hoona6 Posted October 6, 2012 Report Share Posted October 6, 2012 Rode the bike to its MOT a few weeks back and the front brake was err nasty. Pumped up it was fine but within 30 seconds or so it was nasty soft again so earlier this week I ran new fluid through to replace the old. Bit of air came out but nothing enormous. Good feel back at the lever...for two days. Took the bike to work yesterday and riding home inthe rain it was worse than ever, to the point of being dangerous. Question is why? I know that brake fluid absorbs water (?hydroscopic). Is it possible that there is air getting into the system somehow? Presumably via the master cylinder? Advice please folks. May have to take it to work again Monday. Fankuwe. Link to comment Share on other sites More sharing options...
grahamster Posted October 6, 2012 Report Share Posted October 6, 2012 Had this same problem on my CBR 400. In my case It was the master cyl seals. A repair kit from david silver sorted it. An easy job to do aswell. Graham Link to comment Share on other sites More sharing options...
porter_jamie Posted October 6, 2012 Report Share Posted October 6, 2012 it's hygroscopic... the below is all true for clutch systems anyway, i i have data to prove it, not sure if it is true for brakes. have you defo got all the air out? try pushing new fluid in from the bleeders with a syringe. pumping the lever lots turns the air bubbles into smaller ones which then take time to come out into bigger bubbles again. the smaller bubbles are less compressible than the bigger ones (somehow, not sure on the physics).so you think you have a well bled system but if you leave it overnight you don't. interesting fact - when cars are filled in the factory, they vacuum all the air out of the brake and clutch system (common reservoir) and then hose in the fluid, from the reservoir end. The fluid is actually subjected to a vacuum to remove and air from it before it is pumped in. this is because in it's natural state brake fluid has about 3% odd air. This is it's equilibrium if you will. The fluid once in the system then accepts air into it which might be left laying around in the system that didnt get evacuated during the vacuum stage. Link to comment Share on other sites More sharing options...
BudgetBoy Posted October 6, 2012 Report Share Posted October 6, 2012 If air can get it them the fluid can leak out in theory. Take your calipers off the bike and clean your pistons..(leave attached to hose and leave pistons in the caliper,just pump them out enough so you can clean around them) The build up of brake dust and road grime can cause the seals to stick and not return fully. Once you have done that refit and try again. Air can get trapper in the MC and sometimes i will take the MC off (inc lever) hold it at a slight angle and tap it as i pump to dislodge any tiny air bubbles that might be trapped. You might have bad seals in the MC or calipers or you might have a warped disc. Those are my thoughts anyhow.. Link to comment Share on other sites More sharing options...
hoona6 Posted October 6, 2012 Author Report Share Posted October 6, 2012 Ta folks. Will investigate on the morrow (after motocross-vroom!) Link to comment Share on other sites More sharing options...
Evilchicken0 Posted October 7, 2012 Report Share Posted October 7, 2012 Sounds like you've got a leaking seal some where Link to comment Share on other sites More sharing options...
hoona6 Posted October 8, 2012 Author Report Share Posted October 8, 2012 Sounds like you've got a leaking seal some where Could be although I see no evidence of leaky fluid... Link to comment Share on other sites More sharing options...
ste996 Posted October 8, 2012 Report Share Posted October 8, 2012 The seals grabbing the piston and holding them back just a fraction will give a similar sympton. They feel ok when pumped at the lever because the fluid is forcing them against the disc, but when released the seal pulls it back into the bore just enough to give the soggy feeling again. Pop some new seals in the caliper as suggested before. Link to comment Share on other sites More sharing options...
Tomfoolery Posted October 8, 2012 Report Share Posted October 8, 2012 Could also be dirty calipers, worn pads, worn discs and old spongey lines. Maybe a small break in a rubber line that only opens at a certain point. Link to comment Share on other sites More sharing options...
Gregorious77 Posted October 8, 2012 Report Share Posted October 8, 2012 Burn it with fire Link to comment Share on other sites More sharing options...
awood12345 Posted October 8, 2012 Report Share Posted October 8, 2012 My suggestion on checking brakes as I had a similar issue with my race bike. As it cheap try new fluid and one bleed using one way bleeder valve (cheap of the bay). keep swapping between the calipers and master cylinder. The check the front wheel bearing for play as this can pump pads if its gone. Check the disks are straight by spinning the front wheel on a paddock stand. Remove the calipers and pump pistons out a bit. Clean using brake cleaner and push them back in (using something thats plastic). At this point its probably the master cylinder, so you will possibly need a re-build kit. Worth trying to strip and re-assemble if you know wat you are doing. Or if you feeling flush an after markent master cylinder will blow you away with the improvement (accossato or brembo are £180 upwards). For a bit more braking power remove the caliper and pump out the pistons, now try and push them back in by hand. If you can't then then are due a strip. not complicated by an airline is useful. With the caliper removed use an airline down the fluid hole to pump out the pistons, something weged so they only come out one at a time. Remove the seals and clean the piston/seals/cylinder using brake cleaner. Use a small amount of red rubber grease on the seals and re-assemble. The should slide in smoothly. Link to comment Share on other sites More sharing options...
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