I was riding home from work this morning, doing about 85 km/h. I was about 600 - 700 metres out of town, just about to crest a small hill when my front brakes came on by themselves for no reason. My fingers were nowhere near the brake lever. I had nowhere to pull over until I got past the 50 km/h signs about 500 metres later. When I stopped, I saw smoke bellowing from both brake discs and both calipers. The front brake lever had almost no movement until I my dad got to me with a trailer about 5 minutes later and I could my bike with no problems. So I ended up riding the last 400 metres home. After the brakes had stopped smoking and while I was still waiting for my dad to show up, I had a good look at the discs and for a purple looking colour in the middle of the shiney part of both discs. Dad told me that air might have been in the lines, and showed me me a crack in the brake fluid level window. What do you guys think could have caused this to happen ?
Is it an aftermarket brake lever? Poorly made levers can hold the master cylinder piston slightly engaged and over time the brakes will lock on as the brake fluid heats up and expands. Another possibility is too much brake fluid in the system - same result, fluid heats up, except it will just push the pistons out further. This can happen when people replace their brake fluid, but don't account for pad wear when setting the brake fluid level. Then the pads get replaced and too much fluid is in the system. First thing I would do is replace the brake fluid and set the level properly. Then I would also remove the pads and pump the pistons out a bit to see if they are sticking, and if they easily retract. If it's all filthy then pump them out a bit and scrub with hot soapy water and an old toothbrush. Compress and pump out pistons repeatedly until they all come out evenly and retract easily. With the age of the bike, it may need new seals. The dust seals especially are prone to getting corrosion behind them and creating excess friction on the piston. They can also swell if people use brake cleaner on them, same with the rubber diaphragms for the reservoirs.
Did you end up replacing your lever's ? I remember you were looking at some on aliexpress or ebay a while ago. If so i reckon the part that of the new lever that pushes on the piston in the master is a bit too long, it would be pushing the piston into the master too far and blocking the small brake fluid return port, not allowing fluid to return to the brake master. You might be able to still use them if you file some off the end to make it the correct length
I would be checking that the return port is open in your brake master then, it's the small hole in the master cylinder reservoir at the opposite end to the throttle It would pay to flush the old fluid out and refill it with new stuff, should really pull the piston out and at least clean up the seal's too while you're at it So check hole B is open in this picture
This looks like total shite ... full of crap and corrosion everywhere ... needs to be totally pulled apart and cleaned and the lever area lubed up ... hate to think what it looks like inside the master and the calipers
The disc will be fine if you give the surface a good clean up. The rest needs a full overhaul before riding again.