the importance of correct brake adjustment...

[Laddie907]

just a few quick notes on how I nearly killed myself..

I adjusted the rear brake pedal height but somehow didn't notice I'd managed to also adjust out the con-rod freeplay at the master cylinder - bad idea.
The wheel spun freely and brakes operated OK - maybe a little spongey - possibly some air in the line. The brake functioned OK but i don't use the rear brake much so I couldn't say if the braking performance was 100% or below...

anyway, maybe 300 kms AFTER the adjustment I was riding at about 100km/hr on the freeway...
the brake had not been used at all at this time - just straight line, level cruising.
I could tell the engine was loading up and I was automatically increasing throttle like you would to go up a hill to maintain engine revolutions...but the road was dead flat.
obviously something was wrong

The braking effect increased and I had to pull over immediately as the bike was slowing down quite quickly without any brake being applied by myself. Lucky there was some room to pull over on the edge of the freeway...

The wheel locked as the bike slowed to a stop.

The rear disc was very hot and quite blue - obviously it had been over-hot.
I expect it was near red hot.
The brake pedal could not be depressed at all - it was rock solid.

I expect that the brake must have been dragging slightly, heating the pad, caliper and of course the temperature expanding any air in the brake system. The expanding air would cause more pressure in the line and increase the pressure, heat, braking...ending up with a lock up.

Like i said, this all happened without any use of the brake by me prior to cause heat build-up.

I cracked the rear caliper nipple to bleed off excess pressure and the wheel released.
I also backed off the push rod between the master cylinder and the brake pedal when i discovered there was no free-play.

I think I have cooked the pads in the process...

the master cylinder was not over-full.

I have since re-bled the brakes but the pedal is still a bit spongey...I'm convinced there is still some air in the circuit.
It appears that if the free-play is adjusted out of the pedal to master cylinder linkage that the line will pressurize.

[Skins]

Is your disk still true? The slightest distortion is dangerous. It might need machining.

[gail]

A little while back had a similar experience on my 82 ss, did the rear brake adjust thing,I think we all like things on the tighter side of snug , went for a hoon out the road and after 10 km it started doing as you said and pulled up rapidly , Did not do any damage , but is a bit of a shock when you feel the adjust is good. Did not look up freeplay specs for it but that could be a clue. Sounds like it pays to have a bit more free play to allow for expansion etc. .. It might pay to err on the side of loose to alleviate this endearing trait. ....Weren't sneaking off on a tour without your mates was ya? ...... Marty

[Laddie907]

I guess I didn't explain fully...

the master cylinder piston MUST be at the bottom of it's stroke (i.e.: free play) to allow fluid from the reservoir into the piston. When the brake lever is depressed the piston shuts off the master cylinder port back to the reservoir and the pressure goes to the slave cylinder to operate the brake.

The important part is it also MUST be in this position to allow any expansion of brake line fluid due to heat BACK thru the master cylinder to the reservoir.
This is why there is a maximum fill line marked on the reservoir.

and I only went for about a 400km "test" ride...not quite on tour...
aside from the brake issue and battery/starting problems the bike ran faultlessly...lol