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Not claiming to have any sort of magic solution, but if the increase in lever throw followed a sustained pull (for example, on the steepest part of the trail) then it could be a cracked inner liner in the hose. That can lead to fluid leaking into the braided reinforcement layer, without any visual indicator, other than perhaps a slight bulge in the outer polymer layer. Even weirder, the elasticity of the outer polymer can gradually squeeze most of the fluid back through the crack, into the inner liner, leaving no evidence if you don't catch it in the heat of the moment.
Normally, the easy test for this is, while the bike is at rest, simply to squeeze the lever so the pads engage the rotor for an extended period, and see if the lever engagement point gradually creeps inward toward the bar. You have already essentially done this though, by strapping the lever to the bar overnight, and it doesn't sound like it crept in (unless you had it close to bottoming under the strap pressure and therefore couldn't notice the slight additional movement to a fully bottomed position when you checked it the next day). One possibility that occurred to me is that strapping the levers down is totally static, whereas, since it is the front brake, that hose is flexing quite a bit as the fork cycles on the descent. If you had a hairline crack, it might only open when the hose is in a flexed condition, so you may want to push on the fork repeatedly while squeezing the lever, in case hose flex opens up the crack allowing you to replicate the problem.
If that's not it, I think you're right on with the thought of looking to the lever as the likely culprit. If you had fluid blowing past the M/C seal, then that could cause the problem, but why that would only happen after 2min of descending is non-obvious to me.
While bleeding, have you also pushed pistons in and out trying to free any possible trapped air behind them?
I had the same symptoms on my rear one, after many bleeds still was getting the lever to the bar on long descents, also some crashes. Then I read somewhere that some batches of T4V4 had some tolerance issues at the master cylinder, probably a seal or something like that, sent the brakes back to hope and they fixed them and sent them back, no problem since
I had a set that did that on one side - would feel great and then spongy, pump it back up to be great, then spongy.
Contacted Hope and they recommended changing out the diaphragm (https://www.hopetech.com/_repository/1/documents/Hope_TECH_4_Assembly.pdf HPSP405) which was pretty inexpensive and that cleared the issue.
Would definitely hit up Hope and see what they have to say, they were very responsive when I contacted them including being willing to hop on a call and check through various things that might be causing the issue if the diaphragm didn't cure the issue.
Question for the panel: what's the consensus on cleaning and lubing SRAM pistons with mineral oil and a q-tip? Inspired by this SRAM "Sticky Pistons" thread on here, and this video: https://www.youtube.com/watch?v=AY80Zt47M3A
When I interviewed SRAM's Chris Mandell for the DB8 review he told me to be pretty hands off with pistons, to not even clean the pistons unless I absolutely had to, because it's easy to dry out or contaminate the pistons and seals if you're poking around too much. The official line is to do the piston massage as a way to evenly lubricate pistons and reset seal friction, and otherwise don't touch them.
Not cleaning them seems odd to me, because I feel like every bleed I've ever seen starts and ends with hosing the caliper off with Isopropyl alcohol, but maybe SRAM knows something I don't? The cynical take would be that SRAM doesn't trust home mechanics to not contaminate their brake pads by failing to clean the excess mineral oil off caliper surfaces after a q-tip job.
I've always cleaned SRAM pistons and massaged them as regular service. A simple wipe with q-tips and some iso alcohol. I can see the issue of using fluid on the extended piston intorducing dirt into the system and the massage would seem to accomplish the same thing.
In my experience with other brands of brakes (Magura & Hayes), lubing pistons did nothing to resolve sticky pistons long term, and if anything seemed to necessitate doing it more often. The only thing that really resolved sticky pistons on the Hayes was replacing the piston seals. For Maguras, since they don't sell seals & pistons to service them, I had to replace the whole bloody caliper under warranty.
My philosophy is: there's no point in lubing them from the outside because the caliper is such a dusty environment, all you'll do is attract more dust to the pistons. Instead what I focus on is remembering to clean the exposed pistons with Iso before I push them back for a bleed or fresh pads. That way I'm not pushing gunk into the seals.
I'm 100% against lubricating pistons with anything at all because it's going to end up being trapped between the seal and the upper edge of the bore(which will be a continued problem with contamination). It doesn't matter if it's silicone grease, dot/mineral oil or if it's strawberry jam. If one cannot fix piston issues through cleaning, massaging them etc then something else is wrong, and then I will just recommend pulling them apart to inspect, fully clean as well as perhaps changing pistons/seals depending on the condition.
Cleaning them however, that's a given = isopropyl alcohol and q-tips.
Pistons + seals are fairly inexpensive items and normal wear items as everything else which means they should be changed at some point. No point skimping out on this, it's just not worth it.
Most people should easily be able to split a couple calipers to have them refreshed. It really is a simple job, that just takes a bit of time because being thorough pays off so you don't have to do it twice.
You're grossly overestimating either the ability of people or at least the confidence and comfort of going above what they feel they can do.
As for lubricating, technically if the piston should be grabbed and pulled back by the seal, it shouldn't be lubricated then. It should be a bit "sticky" for normal operation. But not too much.
If that was aimed at me I can simply say that I do mean that it is easy because there are very few parts involved and most often you only need very basic tools. My calipers are one piece, so a two piece design will be even easier because you obviously can split it.
Here's one of the calipers I recently pulled apart because I had to clean it all out, and as you see there are very few parts as I did say. You only clean them and(or swap them for new if needed) throw it back together again. It really is a simple job, you just got to do it.
A vice can come in handy, but for the most part you need compressed air to pop out the pistons + some basic picks to pull out the seals. That's all there really is to it.
I'm a home mechanic like most, and most people should be able to do this job as it is honestly simple.
It's not really possible to lube the seals from the outside but you can lube the interface between the piston and caliper bore. This needs to have low enough friction so that just after breaking, when the pistons have been pressed against the bore, the seals can pull them back without slipping. If the seal slips then the pad stays where it is.
I would say applying fluid to the external surfaces is a last-ditch effort to get em to slip better. Residual fluid can weep onto the pads at worse, but will attract grime, and having brake dust settle on a metal surface is a sure fire way to induce corrosion on metal surface regardless of fluid. If the massage doesn't work I'd look to stripping the caliper down, pistons and all, and rebuild it, insuring to wipe the square seals in the bore with fluid as well as the inner portion of the pistons. That would guarantee the system gets lubricated proper and prevent residual fluid from causing any issues. If that doesn't solve it, then I'd suspect a tolerance issue at play.
Damn! Did we work together? :D
Only bit I would add is pull the lever slowly/lightly. If there's an internal leak, it may hydrolock under fast/hard pulls and give a false negative for the internal leak. A couple pages back I listed out the steps for diagnosing a mc piston leak, hose leak, or reservoir issue. Situ can be modded here on by setting the lever in the bleed position and popping the cap off to observe what the fluid is doing.
I'm well aware of how hard or easy it is to service stuff. I do everything on my bike. And most people are in complete awe at what I do. And I find it very easy.
Problem number the two, most people don't have the tools, let alone the space to do stuff like this.
I never said it's hard (it is not), i said you're overestimating people.
Yeah I would love to service my calipers but the space and tools and time are just not there. I can't really afford to screw up, either. I cracked a piston just trying to push it back into the caliper and that was a whole ordeal. Also, off-hand mention of air compressors as though every rider has one... another thing I am sad to not own but would have nowhere to store. I don't have a bench mount vise or even a bench. Myself and others would love to work on all of the parts on our bikes but its just not in the cards for a lot of us right now.
Wacky idea:
As we all know, shimano brakes get more snappy and powerful the further you adjust the lever away from the bars. I guess the lever adjust changes the leverage ratio as well.
It would be sweet if you could have the feeling you get with the levers far from the bars but actually have the lever nice and close to the bars. Maybe via a different pivot in the lever or by doing a really silly mod and filing down the bracing post and somehow changing the clamp angle to allow the lever to be angled in.
I'm pretty sure to do what you want would require a different lever body, with the bend in the middle modified relative to the pivot point. If you wanted it to be adjustable, you'd end up with something like the Magura HC3 lever.
Does anyone make a replacement lever for shimano that might do that?
Freedom Coast and Oak Components make aftermarket levers but I'm not sure how they change the shape brand to brand. I know for TRPs they made it so that people without XL mitts could reach the lever.
Wow! I have been wondering why it’s so commonplace to run levers super far out when it is more ergonomic to run them closer to the bar. I love running my code RS as close to the bar as possible. There is a kind of culture in mtb though that thinks that this is suboptimal, and I think I can now see why.
Also, I have been toying with the idea of switching to shimano but maybe I’ll just stick to my original plan of moving to maven base.
I just learned today that codes get spongier as you bring them in as well. Stupid design
This is why I find Code RSC to be more powerful that XT 4 piston brakes. I like the levers close to the bars and the XTs lose power when you run the levers close.
Hey!
I have a question regarding mavens compared to radic kaha.
I recently got onyx hubs which has brought the rubbing more apparent that I have with my kahas.
The short dead stroke and light lever feel has been good but I can’t stand my brakes rubbing.
Can I achieve a short (max 20mm) of dead stroke with maven ultimates and also have good pad clearance? Currently I can’t get my kahas not to rub in the stand yet alone when riding.
I don’t mind a heavier lever pull but having a firm bite point is a must.
Also a question to those who have ridden both brakes. Are mavens more powerful than kahas?
TBF, leverage rates change (increasing, generally) on all brakes as the lever gets position closer to the bar. This ain't a Sram thing.
I know it’s not just a sram thing. I thought it was only a shimano thing until recently.
I feel like you should be able to make a lever adjust that doesn’t change the leverage ratio.
Also, how come no one makes a brake lever that gives you the snappy feel but with your lever close to the bars. It would be a simple different shape.
Is that due to the design of the adjustment mechanisms or simply because the lever moves in an arc?
https://www.hopetechhb.com/evo
The new Hope brakes will have 3 levers to choose from.
This is sounding like the long rear center discussion lol. My favorite part of riding mountain bikes is joining in with the half century of research and development that every rider gets to be a part of. I think we are starting to figure out something about brakes, but it might take a while to get to that sweet spot.
In Sram's case adjusting reach does not change the leverage. You just move the complete stroke of the lever further away or closer to the grip.
How the pads contact the rotor, when and so on, is determined by the contact point adjust by moving the piston in or out. The swing link doesn't move in any case, neither with the reach or contact point adjust.
Because of the arc as it changes the pull ratio. Easiest way to think about it is the way mechanical disc brakes for road levers pull on a shorter actuating arm than those for linear/V brake pull ratios. That isn't exactly how it works, but for basic visualization it should get the mind working. The interplay of the lever's home position and contact adjustment can help mitigate but the leverage effect will always be there. And of course we can't forget about the cam mechanisms/swinglinks that Sram, Hayes, Shimano and others employ between the lever and the MC piston. Personally, I ride my levers closer on my 'summer' bikes than I do on my fat bike. Part of that is winter gloves being the bulky beasts they are, but I also don't want as much leverage acting on the brake in loose conditions (sand and snow). On the other bikes, I prefer close and light action. Not saying which is better, just a preference, a "different lever strokes for different folks' if you will :D.
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