Rear brake locked up because...?

[Mr Earl]

The bike had been parked in full sun, under a fabric cover, all day.  It hit about 92°F this afternoon.  When I left work, the speedo wasn't registering the speed appropriate to the rpm.  I thought the electronics must be affected by the heat.  When I reached a traffic light (about 3 miles), I noticed the rear brake pedal had no movement, and the bike wouldn't roll.  I muscled it through the intersection, then let it cool about 5 minutes.  The rotor freed up, but the pedal still had almost no play.  Rode it the last mile or so home and parked it.  An hour later, and the pedal has about 3/4 of its normal play.

Brake fluid isn't supposed to expand when it's hot, so this logically means air in the system.  But the pedal is very firm, and I bled it about 3 months ago.

All thoughts welcome...

[ducpainter]

Put some free play into the pedal by shortening the pushrod.

[Speeddog]

Put some free play into the pedal by shortening the pushrod.

+1

[teddy037.2]

+2

bad things can happen if it goes too long like that

[Slide Panda]

You might also want to inspect the pad, and pad guide pin while you're at it, once you've followed the previous suggestion.

I had a pad hang on a dirty pin and the result was a cooked rotor, thrashed pads and an exploded brake line.  The symptoms were similar to what you described, followed by a total lock up of the rear (when I stopped).  But, ambient temp wouldn't have any direct effect in my particular scenario, so look at your pushrod 1st.

[Mr Earl]

Thanks guys, will check it all out, shorten the rod, and report back.

[Ddan]

How old is the fluid?  You could have absorbed enough water to have it do what you described.

[erkishhorde]

Also, not sure what the rear brake looks like on your bike but on my bike the rear brake can't be bled while it's mounted since the bleed point is the lowest point. Stupid design but the first time I changed the fluid I wasn't thinking at all and tried to bleed it while it was mounted.

[Mr Earl]

How old is the fluid?  You could have absorbed enough water to have it do what you described.

The fluid was just changed a few months ago - like April.  And it looks to be in excellent condition.

Also, not sure what the rear brake looks like on your bike but on my bike the rear brake can't be bled while it's mounted since the bleed point is the lowest point. Stupid design but the first time I changed the fluid I wasn't thinking at all and tried to bleed it while it was mounted.

The M800 has the caliper mounted at the 12:00 position, so it's really easy to bleed.

I think I may have a clue on this.  When I got the bike (3 years ago, wow) I adjusted what I though was too much slack out of the rear brake pedal linkage.  Then, late last year, I replaced the rear pads with no other changes.  Then, in April, I flushed the fluid and replaced it all with new DOT4, topping off the reservoir.  It may be that the combination of these left too little play in the linkage.  At least that's my story, and I'm sticking with it unless someone's got a better one?

Thanks for the input.

[teddy037.2]

... I adjusted what I though was too much slack out of the rear brake pedal linkage. ...