Hydraulic brake service questions

[Jules59]

I have Shimano BR-R8070 brakes. (Ultegra hydraulic)

Just wondered:

1) Should the mineral oil brake fluid be replaced routinely ?

2) Is there a recommended minimum pad thickness ? (there's not a lot of pad to start with, but they do last a long time.)

[Nearholmer]

This is the service manual https://www.manualslib.com/manual/15956 ... =10#manual

On a quick skim it seems to say that the pads can run down to 0.5mm, but have a careful read yourself.

Personally, I’ve never “routine changed” the mineral oil in Shimano brakes, it seems to last “forever”.

[Brucey]

....Should the mineral oil brake fluid be replaced routinely ?.....

the usual enemy here is water. This s absorbed by and dissolves in most DOT fluids which is why you are advised to change the fluid regularly. Water does not dissolve in mineral oil, so you might be wasting your time. I have seen puddles of water in mineral oil brake fluid, but for it get in like that requires a very special fault.

So by all means chage the fluid, but if it comes out clean a) don't be too surprised and b) be aware that you might be wasting your time.

[Jules59]

I appreciate both your replies - thanks

I'll get the micrometer out.

SWMBO says I spend a lot of time doing things that are a waste of time - I say, they are my hobbies and I enjoy doing them.

[Nearholmer]

My SWMBO contends that I spend ALL of my time doing largely pointless things, except the one day a week when I still do paying work, so I don’t think your’e too badly off.

[Jules59]

I've got 1.1mm of pad left - plenty.

[TheBomber]

the usual enemy here is water. This s absorbed by and dissolves in most DOT fluids which is why you are advised to change the fluid regularly. Water does not dissolve in mineral oil, so you might be wasting your time.

Presumably in a non-faulty system the moisture enters via the seals on the pistons and this is just as likely with mineral oil as DOT. In a DOT setup it is absorbed into the oil while in a mineral system the water would still be there, just in droplets/pools within the callipers/lines? If so, isn’t this also a problem for braking performance?

[PH]

The theory is that water, being heavier than mineral oil, pools at the bottom, in the caliper. This doesn't effect braking unless they get very hot, water having a lower boiling point than mineral oil. It doesn't help that the standard Shimano bleed instructions are to do it from the bottom up, pushing any water through the entire system. You don't have to follow that, I simply do a gravity bleed.

[PH]

I've got 1.1mm of pad left - plenty.

I change my pads at 0.7mm. partly because that's the thickness of a handy spacer I have.

[Airsporter1st]

the usual enemy here is water. This s absorbed by and dissolves in most DOT fluids which is why you are advised to change the fluid regularly. Water does not dissolve in mineral oil, so you might be wasting your time.

Presumably in a non-faulty system the moisture enters via the seals on the pistons and this is just as likely with mineral oil as DOT. In a DOT setup it is absorbed into the oil while in a mineral system the water would still be there, just in droplets/pools within the callipers/lines? If so, isn’t this also a problem for braking performance?

Isn’t the absorption itself the mechanism by which water enters the DOT system? That being the case, water entry into a mineral oil system is much less likely.

[Brucey]

the usual enemy here is water. This s absorbed by and dissolves in most DOT fluids which is why you are advised to change the fluid regularly. Water does not dissolve in mineral oil, so you might be wasting your time.

Presumably in a non-faulty system the moisture enters via the seals on the pistons and this is just as likely with mineral oil as DOT. In a DOT setup it is absorbed into the oil while in a mineral system the water would still be there, just in droplets/pools within the callipers/lines? If so, isn’t this also a problem for braking performance?

moisture is absorbed so quickly by glycol-based fluids that they recommend that you just chuck the fluid out if you leave the lid off overnight.
In cars, moisture mainly enters via the master cylinder. It also diffuses in through the flexible hoses. Once it is in, it tends to stay in, such is the affinity of glycol and water. Every time the slave pistons extend they do so whilst covered in a very thin layer of brake fluid, which attracts and holds water like crazy. Needless to say, when the pistons react in again, any water is likely to be dragged in too.

Bicycle brakes are different to car brakes but water can still enter the system, eg via the third route outlined above or via diffusion through the flexible rubber MC reservoir cover. Neither mechanism is at all likely to work in a mineral oil system.

Whilst I have seen liquid water in a mineral oil system, I think it took actual suction to put it there. If anything jams the slave piston during retraction, the system will develop suction whilst the compensation port is closed. In the system I found water in, I also found a sticky caliper piston, which freed up nicely once I'd removed a tiny piece of swarf.

There is a long and fairly complex argument against the formation of liquid water within mineral oil systems, based on surface energies, nucleation, and growth, but I won't go into it here.

So it is by no means impossible for liquid water to get into a mineral oil system, but it is highly unlikely. By contrast, in a glycol system it is more or less inevitable that water will get in somehow and depress the boiling point.

[tim-b]

Assuming that the braking system is healthy, as yours seems to be, an important service item is to clean around the exposed pistons before replacing the brake pads.

That means that when you retract the pistons to fit new, thicker pads, you don't push grot into the seals to damage them and the pistons

If you have threaded pad axle bolts take them out once or twice a year and lightly copper grease them to prevent them corroding in place, but not so much that it's a threat to braking

[TheBomber]

Thanks Brucey - a comprehensive reply as always. I'd always preferred mineral oil just because it is nicer to work with. Now I wonder why anybody would make bicycle brakes that use DOT.

[rareposter]

Now I wonder why anybody would make bicycle brakes that use DOT.

Because it's very widely used in the automotive industry and it's cheap. It's also highly regulated independently so it means no matter where you buy DOT fluid, it's the same.

I have Shimano BR-R8070 brakes. (Ultegra hydraulic)

Just wondered:

1) Should the mineral oil brake fluid be replaced routinely ?

2) Is there a recommended minimum pad thickness ? (there's not a lot of pad to start with, but they do last a long time.)

1. Every year or so is regarded as standard although you can go longer without much adverse effects. On the other hand if you're using brakes hard (like full on DH racing) then much more frequently. It's not "water" in the system you need to worry about as such, it's air (which obviously contains moisture) and general degradation of the fluid.

2. Shimano say 0.5mm minimum on the pad. Again, if you have to ride home with 0.45mm on it, that's not going to be the end of the world; where it becomes a problem is if there's 0.1mm left and you end up scoring the rotor with the backing plate or popping a piston out.

[Jupestar]

Now I wonder why anybody would make bicycle brakes that use DOT

DOT has a higher boiling point, so it performs better under extreme use. but needs servicing regularly.

I've had DOT brake pads pushing onto the rotor as the system absorbs additional mosture, thats the typical point of service required.

Never had this with mineral oil, in fact only failures i've had are when fluid leaves the system. via a leak. if it not the hose, its normally caliper seals. At this point its a repair (and not usually a simple one) rather than a service.