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Trying to work out if I've done something wrong with a bleed of my front brake..?
M8000 series. Got back from doing the Lakes 4 passes on Saturday. I gave my bike a light wash down, that dust gets everywhere! And left it to dry in the sun for a couple of hours. The front brake had seized on, with no travel to be had at the lever.
It's done it before, and that time I bled it as the lever had "stuck down". Anyway after putting it in the shade of the garage, it's gone back to normal.
Too much fluid in the system?
Always love a good too hot thread 😉
Bikes also work in 40c+ deserts in the US and many other places...
Could be, I've had similar from bleeding other brakes without bleed blocks and then replacing the pads at a later date - had to let a little fluid out to allow the pads to retract fully.
Ok Mikewsmith, yeah we get that. I'm just wondering if I needed to leave a bit of "slack" in the system to deal with expansion.
Obviously brake calipers get silly hot during use. What I'm wondering is has the fluid in the lever warmed up pushing the pistons in?
Never had to leave a bit of slack normally and that is covering 40c temp ranges and 4000m altitude range too. A proper bleed should be just that though generally all bets are off for me with Shimano brakes 😉
Never had to leave a bit of slack normally and that is covering 40c temp ranges and 4000m altitude range too. A proper bleed should be just that though generally all bets are off for me with Shimano brakes
I wouldn't say you needed to leave slack, just something odd could've gone wrong with the bleed - maybe a bit of air or something particularly volatile in the fluid causing issues. Anyway I'd give it a bleed but from the internet (and to be fair one friend's experience) M8000s can be "fussy", so it might happen again next time it gets hot.
I'm a bit surprised that thermal expansion of fluid can move the pads far enough to jam the brakes on, but I can't think what else it'd be. A bleed block is slightly fatter than new pads + disc, so if you do the bleed with a bleed block, you'll always end up with a little bit of space for expansion. If you bleed with pads + disc in place, you'll end up with more fluid in the system.
If the brake is jamming on in the heat, I'd suspect some air trapped in the hose expanding in the heat. Fluid on the hose wont normally get hot through use, only in the immediate area around the caliper with conducted heat from braking.
LBS are pretty sure it'll be the piston in the lever sticking. So warranty job...
Sram have a major problem with lever pistons sticking in the heat. I've just 'modified' mine as per one of the videos on Youtube.
just more shitty shimano lever problems, had it happen to mine, so did a week in france hanging my helmet over the front brake when stopped so it didnt lock on in the direct sublight.
throw them away and get something decent.
Heat is the SRAM issue with the pistons - modded one of mine, but also bought the revised piston and fitted it when it arrived.
Yeah, grit stuck in the lever piston seal, almost certainly. Get it warrantied and make sure you don't point the hose at the new lever (there are no seals stopping crap from being forced into the piston seal); I keep the lever that faces into the slipstream when the bikes on my towbar rack covered when I'm driving along.
It's a warranty job if it's within 2 years of purchase, if not this might help
https://www.bikehub.co.za/topic/155987-fixworkaround-shimano-xt-m785-brake-lever/
I got one of my bikes out of the garage the other day, I hadn't used it for a few weeks and it hangs high up next to the roof. Brakes on that were jammed on, eased of after a while but they fine when it was put away. Formula Rx's though.
Could well be air in the system but they were working well when I last used it.....
My Formulas are a bugger for it
i have to let a bit of fluid out after a real hot day
My Formulas are a bugger for it
i have to let a bit of fluid out after a real hot day
Now I'm sure my thermodynamics are a bit rusty but sounds like it's air in the system expanding not the fluid itself at that stage as the expansion rates are much higher for liquids, given the temps that these fluids will hit in Auto systems too.