very hard use definitely changes brake lever feel as the fluid heats up and starts feeling "mushy".
I took a close look at the GP11 Desmosedici that was on display at Ducati Island at the Indy MotoGP this year. Nothing too fancy - a megabuck Brembo master cylinder, a remote lever adjuster, brake lines, calipers, and carbon rotors. I think it is very notable how MotoGP bikes have the brake lines hanging way out in the air - to help cool the things I'm guessing. Having to adjust the brake lever with the left-hand remote means that brake lever travel gets mushier and longer as the race progresses.
So if these million-dollar machines can't beat brake lever mush, we don't have much of a chance!
My lever is firmest just after the bike has warmed up (hydraulic fluid does expand some). On the street, it stays about the same. But after a day of doing top speed runs, I definitely started feeling some "mush". I wasn't losing braking power (definition of fade) but I was having to squeeze harder to get the same results. Stopping from 180mph in as short a distance as possible puts enormous strain on the brakes - all of that kinetic energy of a 180mph flying missile converted into heat.
That being said, how good the brake bleed has been done has a huge effect. On my 2010, I could never get someone to bleed my brakes to my satisfaction. On my 2012, which has the RCS19 and M4 calipers carried over from my old bike, the tech did one hell of a job and I have as consistent a feel as one can expect. But hard riding on a hot summer day - the lever feel will change, either slightly or dramatically, but it will.
2012 S1000RR, sapphire black, DTC and HEATED GRIPS
(also a LOT of other mods...)
2000 Ducati ST4 Desmodromico
2010 S1000RR (sold)
2007 F800S (sold)