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Would be pretty cool to build a thermal test system to subject heat to pistons and measure the fluid pressure loss based on various line pressures. Based on the fact that most fluids exhibit similar characteristics where as heat increases the inverse of the bulk modulus of the fluid decreases, thus compressibility increases, giving you some sort of predictability in the system response. It wouldn’t be that hard to measure the thermal effect in a system once you a lookup table to account for the density change.
As others have said, redo a fresh bleed and check all the fittings.
It sounds like you have a leak somewhere since as you add pressure during use the levers creep closer to the bar.
Which brake and which bike do you have? A loss like that points to a leak in the system. Possibly the reservoir bladder, possibly the hose. As others have said, check torque on the hose fittings, but there's a quick diagnostic you can do to determine if it's an internal rupture on your hose. Pull the wheel out of the drops and insert a pad spacer. Set the lever up into the bleed position and attach your syringe to the lever without the plunger in it. you'll need to keep that syringe vertical, so you'll need to figure that out. With the syringe vertical, add a bit of fluid to it, maybe a cm or so. Slowly and lightly pull the lever. This is key. If the lever goes to the bar, watch the fluid level in the syringe. and pull it the same way a few more times. If you notice the fluid level is dropping but not returning to the same height, stop pulling the lever. This is indicating you have an internal rupture in your brake line and it needs to be replaced. This is generally not a warrantable condition as it's related to damage, be it form a crash or poor hose routing. Most bikes have routing down pat these days, but I wouldn't say all do, so if it's the rear brake I'd look where you could make the hose arc smoother.
If the lever is going to the bar but the fluid level is maintaining volume, that indicates a MC piston seal failure. Warranty that bugger.
If the lever firms up, I would inspect the bladder. I've seen brands develop slight tears at the seal surface that would not allow fluid to leak but in use would allow the res to ingest a smidge of air or overzealous syringe use causing the diaphragm to pop out of its sealing surface resulting in the same slow air ingestion.
Super curious about today's BOTD brake set up. Maven calipers matched to Lewis levers. Say what you will about Lewis, but their levers feel really good. Very light and smooth. I'd definitely be interested to see how much of a faff it was pairing them, or if it went pretty smoothly.
When cleaning contaminated rotors I boil a pan of water and dish soap on the hob (use an old pan, or feel the wrath of your partner) and leave the rotor in there a few mins bubbling away. Got the tip from the yeti race team mechanic years ago.
When you say another bleed to get air out of the system, did you actually pull air out? Or was it just a general term?
If you are sure sure sure there is no more air while doing a bleed and then you have air in the system the next time you bleed, you have a leak somewhere. I'd do the things others have mentioned.
Regarding the nut, thanks for that one, I have code rscs that seem to be pulling some air slowly (also have a wandering bite point a while after a bleed) and is for sure a very easy thing to check. The old set I had (that GF uses) still functions flawlessly and it's been in use since 2019, had two bleeds and then a full rebuild in 2024.
The 800degC temp is the rotor to pad contact. They make sure to both keep them warm enough but at the same time have enough cooling. As you can see in the video, they take away everything from the caliper they can. This is for weight savings (unsprung mass and all) bu to also bring the fluid as close as possible to the cooling air. So much so they have cooling fins, ribs and barbs to ensure the caliper itself is cooled. So caliper and by extension oil is cooled, pad and rotor cooled, but kept warm enough.
As for System pressures and boiling prevention, all is fine and well as long as you have said pressure. QED PWR nuclear reactors where water is under 150 bar to prevent boiling at 350°C. But as soon as you take away the pressure, the fluid will boil. With brakes that happens every time you let off them.
Not Mavens, but I'm using Lewis masters paired with Magura calipers. I'd been intrigued about eeking more power out of the stock LH4 I was initially using. The LH4 caliper uses 17 & 14 mm pistons, so I gained quite a bit with the quad 17 mm pistons in the Magura
I was thinking about pairing them with a Cura 4 caliper or a Maven base caliper, but was unsure if the 18 mm pistons would mean I'd have too much lever throw before the pads would contact and didn't want to risk it. Plus I found the Magura calipers for cheap
My Lewgura set up has more throw than stock, but the power bump more than makes up for it, they have immense power and a really nice light lever feel
Getting some pretty horrendous pad rattle coming from my shigura brakes - have been running the same setup for ~18 months now but it’s only been over the last 3 months or so that I’ve been getting it since buying a fresh set for a new bike.
Using Galfer pro pads (which I’ve always used) - wonder if I’ve got a bad batch in terms of tolerances or the backing material isn’t working well with the magnetic piston?
Either way it’s driving me mental, brand new bikes with new brakes sounding like a filing cabinet being pushed off a cliff through any sort of rough corner. In a straight line it’s not quite as bad.
Anything I can try before just buying mavens?
trickstuff power or sinter pads, one piece per side rather than 4, never had any issue or noise with either
This seems to be a galfer thing with their springs. Try bending the tangs of the pad spring into the sides of the pads so that they're held more tightly in place I.e. "preloading" the pad spring. Just be careful to keep the tangs flat with the pad backing and stop them sticking out into the rotors when the pads wear down.
I usually do this before fitting new pads as the rattle drives me nuts if I don't.
The galfer pads are often a little bit short in the length department, They rattle Bad in shimano's and gets worse as it wears the caliper.
Opening the spring helps alot.
It's for a magura brake unfortunately so there's no spring involved, just magnets!
Will try the one piece trickstuff pads
Hey hey folks, having an interesting conundrum with my Hope T4 V4's and am interested to see if any of you have had similar issues. I picked up my set last August and had run them without complaint until the other week when my front lever started pulling much farther into the bar about 2 minutes into a mild descent. I inspected the brakes and couldn't find any sign of fluid leaks or anything of the sort. I subsequently performed a gravity bleed and had the brakes feeling new again. Next ride, exact same issue! At this point, I was sure there had to be a leak somewhere, so, rebled them and strapped the lever to the bar to pressure test overnight and sure enough, they held perfect pressure. I did notice that when I opened the reservoir to bleed them after the rides, they did seem to be down a small amount of fluid and I was also able to "pump" them back up to a normal feeling (similar to Shimano's wandering bite problem). My conclusion is that there must be a leak somewhere in the system, just not exactly sure how to deduce that problem yet. I think it must be up at the lever as the pads and rotors are not being contaminated, but I can't see any signs of leakage. Let me know what you guys think.
FYI: I have checked the torque and fitting of all the barbs/olives and brake hose fittings, definitely not the issue
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.
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.
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.
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.
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