Supermooser Posted June 29, 2011 Share Posted June 29, 2011 I've just rebuilt the rear brake calliper on my 1999 DT125R, but BEFORE I RECONNECTED THE CABLE at the cable to the calliper at the calliper end (with bolt and copper washers), I thought I would check that the master cylinder was correctly pumping fluid from the reservoir - through the master cylinder - and along and out of the tube I had disconnected by the calliper. No matter how many times I pump the rear brake pedal/master cylinder, no brake fluid comes out of the disconnected end of the cable, and the level of brake fluid in the reservoir remains the same! Do I need to rebuild my master cylinder and/or renew the seals, or is there something obvious I am missing? I cannot understand why the master cylinder is not working now/pumping brake fluid properly through the system, as it was working fine before I removed the rear brake calliper! If I do need to rebuild the master cylinder, is this an easy thing for an amateur (such as myself) to do - with the help of Mr Haynes anyway? Link to comment Share on other sites More sharing options...
NEV Posted June 29, 2011 Share Posted June 29, 2011 I've just rebuilt the rear brake calliper on my 1999 DT125R, but before I reconnected the cable to the calliper at the calliper end (with bolt and copper washers), I thought I would check that the master cylinder was correctly pumping fluid from the reservoir - through the master cylinder - and along and out of the tube I had disconnected by the calliper. No matter how many times I pump the rear brake pedal/master cylinder, no brake fluid comes out of the disconnected end of the cable, and the level of brake fluid in the reservoir remains the same! Do I need to rebuild my master cylinder and/or renew the seals, or is there something obvious I am missing? I cannot understand why the master cylinder is not working now/pumping brake fluid properly through the system, as it was working fine before I removed the rear brake calliper! If I do need to rebuild the master cylinder, is this an easy thing for an amateur (such as myself) to do - with the help of Mr Haynes anyway? I wouldn't have thought you'd need to rebuild the master cylinder unless it was actually leaking from it.I take it the internal plunger is going up and down, as it ought to? One thing I have noticed myself whilst bleeding brakes, is when you open up the bleed nipple right open it doesn't come out as easily as when you just open the bleed nipple, ever so slightly, a bit like the difference between putting your mouth over a kitchen roll tube and blowing, and blowing through a drinking straw, if you get my drift. Try just opening up the bleed nipple just one turn, see if it makes a difference. Hope this helps. Link to comment Share on other sites More sharing options...
Supermooser Posted June 29, 2011 Author Share Posted June 29, 2011 The caliper isn't connected remember. I'm just trying to see if the master cylinder pumps brake fluid from the reservoir but it doesn't! The master cylinder seems to be functioning but the reservoir doesn't empty no matter how many times I pump the brake. Presumably the brake fluid should be being pumped out where the cable should be connected to the caliper - but isn't! Link to comment Share on other sites More sharing options...
NEV Posted June 29, 2011 Share Posted June 29, 2011 Presumably the brake fluid should be being pumped out where the cable should be connected to the caliper - but isn't! Aye, you would have thought so yes, have you tried putting the brake pipe, and the caliper back on, and trying? seems daft that it isn't pumping out the top end, and like I say the plunger is going up and back down again, inside the cylinder? I woulda thought it all had to be all connected up , so as to create pressure within the brake fluid. I say this because when bleeding from the bleed nipple, the tube you use to bleed with has to have its other end in some brake fluid , so as to create a type of vaccuum, Oh aye, as a p.s, if you do put it all together with caliper, put something wooden, between the caliper pistons so you can open them back up again Link to comment Share on other sites More sharing options...
Supermooser Posted June 29, 2011 Author Share Posted June 29, 2011 I had thought of this but haven't tried. Good point I suppose. The only reason I haven't done is that someone told me that if I connect the cable to the caliper, unscrew the bleed nipple and pump the foot brake 30+ times- brake fluid would/should spirt out the nipple and need replacing in the reservoir. When I tried this before I rebuilt the caliper/piston, it didn't - so i presumed I had a problem with my master cylinder. Also, if it is supposed to pump fluid out doing this I would have thought it would have pumped fluid out of the cable I disconnected from the caliper! Spent too long trying to repair back brake now - driving me nuts haha! Link to comment Share on other sites More sharing options...
NEV Posted June 29, 2011 Share Posted June 29, 2011 I had thought of this but haven't tried. Good point I suppose. The only reason I haven't done is that someone told me that if I connect the cable to the caliper, unscrew the bleed nipple and pump the foot brake 30+ times- brake fluid would/should spirt out the nipple and need replacing in the reservoir. When I tried this before I rebuilt the caliper/piston, it didn't - so i presumed I had a problem with my master cylinder. Also, if it is supposed to pump fluid out doing this I would have thought it would have pumped fluid out of the cable I disconnected from the caliper! Spent too long trying to repair back brake now - driving me nuts haha! Ha, so now you understand what Barkwindjammer is on about, when he welcomes new members to the ward, lol Link to comment Share on other sites More sharing options...
Supermooser Posted June 29, 2011 Author Share Posted June 29, 2011 Haha yea - indeed Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 29, 2011 Moderator Share Posted June 29, 2011 you cant just pump without blocking off the banjo with your fingers each time you release the lever Link to comment Share on other sites More sharing options...
Supermooser Posted June 29, 2011 Author Share Posted June 29, 2011 Oh really, Kinna like bleeding. So bleeding should bring fluid back into master cylinder from resevoir when cable connected and pressure? Or do I need a midi vac or whatever it's called Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 29, 2011 Moderator Share Posted June 29, 2011 yes just like bleeding, if you dont do that the fluid will just travel back and forth within the pipe...thats pipe...as in NOT cable Link to comment Share on other sites More sharing options...
Supermooser Posted June 29, 2011 Author Share Posted June 29, 2011 Haha ok - pipe it is So you reckon I can just bleed it and the pressure will return? If so, how long/many times do I need to bleed usually? Last time I seemed to do it ages and began to wander whether it was working! After the first pump/bleed (which expelled loads of air), very little/almost no air came out when repeated procedure, although this was before I rebuilt/caliper and replaced piston/seals Link to comment Share on other sites More sharing options...
KirriePete Posted June 30, 2011 Share Posted June 30, 2011 Master cylinders are bastards to get going in my experience, so I don't bother anymore I just push fluid up the pipe instead: 1. Put it all together and empty the master reservoir. b. Get a clean syringe (the ones they use in those B&Q paint tint kits are perfect) and fill it with brake fluid. iii. Connect the syringe to the caliper bleed nipple (bit of clear tubing), open nipple one turn x. Pump fluid from syringe until it appears in reservoir. 99. Close bleed nipple, top up reservoir, test brake. Doing it this way virtually eliminates the need for bleeding as the fluid pushes any air out ahead of itself, but you could do a standard bleed just to be sure if the lever feels a little spongy.... You're welcome. Link to comment Share on other sites More sharing options...
Supermooser Posted June 30, 2011 Author Share Posted June 30, 2011 Hey that sounds good - nice. How does the air escape though - from the piston/master? I presume I leave the top off the reservoir? How does the fluid pass through the master cylinder up to the reservoir? Doesn't matter I suppose, but it does?! I suppose you could do it the other way injecting from the reservoir pipe and leaving the nipple undone p.s Nice photo Link to comment Share on other sites More sharing options...
2 Wheels Posted June 30, 2011 Share Posted June 30, 2011 If master cylinder was working before then it will be fine. The DT rear calliper is hard to bleed, cause it is almost at the same leavel as the master cylinder. Conect everything backup, but take off the rear caliper and let it hang down towards the ground, dont damage the pipe. Fill the resevoir and pump the the peadel with the nipple closed till you get a spoungy resistance. Open the nipple a bit and if there is a lettle oil comming out then You can put the calliper back on. Then its normal bleeding poceedure. Pump peadel few times then keep it held on. with your other hand open the bleed niple a bit, some oil will flow out with bubbles, Close nipple before brake peadel gets to the bottom and looses pressure,,,otherwise you will draw air back into the system. Repeat this til no more bubbles comes out bleed nippel and only oil. When your finished the brake peadel should feel firm and not soft or spungy. If ya ont want to take calliper off then do it like KirriePete is saying Oldgit is correct,,,, no oil will flow out of a pipe if it isnt conected to the calliper. Good luck and keep posting if your in boather. Link to comment Share on other sites More sharing options...
Supermooser Posted June 30, 2011 Author Share Posted June 30, 2011 Thanks mate. If I leave the caliper on and use a syringe, do I need to pump the peddle at all or just let it travel up through the cylinder to the resevoir on its own/with syringe pressure? Does it matter whether the pedal is up/down? I leave the reservoir cap off yea? I'll give it a go on Saturday Link to comment Share on other sites More sharing options...
Moderator Airhead Posted July 1, 2011 Moderator Share Posted July 1, 2011 Thanks mate. If I leave the caliper on and use a syringe, do I need to pump the peddle at all or just let it travel up through the cylinder to the resevoir on its own/with syringe pressure? Does it matter whether the pedal is up/down? I leave the reservoir cap off yea? I'll give it a go on Saturday If youre filling it in reverse dont press the pedal. Also if the pipe that attaches from the syringe to the nipple comes off you will risk soaking your brake pads in brake fluid, I would wrap the caliper in something, rags or polythene in case this happened. You may find a little PTFE tape on the nipple thread would also help to stop fluid leaking through the thread Link to comment Share on other sites More sharing options...
Supermooser Posted July 1, 2011 Author Share Posted July 1, 2011 Right folks.. I used the syringe method and filled the system with fluid fine. When I then went to bleed to stiffen up the brake, I heard a noise of air coming from the master cylinder. When I investigated further I found a small hole in the bottom rubber seal on the master cylinder. As I pumped the brake/tried to bleed, I could hear a jet of air coming in/out of the seal. I presume this is where I'm loosing pressure? If this is the case - do I need a master cylinder repair kit (aprox £20), just a new master cylinder rubber seal set (if available), or shall i just buy the complete used master cylinder on eBay (also around £20)? At least I feel I'm moving forward... slowly Link to comment Share on other sites More sharing options...
KirriePete Posted July 1, 2011 Share Posted July 1, 2011 Master cylinder rebuild kit from WEMoto - £24 plus postage. If you go for the ebay 'pre-loved' master cylinder, it could also be needing a rebuild ... best to stick with what you've got and refurbish, at least you'll know it's done properly. Each tiny step forward is one less you have to take, and one more bit of knowledge you can pass on to others later - how do you think all us wise ones got this way in the first place? Carry on. Link to comment Share on other sites More sharing options...
Supermooser Posted July 3, 2011 Author Share Posted July 3, 2011 Funny - It was alright actually, was only the dust seal making air noise. Friend of mine who's a mechanic happened to have a look and bled it in 30 seconds so nice and tight (after I'd put the fluid in the nipple via injection). Funny watching someone that knows what there doing. He released the pedal from the master so was pumping the master by hand (as more sensitive and had a better pump distance). Then instead of undoing/tightening the bleed nipple - he undid it slightly and just put his thumb over the end of it when required. Only took him a min and made me feel a little stupid haha! All's well that ends...finally, will have a proper burn tomorrow :-) Thanks for your help everyone :-)) Link to comment Share on other sites More sharing options...
NEV Posted July 3, 2011 Share Posted July 3, 2011 Funny - It was alright actually, was only the dust seal making air noise. Friend of mine who's a mechanic happened to have a look and bled it in 30 seconds so nice and tight (after I'd put the fluid in the nipple via injection). Funny watching someone that knows what there doing. He released the pedal from the master so was pumping the master by hand (as more sensitive and had a better pump distance). Then instead of undoing/tightening the bleed nipple - he undid it slightly and just put his thumb over the end of it when required. Only took him a min and made me feel a little stupid haha! All's well that ends...finally, will have a proper burn tomorrow :-) Thanks for your help everyone :-)) Good to see you got it sorted eventually Super, plus now you've seen it done, maybe now you can pass that knowledge on to someone else who needs help. Thats when the feelgood factor of working on bikes kicks in Link to comment Share on other sites More sharing options...
Recommended Posts