Routine brake bleeding
 

These are my first bikes with hydraulic brakes so other than being able to install them successfully, I have no experience with maintenance other than adjusting the calipers and knowing how to replace pads. My wife and I got them 3 years ago (spring 2021) and I built them up with Sram AXS Force 2x with the standard Force hydraulic discs (DOT fluid). So, 3 seasons of use. We have other bikes we ride quite a bit, so I imagine they might have 2,000 miles each, not a lot.

My simple question is: Is there any reason to do routine brake bleeding and/or fluid replacement on DOT brakes if the levers are still firm and the brakes work well? I know that if they get squishy, it's time, but how about routinely in the absence of any issues?

Thanks.

Do you own a car? Have you ever bled the brakes other than after replacing the discs?

Same with bikes. There's no reason to bleed brakes unless you have symptoms of air in the lines, ie. spongy feel, brakes drag on hot days, etc.

Brake fluid does break down over time and extreme use. Pump out the bubbles at the levers if they get spongy and check periodically for oxidation. If the fluid is turning black, it's getting time.

DOT fluid absorbs water over time. I've seen recommendations to bleed them twice a year, especially if they've been exposed to wetter conditions, but minimum of a year is the common guideline.

I waited 4 years to bleed the only bike of mine that has SRAM hydro brakes, and the old fluid was slightly discolored but braking wasn't too bad prior. Hopefully my procrastination didn't lead to any damage of my components.

I've heard once per year, but my guess is that as long as they are working as well as on day 2 (allowing for bedding in here), I wouldn't worry about it. DOT fluid is hygroscopic.

Cars are every 3-5 years depending on how hard you use the vehicle. I wouldn’t be doing my bikes every year unless they feel spongy. I just did a mt bike to reduce some travel, worked OK, it’s 2 years old, but are low level SRAM things.

My Ultegra front brakes haven't been bled in 9.5 years and is still fine, but Shimano uses mineral oil, which is much less hygroscopic than DOT fluid.

Just for clarity, "bleeding" is getting the air out - which is different than replacing the fluid and then bleeding it.

Right. That's why I posed my question:
Is there any reason to do routine brake bleeding and/or fluid replacement on DOT brakes if the levers are still firm and the brakes work well?
So far, I'm inclined to follow your advice above - no reason to do either if the brakes are working well.

But I was wondering if there's any reason to actually replace the fluid at any point.

I don't bike with hydraulic brakes, but the idea of replacing fluid every so often in a car is not only to have clean, fresh fluid, but also to ensure that your fluid doesn't boil during heavy braking. When automotive brake fluid is pressurized, it can get very hot (350-500F).

Edit: I saw some impressive braking recently at a monster truck show. The brakes are visible in the drivetrain under the truck and can glow very bright orange and catch fire.

I'm not certain, but my recollection is that DOT breaks down into chemicals that damage the seals in the brake system, hastening its demise. But it also just doesn't work as well. If it didn't matter what's in you hydraulic system, we'd use water.

Mixed with anti-freeze in some places.

Any liquid is in-compressible and would work in principle. The different grades of fluid lubricate the parts and battle corrosion and boiling.

Again, I don't bike with hydraulic brakes, but I've done hydraulics on cars plenty of times.

The cool thing about hydraulics brakes, and I'm not sure if this is common knowledge here, is that they can work just like gears and a chain. A long and narrow cylinder that you press on can provide a massive mechanical advantage to a shorter, wider cylinder that is actually doing the braking.

Yes, it is very similar to how a long brake lever interacts with a caliper through a cable.

I would not bleed the brakes if they are firm.

Right on. If it ain't broke, don't fix it.

However, the danger is that the brakes might become spongy so gradually that while your perception is that they are still great, the reality is that enough air has entered the system to affect performance.

we had a bike that sat for around 10 years in the garage untouched / unused

hydraulic disk brake bike - low / mid level Shimano

just got it out this past summer … did a quick clean of the rotors with iso alcohol - thats it

littlest t2p rode the bike remainder of the summer - including nasty off road downhills … brakes worked well - firm feel - no squeal (whew)

Update: of the four hydraulic brake levers in the family, three are dead solid and the pads are also plenty thick. If I wasn't a BF denizen and more of a normal bike rider who just rides and only pays attention to stuff that isn't working right, I wouldn't give them an iota of thought.

One lever is a little spongy although it brakes before hitting the bar. If I were the non-BF person described above, I'd ignore it I'm sure. I'll probably bleed it because it's spongy, plus it's something to do while I wait for spring and will reinforce my very beginner hydraulic skills.

You'll find lots of helpful videos on YouTube. It probably doesn't need a full bleed; I would look for instructions on 'brake lever bleed' first and only go on to a full bleed if that doesn't work.

If you plan to keep the bike for 30 years, flush and bleed the system on a regular basis, perhaps every 2 years. If the bike is a disposable unit, one that you will get bored with in 10 years just let it go and pass it on to the next owner.
Just like an automobile the brake fluid is not only used for compression it is used as a lubricant within the system. Fresh lube keeps things working well for a very long time. I have a 40 year old Volvo that gets regular fluid flush and bleed and still using the original calipers and master cylinder. The bicycle is no different.

I like the Park webpage instructions and the Shimano dealer manual for the STI shifter.
As a hydro-newbie I found written directions better than video, as there's a lot of steps and with text it was easier to keep track of where I was in the process.

I did remove the rear caliper from the frame so it could hang as low as possible.

My take is that if you are the kind of rider who barrels down mountains like you are immortal, then you better know the manufacture's recommendations for service, and either be a darn good mechanic yourself or have one on standby. The way I ride, I could likely stop with my foot even if they failed, which they likely won't, just all at once.

Even the most daredevil rider racing down alpine switchbacks, needn't worry.

Contaminated brake fluid doesn't cause sudden brake failure. As the fluid absorbs water it becomes compressible. This eventually manifests as spongy feel, and gets worse very slowly giving you plenty of time to deal with it.

Not only all of this, but as far as I know, you also always have two completely independent systems. The front and rear brakes don't share any point that might fail.

They're SRAM brakes. At some moment the issues will came from nowhere and you'll be bleeding them at least twice for each couple of pads you use. After more miles/time, you can expect having to do a master cylinder overhaul and maybe calipers too. I have had too many SRAM brakes (and still have a road brake with Rival HRD) and they all deserve to burn in hell.

That being said: yes, there's a reason to replace fluid regularly even if they work well. DOT fluid is hygroscopic. That means that it absorbs moisture. If the water content on the fluid is too high, it can boil when you brake and you definitely don't want that. That's the same reason why brake fluid is regularly replaced on cars.

In mineral oil brakes, liquid also has to be replaced because, despite the liquid not being hygroscopic, moisture still can get inside and, water being heavier than oil, it tends to pool at the lowest point of the system (the caliper), posing the same boiling risk.

This being said, I used a Shimano SLX for 7 years without bleeding. I could only wish my SRAM brakes lasted half of that without an issue popping up from nowhere... they're 6 years old and I've lost count of the times I have bled them. I think I'm bleeding them at least a couple of times a year.

Theory:
Brake fluid is used to actuate the brakes, but it also lubricates the whole system and protects it from rusting inside.
Water and dirt enter the system over time (seals are not perfect).
Water lowers the boiling point and can allow rust to build up, while dirt can make the calipers not move freely enough.
Water and dirt are probably not helping the seals last longer either.

Practice:
I've seen both DOT fluid, and mineral hydraulic oil brakes work fine after years of non-servicing.
I've also often replaced fluid in brakes that started working poorly after years of "negligence," and they worked fine for years after the fluid change (so, there was probably no permanent damage due to non-maintenance).
It is a fact that very ugly & dirty looking fluid usually comes out of such brakes.

Conclusion?
Based on my knowledge & experience, I recommend to most people to stick with good old rim brakes or "at least" use mechanical disc brakes. :)
But, for those who insist on hydraulic disc brakes, I would still say that once a year is a reasonable servicing schedule that one can easily stick to (every two years is OK, unless you mix up if you do it on odd or even years :) ).
Can you replace the fluid less frequently?
Yes, and it will most probably be fine - just don't blame me in case of any problems (that are really not likely, at least in my experience - no sarcasm).

P.S.
Brake performance can degrade gradually over time.
Some people whose brakes "are fine," notice the difference once the fulid is replaced (and any air removed, of course).

Relja