New brake fluid Cleaned rear caliper Rear brake does not fully release when the fluid is hot , I know these are susceptible to hot brake fluid ; I rarely use the rear brake ; other than trying to move the brake line away from the engine , cannot think of anything else Caliper looks all good , everything works as it should when cold I can push the pistons back in by hand with a bit of force Before After
I would suggest your brake pedal hasn't got any free play and is leaning on the pushrod ever so slightly. When it all gets hot the master cylinder piston can't return fully and hence it's building up pressure in the system. I'd make sure the MC pushrod has 1-2 mm free play before it engages the piston.
How old are the piston seals? it maybe they have aged/swollen causing more drag on the pistons. Also make sure there is free play at the foot pedal (as post above).(and that you dont ride with pressure on it ) PS I have had the swollen seals issue albeit on Mtb brakes (using M/C dot fluid -as per their oem design) the vendor suggested a brake piston grease on these to alleviate the sticking pistons. This was only a short term fix, new seals sorted it until the next time they swell up due to age etc
Had a similar experience on my RS first time out at Brands with a nasty smell, brake pad dust and blue colour on the disc. New seals, fluid and pads sorted it.
Free play is essential for the pedal both in terms of the pivot pin and also contact with the master cylinder. I had a rear brake literally catch fire on a track day because I had adjusted it too tight. As you have an aftermarket brake lever I would make sure it can move freely on the pivot pin as it could stick when hot and lose movement and bind.
Agree on the freeplay. My bike had the freeplay 'adjusted' at the first service as the shap said there was too much, they obviously adjusted it too much because on the ride home the bike started to get sluggish due to the brake dragging. Within a couple of miles the rear wheel completely locked up and the brake pads were on fire!
as said above, very good chance that this is a master cylinder free play issue. I can't remember the figure, but one hot summer i found that I had to exceed the manufacturer's recommended free-play figure on a 748 to prevent rear caliper gradually locking on, and to confirm:- it wasn't a sticking caliper piston.
I had this a couple of years ago after I fitted some rearsets. Neglected to double check freeplay and then melted caliper and shagged disc.
