So, a strange one. The 748 front brakes are really great until they disappear completely and the lever comes all the way back to the bars. Gets a bit worrying as you can imagine when out and about. The first time, the bike got very hot so I thought I might have boiled the fluid so I bled them when I got home which seemed to fix it for a bit.The last they I was moving it around the garage a week after riding it and no brakes. I have bled them several times now, lots of fresh fluid, lots of brakes then none. Calipers aren't leaking, there's no air in the system, thoughts? Master cylinder??
if you are happy that system is fully bled with the right fluid and no air in the system, with no external leaking it has to be master cylinder seals failing/partially failing and/or worn bore.
I concur. Could you zip tie the lever back overnight when the brakes are firm and see if it goes soft overnight? If you have bled them correctly it cannot be air or old fluid, although I can't see what difference heat would make to the master cylinder seals. Does it get that hot up there? Good luck. Upt.
First the obvious - Be incredibly careful until you find the issue. If the feel suddenly changes then it isn’t down to bleeding. If it only happens after riding then it’s possible heat soak is boiling the fluid. Things to check x are the discs VERY hot? If so a partially sticking caliper could be the issue. Do any of the brake pipes run close to the engine/ exhaust? Does the brake pressure come back when the bike is allowed to cool? This would suggest heat expansion of either the fluid (boiling) or component is the issue. If ypu can 100% rule out the fluid issue then it’s time to do some in depth checks on the master cylinder and calipers.
the OP has said that he had no brakes when moving the bike around in the garage when cold. There's a small chance that things had got overheated the last time he rode it a week earlier, but I doubt it.
First thing I’d do is rebuild the master cylinder to rule that out then go from there. I presume a rebuild kit is available?
The first post suggests the brakes weren’t bled between riding and moving it a week later, therefore it’s possible that boiling of the fluid has created bubbles that are now causing the issues - hence my suggestions. Had he bled it between riding and finding them gone it would not be relevant. The key point is the possibility of drag on a calliper/ incorrectly routed brake lines causing heat build up that then soaks into the fluid - so the brakes could be ok on the move and then die a period after the bike is stopped. They are simple (and free) things to check so worth doing before the OP starts throwing money at things. And cleaning and servicing brakes is a win anyway even if it doesn’t solve this particular issue.
best agree to disagree - apart from ** ** "There's a small chance that things had got overheated the last time he rode it a week earlier, but I doubt it"
I wasn’t aware we were disagreeing? So you disagree that checking brake calipers etc are working ok is worth doing? Very strange, personally I think it’s ALWAYS worth doing easy and free checks before chucking money at a problem, especially when those are checks worth doing anyway as part of routine servicing. It’s how I was taught 40 years ago and has always worked for me. But hey, if you want to rebuild random parts before you have properly diagnosed a problem, fill your boots, your money to spend how you wish. I wouldn’t advocate that approach but have seen it done on so many occasions, even by some in the trade……..
best not go there as far as experience is concerned - i can only repeat what i've already said:- UNLESS things had got overheated the last time he rode it a week earlier - i.e. calipers showed no signs of overheating caused by sticking/staying on leading to air ingress, then brakes unlikely to fail completely when moving bike (cold) a week later unless - Post #3 - (i'm not typing it out again!)
But how can he know there are no signs of overheating unless he checks it? Even a small drag on a disc that is unnoticeable can cause significant heat build up. That may not cause any issues when there is air flow over the front of the bike, but then cause issues once the bike is sat and the heat in the caliper soaks into the system. Similar with brake lines too close to hot components e.g. exhausts. Fine while moving, issue when static for a period. The OP doesn’t state that he’d checked such things? So why wouldn’t you rule these out before going onto more complex/ more expensive stuff? The same thing can happen with vapour lock in fuel systems. Absolutely fine while running, stop the engine for a short while and you get heat soaking leading to complete failure to start. The issue isn’t whether this is likely to be the fault - only a full inspection can determine the cause. The issue is that these are very simple and quick checks that can rule out the issue, or if you are lucky, give you a free solution without chucking money at stuff that may or may not help. Your last post shows that you understand the principle, so why not just accept that you misunderstood the first post? No harm, we all get stuff wrong sometimes……. Anyway, I have spent enough time mansplaining this to you. Hopefully the OP has some stuff to check and will sort the bike quickly and get back out riding.
Wow, thanks for all the replies. I hadn't considered a sticking caliper boiling the fluid, so cleaning them is now on the list. That would explain them disappearing on a ride. I have just been in the garage and tried bleeding it from the master cylinder nipple. No air came out of there, just clean fluid but after a bit of bleeding and some pumping, I have brakes again. I can't check the calipers yet as I will have to drag it out of the garage and that's easier said than done as it's in front of one bike and wedged between another... As for routing the brake lines, it's one from the master cylinder and the other then loops over the mudguard, which I don't really like and it makes it a pain to bleed. If the loop is now full of air, that would explain why I have brakes after pumping. The lines don't look too close to anything hot from what I can see but I may look to change them for 2 from the master cylinder anyway. I can have a better look when it's out of the garage. Then hopefully after replacing an injector, cleaning the other and a new fuel filter, it will run for long enough for me to check everything still works
Most people are instinctively straight on a sticking caliper - carefully take it for a short ride and brake normally a few times and then stop and (again carefully) check the calipers and discs for heat. A little bit more difficult to assess in the current climate. If discs and/or calipers too hot to touch despite only light braking then you have your answer. You need to also check brake hand lever for excessive travel before you set off on return and obviously keep your distance/be ready with rear brake and/or down changes*. * if we are covering all options then you should take a large bottle of water with you in a rucksack!
i think you are overcomplicating things - (moo) I think you just need to read through O.P.'s original post again - the bit where he merely moved the (unridden,cold) bike in the garage but had no brakes with (apparently) no air in the system.
Okay, so I have taken the calipers off (left them connected to the hoses) to have a look. One of the pads seemed to be a bit thinner than the others but it was barely noticeable, but I can't remember which side it was on. A couple of the pistons might have been a bit sticky but a tin of brake cleaner and a toothbrush seems to have loosened them all off - I could push them all back in by hand at the end. I also hung the one caliper in the air and bled it like that, then I bled it again when it was back on the bike. I bled the master cylinder as well. Now I now have really good brakes again and the wheel freely turns when they aren't on so I don't think they are sticking. And for good measure, I have strapped the brake lever to the bar to see what happens...
Hi team. I’ve only skim read through this thread. Here’s my two pence worth! You won’t boil fresh brake fluid! On the road or track! You’d have to have severe brake binding that you’d feel when manoeuvring the bike around. You can boil the moisture that gets absorbed into the brake fluid quite easily (I know, I’ve done it) so the op states he’s bled the brakes multiple times so I’d guess the fluid has been refreshed with new fluid. Not a half bottle that’s been on the shelf in a damp garage for 6 years. If after bleeding you get a good firm lever action. Great! There’s no air in the system. If that degrades over time. Then obviously air is ingressing from the low pressure side. (Reservoir connection) If the lever suddenly and without warning is coming back to the bars. It’s the master cylinder. Don’t dick about. Either get a rebuild kit or replace it. And while you’ve got it in bits re build the calipers too.
My car brakes were sucking air in round the caliper piston seals...., or at least when I changed the caliper piston seals my brake issue ( soft pedal after a few days) went away.
How are you applying the brake when the lever comes back to the bar- slowly or quickly? When it happens, does the lever come back all the way repeatedly if you pump the brake quickly?
I experienced the same problem on my 1098. Bled the brakes and all fine convincing me there was no air in the system. Would work fine for a while and then out if the blue the lever would come close to the bars. Made no difference whether the bike was hit or cold. Bought a motivac vacuum bleeder and all fine for a while then problem would randomly occur again. In the end I fitted these bleeders which resolved the problem and has been fine for several years. https://www.google.com/aclk?sa=L&ai...OAxW7XUEAHeZaIUAQwg8oAHoFCAkQ4gE&nis=8&adurl=
