bikerpet Posted March 4 Report Share Posted March 4 The front caliper on my '21 TRS has an annoying tendency to drag constantly and I can't seem to fix it. Couple of days ago on a long steep descent it overheated so badly I completely lost front brake. It wasn't from over use of the brake, it's a hill I've been riding a couple of times a week for quite a few years on a variety of bikes. I am very careful how I use the brakes and it's not a problem. But this time the brake was dragging really quite hard - it would start to skid a little on flat, loose dirt with no brake applied. I took it very carefully down trying not to use front brake at all, but it still completely overheated to the point of the disc just beginning to blue at one point! Sometimes it releases a little better if I roll the bike backwards a bit. I'm not sure it hasn't always had a bit of drag from new, but it certainly didn't bother me until recently. Bike has about 250 hours, I ride on some moderately hilly terrain. I've checked the fluid return hole is well and truly open when the lever is at rest. Replaced pads. I've removed the pistons and cleaned bores and pistons. Re-filled and bled. Still got more drag than I think it should - serviceable but not ideal. I'm not totally convinced that perhaps my bleed is leaving a tiny pocket of air that's preventing the pads completely sucking back off the disc, but I'd be a bit surprised. I finished the last bleed with the caliper off the bike with a syringe on the nipple and giving the caliper good taps as I rotated it into different positions trying to encourage every last bubble to the bleed nipple. It's sitting overnight with a strap on the lever at present. Has anyone got any insights? Quote Link to comment Share on other sites More sharing options...
Tr1AL Posted March 4 Report Share Posted March 4 @bikerpet as you obviously know what you are doing all I can think of is that you have a crimped/kinked hose and the crushed part of it is not allowing full flow on the brake fluid return to the reservoir so stopping the brake pistons from backing off, it may be worth a check just to tick it off the list. Quote Link to comment Share on other sites More sharing options...
bikerpet Posted March 4 Author Report Share Posted March 4 11 minutes ago, Tr1AL said: @bikerpet as you obviously know what you are doing all I can think of is that you have a crimped/kinked hose and the crushed part of it is not allowing full flow on the brake fluid return to the reservoir so stopping the brake pistons from backing off, it may be worth a check just to tick it off the list. Good suggestion, I'll take a careful look. Although I can't recall seeing anything remotely looking like damage. You never know if you don't look though. Thinking about it, it seems very unlikely - when I was bleeding with the syringe I could get a very generous flow up the hose with quite little pressure. Hmmm. I obviously don't know what I'm doing because I haven't solved it! Quote Link to comment Share on other sites More sharing options...
lemur Posted March 4 Report Share Posted March 4 Two things I would check, make sure both pads are moving and one side isn't stuck. Second is replace the wheel bearings, the brakes can't work great if the wheel bearings are not great. Quote Link to comment Share on other sites More sharing options...
hugo_furst Posted March 4 Report Share Posted March 4 New piston seals. They compress/flex outward with braking pressure, then flex back inward and move the pistons back. If they are old / cooked from overheating, they won't . Source: "Brake Handbook" by Fred Puhn, Professional Engineer, HP Books, copyright 1985, 178 pages, mostly race car stuff. Quote Link to comment Share on other sites More sharing options...
hugo_furst Posted March 4 Report Share Posted March 4 Caliper worn. Grooves where the steel plate of the brake pad rides on the aluminum caliper. Trials bike brakes usually lack the stainless steel protector found on Nissin and Brembo MX/enduro brakes. Quote Link to comment Share on other sites More sharing options...
lineaway Posted March 5 Report Share Posted March 5 The master cylinder is probably junk. If you had a good brake guy, he could clean up the surface inside and just use a rebuild kit. Myself I would buy a new m/c and start over. I dislike what is normally the quota flow. There are several after market options. Nissin and a few others are available, but pricey and not a whole kit. I threw away the braktec`s on my Montesa 301 as I had been down this path before ( I still have a nissin m/c to an ajp caliper as a spare} I run the large ajp m/c to the TRS gold front caliper. Strong and dependable. You don`t want to ask about my clutch. LOL Quote Link to comment Share on other sites More sharing options...
bikerpet Posted March 5 Author Report Share Posted March 5 (edited) Thanks for all the advice, including a key PM from @konrad I was under the misapprehension that the retraction of the master cylinder piston before the feed hole opened was the main cause of retraction. I believe I've previously heard it's actually the seals, but it apparently went straight out the other ear. The front of the caliper did have grooves from the pads bearing on it. I'd given them a bit of a file to reduce them but @hugo_furst's prompting encouraged me to clean them right up. I'd looked for that due to the way a slight rollback helped to release the drag somewhat. So today I took the caliper apart again. This time I inspected the seals carefully and could see slight ridges where they have been extruding past the groove on the outer side. The grooves appeared quite clean but I gave them extra attention and replaced the seals flipped from their original position so the ridges are now on the interior side. The seals felt quite supple. I filed and scraped the pad bearing surfaces flat and square. I also filed the edges of the pads to remove the stamping roughness and put a small radius on the edges, hopefully this will reduce the scraping on the caliper surface and reduce future grooving. I tried making a stainless wear plate but there is very, very little clearance around the pistons so I would need to do some more excavation of the caliper body to fit any stainless I have here in there - 1mm was too thick. Perhaps 0.5mm sheet might fit, but it needs to be carefully shaped to sit clear of the piston. Another day perhaps. Now reassembled and very thoroughly bled. They feel better than they have for a long time. Complete release with no noticeable drag at all. Hooray. I'll try to remember to report back after they've got some hours on them to say if it's a lasting or temporary fix. Thanks everyone. EDIT. I forgot to add - Konrad sent me a link to this video showing quite clearly that it's the seals not the master cylinder that actuates the piston retraction. I should also say - probably the 'correct' solution for this is a new seal kit. But I'm your typical tight trials rider so I'd rather spend hours faffing about than spend $100 on a kit! Besides, I learnt something new. Edited March 5 by bikerpet 3 Quote Link to comment Share on other sites More sharing options...
bikerpet Posted March 5 Author Report Share Posted March 5 20 minutes ago, lineaway said: The master cylinder is probably junk. If you had a good brake guy, he could clean up the surface inside and just use a rebuild kit. Myself I would buy a new m/c and start over. I dislike what is normally the quota flow. There are several after market options. Nissin and a few others are available, but pricey and not a whole kit. I threw away the braktec`s on my Montesa 301 as I had been down this path before ( I still have a nissin m/c to an ajp caliper as a spare} I run the large ajp m/c to the TRS gold front caliper. Strong and dependable. You don`t want to ask about my clutch. LOL I think I have it licked without any work on the m/c. As the video in the post I just made shows, the m/c doesn't have anything much to do with piston retraction so this problem is unlikely to be caused by anything happening at the handlebars. The brakes always went on a treat, no complaints at all - it was just the release that was a problem. Quote Link to comment Share on other sites More sharing options...
lineaway Posted March 5 Report Share Posted March 5 m As far as the m/c goes the seal does not seat properly, so you screw the tensioner bolt tighter. Once the brakes start heating everything goes bad. Good luck. Quote Link to comment Share on other sites More sharing options...
bikerpet Posted March 5 Author Report Share Posted March 5 Just now, lineaway said: m As far as the m/c goes the seal does not seat properly, so you screw the tensioner bolt tighter. Once the brakes start heating everything goes bad. Good luck. That can certainly be a problem, but I'm careful to keep the required free play, and if in doubt I take the top off to check for that little pulse of return fluid. I did wonder if maybe I had too much fluid in the reservoir so the expanding fluid had nowhere to go, but I backed off the m/c cap screws while it was still hot and holding brakes and it didn't make any difference. Fingers crossed. Quote Link to comment Share on other sites More sharing options...
lineaway Posted March 5 Report Share Posted March 5 Red rubber grease is what is needed to rebuild brake calipers. They sure can stick, but time will tell. Braktecs are known to go bad quickly. Quote Link to comment Share on other sites More sharing options...
faussy Posted March 5 Report Share Posted March 5 I love it when you can fix something with a little knowledge, some elbow grease and no money 1 Quote Link to comment Share on other sites More sharing options...
lemur Posted March 5 Report Share Posted March 5 10 hours ago, bikerpet said: ... I did wonder if maybe I had too much fluid in the reservoir so the expanding fluid had nowhere to go, ... Brake master cylinder reservoir has a rubber baffle in it to keep the fluid separate from the vented air space above the fluid. It would be virtually impossible to over-fill and pressurize the reservoir unless you left the rubber baffle out completely, plugged the tiny reservoir vent hole and over-filled the reservoir. The rubber baffle is in the master cylinder reservoir between the fluid and the vented area in order to reduce oxidation and absorption of water from the atmosphere by the DOT brake fluid. The little stainless leaf spring part is called the anti-rattle clip. Quote Link to comment Share on other sites More sharing options...
bikerpet Posted March 6 Author Report Share Posted March 6 19 hours ago, lemur said: The little stainless leaf spring part is called the anti-rattle clip. If you're referring to the clip in the photo, that's the anti-wear shim I tried to create. Thinner SS required though. The anti-rattle spring sits over the top of the pads. But I've ditched that anyway. Quote Link to comment Share on other sites More sharing options...
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