Brake pedal to the floor after 4 laps.

Took my Coupe to "The Bend", South Australia. What a fantastic facility! More to the point, brake pedal went to the floor after about 3-4 laps on my second run. My question is, can this be something other than the fluid? I haven't done anything different from the previous 4 track events, the only variable is the circuit, which I must admit is hard on brakes but still, it seems unlikely.

Brakes are standard Brembos with Winmax W5 race pads and super dot 4 brake fluid.

First time out I noticed the discs were glazed, noisy LF, a little vibration, nothing too concerning and good stopping power. I tip toed around for the first run learning the circuit, it was my first time out. I decided to try to push the brakes hard on the second run to see if they would come right - and they did, no noise or vibration, good stopping power so I then pushed on and set my fastest 2 laps and by the 3rd or 4th lap the pedal went to the floor all of a sudden. I still felt I was fairly easy on the brakes leaving a lot of margin.

It took 3-4 pumps to get any pedal and then when it came back it was very uneven, pulling to the left, even with the pedal very low the ABS was interrupting, I assume for one of the rears. Let the brakes cool, pedal came back, tried bleeding the brakes, no air. All seems fine again. 3rd run I took it easy for 3-4 laps without trouble, then a few hard laps the pedal went again, this time I continued slowly and the pedal came back up, basically if I was gentle on the brakes they were OK.

Any thoughts?

Sticky caliper. I had a road car something similar. Turned out the rear shoes were able to drag when hot which caused my fluid to boil.

Strip, clean, new fluid

Sounds like a fluid problem. Do you use high temp fluid and change it from time to time? If there was no air and no fluid loss then it cannot be anything else.

It's easy to boil standard brake fluid in a track scenario. High temp fluid is the only way to go.

How old is the Dot 4 fluid? It is hygroscopic, so you may have experienced the water content of the brake fluid boiling in the calipers, which will give the "pedal to the floor" effect.

Before changing any of the hardware, I'd try a fluid change. Make sure you use a top-quality fluid from a household name brand - eg Castrol SRF - there are lots of no-brand brake fluids that have a considerably lower heat range than some of the top-grade stuff

+1 old fluid boils at much reduced temperature

Thanks for the suggestions. I flushed the fluid prior to the event and I replace it yearly. I even flushed the clutch this time too. The clutch fluid was terrible too, I should probably flush it more often!

I've been using Castrol React Performance Dot 4 for the last 5+ years without issue. Boiling pt is 284C which is respectable for a dot 4, minimum is 260C.

SRF is 320C, will that make all the difference? I'm not so sure. I can try the fluid of course. Unfortunately that race track is 400 miles away so not easy to test it out!

Are you using braided brake hoses? Old OE rubber ones can expand easily under heat and pressure.

Had similar situation 4-5 years ago........boiled brake fluid.....