What causes and how to fix wandering bite point on Shimano hydraulic brakes?

Question

For the last two months and less than 600 km of riding I have been using a pair of new Shimano SLX BR-M7120 "high power" 4-piston hydraulic disc brakes. Yesterday I have noticed that the bite point of the rear brake started to wander, i.e. the lever travel required for brake engagement changes.

Specifically, when pulling the lever a few times in quick succession, i.e. when pumping the lever, the travel shortens considerably (up to 1.5 cm at the finger position, depending on how fast you pump), so that the brake bites sooner, with the lever further from the handlebar than normally. The lever never sinks closer to the handlebar than normally, i.e. the bite point always wanders to the outside.

The brakes have been bled very recently and the lever does not feel soft nor is the braking weak. Combined with the fact that the lever never sinks, I am not sure this is due to air in the system.

This is quite a frustrating and unsettling issue in general and especially when hurtling down very steep and rough terrain (which these brakes are meant for) where it is dangerous both to go too fast or too slow, one of which you are likely to do if you cannot tell when your brakes will engage. So as the question says, does anyone know why this happens and how to fix it?

Some additional info:

• front brake is also affected by this issue but to a far lesser extent; the lever has to be pumped faster than it would be during any realistic trail riding situation
• problem first occurred in wet and cold weather (just above freezing), however I've ridden in the same conditions a few times before without noticing it
• the brake is not damaged and, apart from this issue, works great

Answer 1

Originally I gave a tentative answer to this question and proposed the solution without actually trying it (next three paragraphs), and now that I have tried it I expand the answer with a report on the results (last two paragraphs). Spoiler: as already verifed in the meantime, the solution works perfectly.

Firstly, this is not a bleeding problem. I verified this by thoroughly bleeding the brake using fresh original Shimano mineral oil, after first draining the system to make sure I really do use the proper fluid in good condition. I also took the opportunity to clean the pistons and "lube" them a little with the braking fluid, just to eliminate dirty/sticky pistons as a possible cause of the problem. The bleeding was done at room temperature and the results were excellent; great lever feel, sharp engagement, consistent bite point position. However, after I took the bike for a test ride through the neighbourhood, with the outside temperature just around freezing, as the system cooled to the outside temperature the wandering bite point reemerged, i.e. the lever would "pump up" (engagement would begin earlier). The corresponding effect, to a smaller extent, emerged at the front brake as well.

Given this test and the information I have gathered at various places on the web, I conclude that the cause of the problem is the high Shimano mineral oil viscosity at low temperatures (viscosity being, in layman's terms, the resistance of fluids to flow freely; water has very low viscosity and flows easily, while e.g. honey has high viscosity and oozes slowly), combined with the small orrifices of the Shimano levers' inlet and compenstaing ports through which oil flows between the master cylinder and the brake fluid reservoir. Namely, instead of the oil flowing to the reservoir through the compensating port as the pistons are retracted after braking, the oil stays in the main line, so as the lever returns the pistons do not yet retract fully, meaning that on the next braking they are extended closer to the disc and therefore bite sooner, with lever further from the handlebar. If given more time, the oil slowly flows back in the reservoir, the pistons retract fully, and the bite point returns to its usual position. Hence, the bite point wanders to the outside by amount dependant on how often you engage the brake; at low temperatures, you can pump up the lever so that there is almost no free stroke before engaging.

The solution I have read about and intend to apply is to swap the original Shimano mineral oil with a low viscosity oil that has small viscosity variation with temperature (i.e. a high viscosity index or VI; Shimano's VI is pretty low, apparently, meaning it gets "sticky" at low temperatures). I believe this nullifies the warranty, but I have found there are many people, especially in Germany, who have been doing this for years using one particular motorcycle fork oil. In fact, this is so widespread that this oil, Putoline HPX R 2.5, is considered a benchmark on their forums, and if you try to buy it on German Amazon, at the time of writing 7 of the total 8 reviews rave about its use in Shimano brakes, the eighth also lists this as one of the uses it is good for, while Amazon auto-suggests Shimano bleeding funnel as an additonal item to buy with this moto fork oil (!).

After I actually try this, I will update this answer with a report on the results.

UPDATE: Roughly a month ago I finally bled my rear brake with Putoline HPX R 2.5. The bleeding was as easy as usual if not easier, as the less viscous Putoline fills the lines more quickly. Since then I have had several snowy rides at temperatures well below freezing, down to under -6 °C (under 21 °F). The wandering bite point dissapeared completely and the bite point became perfectly consistent, i.e. the lever neither pumps up nor sinks. I cannot even reproduce the wander deliberately by successively puling the lever as fast as I can while standing over the bike, let alone while riding. For comparison, I still use regular Shimano mineral oil in my front brake, and now that the rear brake is completely wander-free, even the small „pump-up“ I previously felt in the front feels huge. As for the performance at warmer temperatures, the highest in the last few weeks were about 15 °C or 59 °F, and the brake felt great. Apart from achieving predictable and consistent bite point in the entire range of riding temepratures, I could not really feel any difference compared to using Shimano oil; the lever action on both sides feels firm and safe. As for the seals, there are no indications of any adverse effects of Putoline whatsoever, let alone leaks.

Given this experience, which has now also been confirmed by others (see answer by Gareth D), and the fact that Putoline is also cheaper than Shimano mineral oil, I can only recommend using Putoline HPX R 2.5 as a solution to the wandering bite point problem to anyone not worried about the warranty of their Shimano brakes. Come next bleeding, I will start using it in the front as well.

Answer 2

My slx rear brake had the exact symptoms of the wandering bite point Bled them properly- no change, changed the fluid to the Halfords one and this made it worse. I even noticed the lever was returning more slowly than the front. Ordered the Putoline HPX R 2.5, preformed a full flush and bleed with it in the rear brake and the problem was instantly cured.The new fluid is noticeably less viscous and the lever now returns with a snap when released. No pumping up of the lever nor wandering bite point just consistent brakes again. I also noticed when bleeding it was much easier to get all the small bubbles out as I suspect the thinner fluid can get into all the nooks and crannies more easily. Overall really pleased with the results.

Answer 3

There’s three factors to this: a design by Shimano which seems to be a lot more sensitive to fluid viscosity than other’s designs, ineffective bleeding of the brakes, and the viscosity of the fluid and how that changes with temperature. We can’t change the Shimano design and presumably you don’t want to replace your brakes. So let’s focus on the bleeding and viscosity.

Proper bleeding is critical to all hydraulic brakes and I address this at the very end.

The rest is about the hydraulic fluid which is something you can change that can reduce the problem especially when it’s worse at cooler temperatures. By using fluid with lower viscosity at cold temperatures, the fluid will move faster through the system. When it moves faster through the system, your levers are less likely to “pump up” (change bite point), at least during normal usage.

TLDR: Maxima Mineral Brake Oil is a safer and more widely available alternative brake fluid than Putoline. It’s the official SRAM mineral oil for their DB8 brakes, which use mineral oil instead of DOT fluid. I recommend using Maxima if you want to try a mineral based hydraulic fluid that will reduce wandering bite point on Shimano brakes. Engineering nerdery follows.

It’s important to understand that mineral oil actually isn’t really one thing, it’s a mixture of various length hydrocarbons, which is why the viscosity and other properties can and do vary quite a bit depending on how it’s processed.

Also, the mineral oil is only the base oil. The additives in it, even if they only amount to a few percent, can have a major influence on the properties of the complete fluid. So even though people commonly say the final fluid is “mineral oil” it is really a mix of some variant of mineral oil and additives. It would be better called “mineral base hydraulic fluid” to make that more apparent.

Typical mineral oil has a viscosity around 35-50 cSt. And it may have no additives or have perfume added (like in baby oil). We need mineral oil in Shimano brakes that has very low viscosity and the proper additives.

When we say viscosity in the context of brakes, we actually mean kinematic viscosity, and it varies with temperature. It’s often quoted at 40C / 104F (ISO Viscosity Grade) and 100C / 212F (SAE condition). But different mineral oils will change their viscosity differently with temperature, even if they have the same viscosity at 40C they can have different viscosity at 0C.

Pour point is a measure of the coldest temperature just before an oil stops flowing like a liquid.

Viscosity Index (VI) is a relative quantified of how much the viscosity of a fluid changes as temperature changes.

So if we want to replace Shimano fluid with something, we should use a mineral based oil that has the same or slightly lower viscosity at 40C and is not as affected by temperature, meaning it has a higher VI, lower pour point, lower viscosity at 100C, or all three. Plus we have to be careful that the boil point is high enough not to cause issues when the brakes get hot.

Oh and it had to have the right additives to play nice with the elastomers (“rubbers”) in the brake system and prevent corrosion, oxidation and foaming.

We can get some useful information from the legally required safety data sheets for the fluids.

Shimano mineral base hydraulic fluid has a viscosity of 8 cSt (1 mm2/s = 1 cSt) and a pour point of -35C per the safety data sheet. Shimano doesn’t tell us the viscosity at 100C nor the VI. The boil point is >200C and the flash point is >130C. https://si.shimano.com/pdfs/compliance/sds/HYDRAULIC%20MINERAL%20OIL-202206-ENG-CLP.pdf https://si.shimano.com/pdfs/compliance/sds/hydraulic%20mineral%20oil-202008-ENG-GHS.pdf

Putoline HPX R 2.5 shock fluid has a viscosity of 6.7 cSt at 40C, 2.9 cSt at 100C, VI of 425, flash point of 87C, pour point of -51C, boiling point not given. Strictly looking at viscosity it is lower than Shimano oil and will stay more viscous at cold temps. It’s a fork oil, so it probably will play nice with the elastomers but I’m concerned about the high temperature performance. That low flash point might also mean that it has a low boil point, which could cause issues in a brake application. https://api.kroon-oil.com/pdf/en/safetysheet/SDS-PF2009-putoline-hpx-r-25w-EN.pdf https://api.kroon-oil.com/pdf/en/informationsheet/part/1735/EXP/PI-PF2009-putoline-hpx-r-25w-EN.pdf

Now for a weird suggestion; let’s look at SRAM mineral oil. But wait! Don’t SRAM brakes use DOT fluid? Yes, but also no. There’s one model of SRAM brakes, DB8, which uses mineral oil, specifically Maxima mineral brake oil. It has a viscosity of 8.7 cSt at 40C, 2.8 cSt at 100C, pour point of < -57C, VI of 202, boiling point of 215C. And it’s specifically designed and validated for bicycle hydraulic brake use, albeit SRAM not Shimano brakes. https://cdn.shopify.com/s/files/1/0555/2957/0594/files/Mineral_Brake_Oil_TDS.pdf?v=1675871564

So I would recommend using the Maxima oil, not the Putoline. It’s safer to use and should still perform better than Shimano oil at low temperatures.

The final kicker is that the Putoline HPX R 2.5W is hard to find anywhere and costs $50 to ship to the US on top of the fluid price. Whereas the Maxima oil is readily available in US and I assume most of the world given it’s the official SRAM mineral oil.

The SRAM bleed instructions video is actually very thorough and the best I’ve seen. If you also ensure you get your brake hose vertical and tap it during bleed, then I think you’ll get the best possible bleed.

Answer 4

I don't think the wandering bite point on brakes is solely a viscosity problem or in need of bleeding. My case is Shimano XT brakes, at 0C the bite point was 62mm (far side of grip) when pumped several times it went to 72mm. Brought the bike inside where it warmed up. Hours later the bite point was 64mm, when pumped it went to 70mm. Temperature helped the situation but didn't eliminate it.

I had let my pads and rotor wear more than usual primarily because I was preparing to change both. I put on a new rotor and new pads and no more wandering bite point at any temperature. I believe the reason is the caliper piston was extended so the chamber was full of oil, a larger volume than usual. A greater volume of oil had to back out of the caliper to get back to resting position. Yes, lower viscosity can help but it also has to do with extended caliper pistons.

Answer 5

I wonder how you got on with this in the end?

I bought some second hand cut m785s a couple years ago. They were fine for a while but then the wandering bite point happened to me too. I had the rear caliper replaced with a compatible slx one and it was fine for a while again but then on a ride, boom, no brake, then just as suddenly it went back to normal. This happened a few times, and I've had the brakes in the shop and they did the regular top down bleed but still the problem persisted.

My solution, which so far has worked, was to do a bottom up bleed... I removed the caliper from the frame and put it on the floor. I removed the pads and jammed a pair of long nose pliers between the pistons as I don't have the little plastic spacer. Using a big syringe, push brand new (regular Shimano) fluid from the caliper, up and out into the normal bleed funnel on the lever at the top, tapping everything to dislodge any air bubbles.. As the funnel filled up, I emptied it with a second syringe.

Once the system was totally full of clean fluid, and the funnel half full, I just squeezed the lever over and over at all different angles tapping the hose and lever until I could squeeze about 20 times in a row with no new air bubbles.. (amazing how much was in there actually, especially having had the brakes bled recently in the normal top down way). Once done, close the bleed nipple at the caliper before removing the hose, plug and remove the funnel, put a drop of fluid into the hole before winding the cap back in. Clean up, pads in, caliper back on the bike. Brake feels good, solid, woody feeling at the lever, just like the front and how I expect brakes to feel.

After reading all the issues about wandering bite point on Shimano brakes, I was convinced it wasn't a bleed issue, bit thought I'd try one last time a different way, just like I used to do it with my old magura rim brakes...from the bottom to the top, the way air naturally wants to travel... So far so good, it feels totally different. Fingers crossed..