Shimano GRX Hydraulic brake issue….

[sevenfourate]

Brand new bike with GRX Hydraulic Calipers.
Bought online and was delivered with a slightly rubbing front brake. I fiddled and couldn’t sort it - so this afternoon i’ve taken it to my trusted and usually skilled LBS.

In all the mechanic has:

Tried multiple times to reset Caliper position / alignment etc
Checked wheel, bearings, seating / alignment.
Trued the brake disk
Taken pads out, exposed and cleaned pistons.

Fiddled, played, gesticulated, pontificated, Googled and pondered some more…..

Considered the pistons (both ?) appeared to be retracting slowly. Not all the way back ? **As you could adjust to get the disc / pad rub as minuscule as possible - haul on the front brake. For the rubbing to then be 10 times worse / constant….😳

So pistons / Caliper cleaned again.(No difference).

At this point he took a small amount of fluid out the Master cylinder to try to gain a tiny bit of disc / pad clearance. Feeling (Literally) the lever was quite firm / tight. This wasn’t really successful or made any noticeable difference.

For the hell if it - my mechanic then bled the system of old fluid. And put fresh in.

The lever immediately got firmer. He said he can literally never remember this problem. Normally it’s a system that is failing / difficult to gain pressure / firm lever feel that’s presented to him.

Whereas I still have the very same (small rub) - but now have a tremendously firm, hard feeling brake lever.

By then he’d been at it 1.5 hours and the shop was shutting. So we called it a day.

Ideas / thoughts / suggestions ?

Strip Caliper completely removing and re-fitting seals ? Buy a new seal kit anyway incase it’s got a faulty / badly fitted / folded seal. Or could it be a Master Cylinder problem.

Where would you head next ?

 

[si_c]

I had a similar issue with a 105 caliper, as you noted it's an issue with the piston sticking slightly, the mineral oil is supposed to lubricate the seal so this doesn't happen but sometimes it fails for whatever reason. I fixed it by removing the pads so I could extend the pistons and cleaned then with disc brake cleaner and then thoroughly wiped them down with a paper towel.

Once that was done I took some WD-40 silicone spray and lubricated the sides of the piston with that, then pushed them in, extended and repeated the process a couple of times. Not had the issue recur.

Edit to add: It would probably have been better if I had stripped the caliper all the way down and rebuilt it but I couldn't be bothered.

 

[Yellow Saddle]

Rule No 1 of disc brakes: Do not expect the brakes to be rub-free or noise-free. If they are, consider this condition as temporary only.

There are many reasons for this but first and foremost is the fact that disc brakes have no pad retracting mechanism at all. The fact that the pads do actually retract ever so slightly when lever pressure is removed, is purely coincidental. Imagine a pistonl moving back and forth in a cylinder and the piston seal is an O-ring (more like a square ring in disc brakes). As you apply brakes for the very first time, the piston moves forward towards the disc and hits the disc. That forward movement is part slide past the seal ring and part, seal ring deforming in the direction of the movement, moving in the direction of the piston travel.. As you release the leaver, the pressure drops just enough to not force the piston against the disc anymore (i.e. it stays in contact but not under force) and then, retract a tenth or two tenths of a mm because of the rubber seal that's relaxing and bringing the piston back with it.

Once you visualise this picture, you can add some unknowns to the mix - dirt, disc run-out, pad uneveness, contamination between piston and seal, etc etc etc. None of this can be engineered out and the discs will rub.

Further, the disc is mighty thin for a plate that's continuously heated and cooled. It does warp a little and unwarps a little. It all depends on how the disc feels on a particular day.

Ever further further...The entire wheel setup is particularly flimsy and your super lightweight (Hunt or DT Swiss or other lightweight nonsense hubs) flex a bit and carries the disc with them when they move, causing little rub rub rub noises. You can even time these noises with your pedaling, as it induces flex in the bike.

Live with it. Hydraulic disc brakes are the dog's bollocks in all aspects except silence.

 

[sevenfourate]

I had a similar issue with a 105 caliper, as you noted it's an issue with the piston sticking slightly, the mineral oil is supposed to lubricate the seal so this doesn't happen but sometimes it fails for whatever reason. I fixed it by removing the pads so I could extend the pistons and cleaned then with disc brake cleaner and then thoroughly wiped them down with a paper towel.

Once that was done I took some WD-40 silicone spray and lubricated the sides of the piston with that, then pushed them in, extended and repeated the process a couple of times. Not had the issue recur.

Edit to add: It would probably have been better if I had stripped the caliper all the way down and rebuilt it but I couldn't be bothered.

That’s exactly what I’m imagining…..needing the Caliper pulled apart and rebuilding. Perhaps with new seals for the hell of it. In saying that a Brand New Caliper can be had for £40. It’s hardly worth the time / bother of buying a seal kit and messing around swapping ? Unless of course it turns out the new Caliper is ‘dry’ and still needs the pistons lubing up as you say.

For £40 it’s hardly worth calling the supplier. And worth just swapping a new one out as a first line of attack. After that - a Master Cylinder / Lever for the front is circa £175. Dealer will be getting a call if it looks like that’s the issue !!

Thanks for your thoughts.

 

[si_c]

That’s exactly what I’m imagining…..needing the Caliper pulled apart and rebuilding. Perhaps with new seals for the hell of it. In saying that a Brand New Caliper can be had for £40. It’s hardly worth the time / bother of buying a seal kit and messing around swapping ? Unless of course it turns out the new Caliper is ‘dry’ and still needs the pistons lubing up as you say.

For £40 it’s hardly worth calling the supplier. And worth just swapping a new one out as a first line of attack. After that - a Master Cylinder / Lever for the front is circa £175. Dealer will be getting a call if it looks like that’s the issue !!

Thanks for your thoughts.

Might be worth just trying a squirt or two of the silicone spray might just lower the friction enough that it stops being a problem and it's only a few quid. I had it lying around anyway as I use it from time to time to lubricate curtain track amongst other things. It should evaporate away leaving a low friction surface coating the ceramic piston which will help it move past the seal better.

@Yellow Saddle is not wrong it's hard to get all of the noise out of a disc brake system, but you shouldn't have issues with the pistons failing to retract properly, you'll just end up glazing the pad and it will fail to operate properly. In that case I'd be looking for corrective measures.

 

[sevenfourate]

Might be worth just trying a squirt or two of the silicone spray might just lower the friction enough that it stops being a problem and it's only a few quid. I had it lying around anyway as I use it from time to time to lubricate curtain track amongst other things. It should evaporate away leaving a low friction surface coating the ceramic piston which will help it move past the seal better.

@Yellow Saddle is not wrong it's hard to get all of the noise out of a disc brake system, but you shouldn't have issues with the pistons failing to retract properly, you'll just end up glazing the pad and it will fail to operate properly. In that case I'd be looking for corrective measures.

You’re probably right. I’m going to order a Caliper anyway however. My time messing around is worth more than £40 ! And I’ll be prepared if I need it.

For reference my calipers are GRX BR-RX400. Pretty sure they are steel pistons. It’s the very top of the line 810’s that have Ceramic pistons. I only know this from much Googling for answers tonight !!

 

[si_c]

You’re probably right. I’m going to order a Caliper anyway however. My time messing around is worth more than £40 ! And I’ll be prepared if I need it.

For reference my calipers are GRX BR-RX400. Pretty sure they are steel pistons. It’s the very top of the line 810’s that have Ceramic pistons. I only know this from much Googling for answers tonight !!

If I was replacing the caliper, I'd probably go all in and get some Hope RX4s, they are apparently better at braking and look nicer too! That being said you can get the RX810 for only a little bit more than the RX400s so I'd probably do that actually.

 

[SpokeyDokey]

Is the caliper properly aligned with the fork ie do the mounts need properly facing?

 

[sevenfourate]

Is the caliper properly aligned with the fork ie do the mounts need properly facing?

Good shout.

And that was considered as an early ‘option’ ! He has the tools to re-face the mount. But chose not to as alignment all ‘looked’ good.

Maybe we owe it to ourselves to do it - if only to get the possibility crossed off the list.

Thanks !

 

[SpokeyDokey]

Good shout.

And that was considered as an early ‘option’ ! He has the tools to re-face the mount. But chose not to as alignment all ‘looked’ good.

Maybe we owe it to ourselves to do it - if only to get the possibility crossed off the list.

Thanks !

As a slight aside. My Shimano RS505 discs (precursor to 105 discs) on my bike here:

https://www.trekbikes.com/gb/en_GB/.../domane/domane-alr/domane-alr-5-disc/p/21571/

Have never suffered any rub or noise apart from on delivery!

They arrived out of skew but, after a few hours of painstaking fettling, I got them perfectly set up.

Similar with the FD, supposed to have four positions but it came with three. Hours of fiddling and it was clear the shop didn't have a clue how to set up the then new 4 position FD's. Spent half a day on the fiddly little bugger and sorted it myself.

Good mechanics with the time to spend on your bike are hard to find.

Watch this outfit, who are located a short ride away from me in Kendal, for some quality mechanical info':

https://youtube.com/@mapdec?si=O7OO1IJsvV4pmf2B

 

[Dogtrousers]

Hang on ... It's a brand new, out of the box purchase and already you're having to replace parts for £40. And presumably you also had to pay your LBS mechanic for an inconclusive 1.5 HR job (and maybe some brake fluid)

I hope you got a good discount on it.

 

[sevenfourate]

Hang on ... It's a brand new, out of the box purchase and already you're having to replace parts for £40. And presumably you also had to pay your LBS mechanic for an inconclusive 1.5 HR job (and maybe some brake fluid)

I hope you got a good discount on it.

Correct.

The other option is to waste my precious time chasing original seller and potential replacement parts / refund etc. *Online purchase as I said. And then wait for parts to arrive with a bike I can’t use until fitted. Or worse still - they demand bike back / want to send a replacement bike......as mine already has lots of aftermarket upgrades fitted. Imagine packaging up. taking to or waiting in for a courier etc.

Or i just get on with it; and dont waste my time......and have a bike I can then use and get sorted out quickly in my own timescale. *Caliper is ordered and will be here and fitted on by Saturday for reference.

Principles are great and I agree with you completely in essence. Until that wastes my time and stops me doing what I want to be doing (Or other things with the time-wasting) / using it for what I bought it for. Or costs me £120 to save £40 😉

 

[raleighnut]

There are many reasons for this but first and foremost is the fact that disc brakes have no pad retracting mechanism at all.

You're wrong there, brake fluid/mineral oil works just as well under vacuum as under pressure, once the brake lever is released the master cylinder returns to it's 'resting' position pulling the brake cylinders away from the disc. It is only a slight pull as the 'self adjusting' element refills the master cylinder from the reservoir but the amount of fluid is carefully metered so that whatever fluid is sucked from the reservoir is less than that which is pulled from the brake cylinders (smaller hole)
I know that my experience comes from larger systems (on Cars mainly) but the principle remains the same as essentially that's how hydraulic systems work they are just 'miniaturised' on bicycles, if they didn't then the system would 'pump'up' leaving the brakes locked on.

 

[sevenfourate]

Rule No 1 of disc brakes: Do not expect the brakes to be rub-free or noise-free. If they are, consider this condition as temporary only.

There are many reasons for this but first and foremost is the fact that disc brakes have no pad retracting mechanism at all. The fact that the pads do actually retract ever so slightly when lever pressure is removed, is purely coincidental. Imagine a pistonl moving back and forth in a cylinder and the piston seal is an O-ring (more like a square ring in disc brakes). As you apply brakes for the very first time, the piston moves forward towards the disc and hits the disc. That forward movement is part slide past the seal ring and part, seal ring deforming in the direction of the movement, moving in the direction of the piston travel.. As you release the leaver, the pressure drops just enough to not force the piston against the disc anymore (i.e. it stays in contact but not under force) and then, retract a tenth or two tenths of a mm because of the rubber seal that's relaxing and bringing the piston back with it.

Once you visualise this picture, you can add some unknowns to the mix - dirt, disc run-out, pad uneveness, contamination between piston and seal, etc etc etc. None of this can be engineered out and the discs will rub.

Further, the disc is mighty thin for a plate that's continuously heated and cooled. It does warp a little and unwarps a little. It all depends on how the disc feels on a particular day.

Ever further further...The entire wheel setup is particularly flimsy and your super lightweight (Hunt or DT Swiss or other lightweight nonsense hubs) flex a bit and carries the disc with them when they move, causing little rub rub rub noises. You can even time these noises with your pedaling, as it induces flex in the bike.

Live with it. Hydraulic disc brakes are the dog's bollocks in all aspects except silence.

Apologies. I missed this.

Avid bike, car fettler here who’s dabbled with hydraulic systems periodically. And a mechanical Engineer by trade - so I comprehend what you’re saying.

None of which explains 2 x bikes in my fleet / 3 total in the household with Hydraulic systems - two of which took minimal adjusting to achieve zero rub ever. The other coming ‘perfect’ from new and has never been fettled since. And remains 100% noise / rub free.

From my experience it’s the one currently rubbing that’s the odd one out. Not vice versa.

 

[Yellow Saddle]

You're wrong there, brake fluid/mineral oil works just as well under vacuum as under pressure, once the brake lever is released the master cylinder returns to it's 'resting' position pulling the brake cylinders away from the disc. It is only a slight pull as the 'self adjusting' element refills the master cylinder from the reservoir but the amount of fluid is carefully metered so that whatever fluid is sucked from the reservoir is less than that which is pulled from the brake cylinders (smaller hole)
I know that my experience comes from larger systems (on Cars mainly) but the principle remains the same as essentially that's how hydraulic systems work they are just 'miniaturised' on bicycles, if they didn't then the system would 'pump'up' leaving the brakes locked on.

You explained the concept well and I've changed my perspective. I have believed (assumed) that the return mechanism is decoupled from the lever's return cycle, which is evident from the bite point gap between lever and piston. However, an internal spring could be what returns the piston.
Nevertheless, I still think the movement in the piston is still small enough to not make the piston slide inside the seal, but just flex it in and out - aided as you say, by vacuum. I'll take my Shimano caliper off this weekend and have a good look again. It will be interesting to put a dial gauge on it and see how much it moves.