! Disconnect the vacuum hose (plug the engine side) and see it duplicates the issue (low speed please)