rich1978 Posted July 17, 2009 Share Posted July 17, 2009 Hello all, New member here, so please go easy on me! I have a technical problem that I'm hoping someone can point me in the right direction on. I have a 56 plate XT125X which I use to commute on (around 100 miles a week) It's done almost 10,000 Kilometres without much hassle but recently I have had a nightmare with the back brakes. it started when the rear brake continued to apply itself after i had taken my foot off the brake. I took it to the garage (authorised Yamaha dealer) and they said it looked like the seal had gone on the rear calliper. it took 2 weeks to order in the parts from Holland (a whole new calliper and disk set costing a lot! - apparently they cant just replace the seal) anyway, after braving public transport while the bike was in the garage i finally got it back yesterday, only for the problem to be much worse! previously it only happened when i used the back brake so i avoided using it (not ideal i know but hey) now, with brand new calliper and disk etc it happens randomly at really inopportune moments (trying to overtake a bus on the old kent road this morning!! not the time when you want your rear brake to apply itself almost fully in the morning rush hour. the problem seems intermittent, happening when the brakes are cold or hot, totally arbitrary. it will engage then pop out (causing me to shoot forward) this is obviously very dangerous and disturbing but the garage has had it back in today, taken it apart and put in new wheel bearings (!?) to no effect. they're stumped and so am i. does anyone have any ideas what it could be, or have had the same themselves? I'm not feathering the brake, the mechanic bled the brakes and all the pats are new now. it's very frustrating, it cost me almost £400 including labour and its worse than it was before! (at least it only happened when i used the rear brake before!) argh! please advise! Cheers, Rich Link to comment Share on other sites More sharing options...
Moderator DirtyDT Posted July 17, 2009 Moderator Share Posted July 17, 2009 If you have changed most things have you looked to see if the piston in the rear wheel brake fluid reservoir is not sticking on? Link to comment Share on other sites More sharing options...
rich1978 Posted July 17, 2009 Author Share Posted July 17, 2009 If you have changed most things have you looked to see if the piston in the rear wheel brake fluid reservoir is not sticking on? not tried that, to be honest I'm not particularly technical minded (although I'd like to change that!) is there anything else it could likely be? I want to go back to the mechanic on Monday armed with as much info etc as possible so that he can check and rule things out. any suggestions greatly appreciated. thanks. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted July 17, 2009 Moderator Share Posted July 17, 2009 Dunno, never heard of this but for what its worth, check the reservoir cap is vented ok if there is a return spring on the brake lever, check it is attached and functioning Lubricate the brake lever pivot point Link to comment Share on other sites More sharing options...
rich1978 Posted July 18, 2009 Author Share Posted July 18, 2009 Dunno, never heard of this but for what its worth, check the reservoir cap is vented ok if there is a return spring on the brake lever, check it is attached and functioning Lubricate the brake lever pivot point cheers guys. all will be duly noted. any other suggestions? Once they (hopefully) get to the bottom of it, I'll let you know what it was Link to comment Share on other sites More sharing options...
Moderator Cynic Posted July 19, 2009 Moderator Share Posted July 19, 2009 cheers guys. all will be duly noted. any other suggestions? Once they (hopefully) get to the bottom of it, I'll let you know what it was The only thing that comes to mind is the master cyl. If the return valve is sticking that is the only way i can see this getting worse with the new parts. New caliper and disc make the brake opperate better than the old stuff so the effects of a failed master cyl will be all the more obvious. The fluid in the brake system has to be able to move, brake fluid doesent compress which is the whole point of it so it can pass on the force of you pressing on the brake pedal. If the fluid cannot return through the master cyl then the pressure will increace in the caliper, causing the pads to drag, this causes heat, that makes the fluid expand, causes the pads to drag, causes heat. You can see where it's going. Then the release valve finally unsticks and the brake works properly. You used the example of the bus, if you wound on the power to get past the bus that will cause the chassis, swing arm, etc, including the wheels of the bike to flex. Perfectly normal but this flexing WILL bring the pads into contact with the disk and we are back to the previous paragraph with heat causing the fluid to expand etc. I had something similar happen on the front brakes on my suzuki where they put themselves on when i got a move on through the twisties. Replacement master cyl fixed that. Oh and if it is the master cyl then you are entitled to ask for a refund of the unessesary parts as the 'proffesional opinion' is wrong. You may even get away with the labour for returning your bike to its original condition if your canny about it. Don't think you will be able to do much about the wheel bearings but the disc/caliper kit that was not required, thats different. (edit) I have VERY little faith in bike dealers, the small experience i have had with ANY motor trade people who i do not know personally has always, always been a let down. Link to comment Share on other sites More sharing options...
rich1978 Posted July 19, 2009 Author Share Posted July 19, 2009 The only thing that comes to mind is the master cyl. If the return valve is sticking that is the only way i can see this getting worse with the new parts. New caliper and disc make the brake opperate better than the old stuff so the effects of a failed master cyl will be all the more obvious. The fluid in the brake system has to be able to move, brake fluid doesent compress which is the whole point of it so it can pass on the force of you pressing on the brake pedal. If the fluid cannot return through the master cyl then the pressure will increace in the caliper, causing the pads to drag, this causes heat, that makes the fluid expand, causes the pads to drag, causes heat. You can see where it's going. Then the release valve finally unsticks and the brake works properly. You used the example of the bus, if you wound on the power to get past the bus that will cause the chassis, swing arm, etc, including the wheels of the bike to flex. Perfectly normal but this flexing WILL bring the pads into contact with the disk and we are back to the previous paragraph with heat causing the fluid to expand etc. I had something similar happen on the front brakes on my suzuki where they put themselves on when i got a move on through the twisties. Replacement master cyl fixed that. Oh and if it is the master cyl then you are entitled to ask for a refund of the unnecessary parts as the 'professional opinion' is wrong. You may even get away with the labour for returning your bike to its original condition if your canny about it. Don't think you will be able to do much about the wheel bearings but the disc/calliper kit that was not required, thats different. (edit) I have VERY little faith in bike dealers, the small experience i have had with ANY motor trade people who i do not know personally has always, always been a let down. excellent advice. that sounds very plausible, thank you. all will be revealed tomorrow I guess (I'm going to ask them to take it out on a 5 mile test etc. - they said it looked OK before but there's no way if they had driven it for a period of over, say half a mile, that they wouldn't have noticed it) its very frustrating Link to comment Share on other sites More sharing options...
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