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Hydraulic Brake Bleeding Interval

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Hydraulic Brake Bleeding Interval

Old 02-01-23, 05:00 PM
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Spandex_fairy
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Hydraulic Brake Bleeding Interval

How often should I be bleeding the brakes on my road bike? Is it like a car, where I can pretty much ignore it?
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Old 02-01-23, 05:27 PM
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when they feel mushy or don't work as well after cleaning them. or in one case when my bike fell over.
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Old 02-01-23, 05:40 PM
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I really depends upon your anal-ness. How much of a maintenance nut you are. I am a maintenance nut, thus I keep my bikes very clean and operational to the Nth degree. On a road bike if it has hydro brakes, flush the system every year, especially if using DOT fluid as it collects moisture from use. Mineral oil systems tend not to do that, thus some mechanics say it is 'forever fluid", meaning mineral spirits don't need flushing for the life of the product. If it were mine, it would be flushed once a season. When I was service mech, I always recommended annual brake flush to the customer. Of course few did that, in fact only one person followed my advice, and his bike was something I would be proud to claim because it was well cared for.
Flushing the system gets the moisture and dirt out of the system. This is imperative for long term service life of the product. On my cars I flush the system every 3 years, motorcycle every year. I have a 40 year old vehicle still running with the stock brake calipers. Don't see that often as most will seize up long before the 40 year mark. Mama said cleanliness is next to godliness.
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Old 02-01-23, 06:18 PM
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I'm using Shimano/ mineral oil; looks like I'm good for some time then. The brakes were mushy from brand new. The service guy told me that's how it's supposed to feel. I didn't believe him so I took the bike home and flushed it myself (multiple times). It seems he was right because nothing changed. I'm using the short reach shimano 105 shifters that came on my 52cm bike. What a difference in pull stroke and force vs performance moto master cylinders, such as a brembo rcs19 or an Accosatto equivalent.
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Old 02-01-23, 06:33 PM
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If you feel like performance has gone down I would re-bleed or if you get air in your lines. I might swap it out every few years at least if I am riding a bunch just to keep it fresh and keep my braking excellent.
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Old 02-01-23, 09:01 PM
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I’d say to basically ignore fluid changes, and maybe just do it for kicks when you need to change rear pads…which should be a good, long, time. There’s no need for routine changes, not in my experience, which includes 10 years on Shimano Hone which are stiil in service with my brother at about 20 years old with only 2 fluid changes that whole time! Okay, maybe three changes…I forget, but it has been a long time of hassle-free service!
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Old 02-01-23, 09:22 PM
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DOT fluid: annually, as it is hygroscopic (absorbs water)

Mineral oil: if/when it has air in the line or leaks

I had a front brake (Shimano Ultegra, so mineral oil) that was perfectly fine when I finally bled it at 6 years. If the fluid is dirty, it is a good idea to change it. But you only really know that if you bleed it.
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Old 02-01-23, 09:23 PM
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I assume this is posted in General as a courtesy to cxwrench
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Old 02-04-23, 11:13 AM
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What are the consequences if I don't flush my hydraulic brakes?
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Old 02-04-23, 12:19 PM
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Originally Posted by Spandex_fairy
The brakes were mushy from brand new. The service guy told me that's how it's supposed to feel. I didn't believe him so I took the bike home and flushed it myself (multiple times). It seems he was right because nothing changed.
Perhaps you are just getting more lever travel because the pads are nearing replacement time.


Originally Posted by zsid
What are the consequences if I don't flush my hydraulic brakes?
since the hose and most everything else is plastic, not much. Brakes systems on cars with metal brake lines will get corrosion from the water that the DOT 3 fluid absorbs. But even in cars that usually takes many years to get to the point where it's and issue for those cars. Then it's quite costly.
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Old 02-04-23, 01:37 PM
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All my brakes are mineral oil. The one DOT system I used was a SRAM that didnít even make it to its one year mark, since i disliked it then sold it so quickly.

I am having a hard time understanding why it would absorb water. The system is sealed. Air canít enter and neither can water. So how does this happen?
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Old 02-04-23, 01:40 PM
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Originally Posted by zsid
What are the consequences if I don't flush my hydraulic brakes?
The fluid will look like one would expect if you fail to flush.
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Old 02-04-23, 01:47 PM
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Originally Posted by rosefarts
All my brakes are mineral oil. The one DOT system I used was a SRAM that didnít even make it to its one year mark, since i disliked it then sold it so quickly.

I am having a hard time understanding why it would absorb water. The system is sealed. Air canít enter and neither can water. So how does this happen?
DOT fluid can absorb water from the air, even if it is just sitting on the shelf.

The hydraulic system is sealed, but contaminants can seep in, fluid can seep out, especially at the piston interfaces. Unlike inner tubes (which are also semi-permeable), there is no net positive pressure when the brakes aren't being squeezed, so there can be a gradual exchange with atmospheric water vapor (or whatever is on the surface of the pistons).

It happens with mineral oil as well, but mineral oil has much less of a propensity to absorb water molecules from the air.
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Old 02-04-23, 02:00 PM
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Originally Posted by Polaris OBark
DOT fluid can absorb water from the air, even if it is just sitting on the shelf.

The hydraulic system is sealed, but contaminants can seep in, fluid can seep out, especially at the piston interfaces. Unlike inner tubes (which are also semi-permeable), there is no net positive pressure when the brakes aren't being squeezed, so there can be a gradual exchange with atmospheric water vapor (or whatever is on the surface of the pistons).

It happens with mineral oil as well, but mineral oil has much less of a propensity to absorb water molecules from the air.
Makes sense, PEX tubing does the same thing so radiant flooring systems have air bleeding built into them. Aluminum lined PEX is supposed to avoid that. I wonder if that would work with brake lines.
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Old 02-04-23, 03:53 PM
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I don't think it is the hoses. The pistons and master cylinder (and possibly bleed port) seem the most likely.
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Old 02-05-23, 03:26 AM
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My experience with autos and motorbikes suggests it is the seals at the caliper and master cylinder pistons that allows contaminants into the system, and if contaminants are getting in there, so is moisture. I'm sure that moisture can also find its way into the system through the hoses and connectors as the system heats up and cools down (expansion and contraction).

Anyway, an ounce of prevention is worth a pound of cure. Flush and bleed your system on a regular basis to ensure optimal performance and long term service from it. Shimano is silly easy to do and leaves little room for excuse not to do it. The others require more effort and patience.
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Old 02-05-23, 07:32 AM
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I'm very new to hydraulic brakes and am not impressed with the stopping power. I wonder if I need new fluid? I can bottom out the levers, the braking force is really no better than rim brakes thru the range of motion.

I bought the bike in November 2022 and it only has about 1100 miles on it, but it sat on the showroom floor for just over a year. I don't know if it uses mineral or DOT. SRAM Red AXS HRD brakes.

Should I buy the tools and learn to purge? Or do I expect too much?
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Old 02-05-23, 07:57 AM
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SRAM uses DOT 5.1 or 4 fluid, not 5. Buy the bleeding edge kit with everything you need. Flushing every year shouldn't be necessary, but try it and see if the fluid is darker than new. If the fluid still looks new, the main objective is to remove air. I switched three bikes over to SRAM hydraulic last year, so I've done several bleeds already. The first Bike will be a year old in July.

If mineral oil is so great cars and motorcycles would use it but almost none do. Water in the system drops to the calipers where it's most likely to boil.

Last edited by DaveSSS; 02-05-23 at 08:08 AM.
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Old 02-05-23, 08:05 AM
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Originally Posted by GhostRider62
I'm very new to hydraulic brakes and am not impressed with the stopping power. I wonder if I need new fluid? I can bottom out the levers, the braking force is really no better than rim brakes thru the range of motion.

I bought the bike in November 2022 and it only has about 1100 miles on it, but it sat on the showroom floor for just over a year. I don't know if it uses mineral or DOT. SRAM Red AXS HRD brakes.

Should I buy the tools and learn to purge? Or do I expect too much?

They should be much better than even the best dual-pivot rim brakes, but some of the cheaper SRAM/Avid brakes seldom live up to the promise. Pretty much any Shimano should "just work." However, new pads (possibly) and rotors (unless something is wrong, 1000 miles shouldn't be a problem with what you have) could go a long way to making it work better. If the brake levers feel squishy, or if there is some other obvious issue, a full bleed is warranted. It might be worth having a competent bike shop do this to begin with, so that you know everything is done correctly, and you will get an idea for how they are capable of performing.

I have no experience, but I think Red AXS HRD should work well, and nearly all SRAM uses DOT fluid, which is corrosive and harder to work with (I personally avoid SRAM brakes because of this).
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Old 02-05-23, 08:17 AM
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DOT fluid, left sitting on painted parts can damage the paint, but it is not the least bit difficult to use. It wipes off with a damp rag. Calling it corrosive is a stretch. I don't bother to wear nitrile gloves when bleeding. I've been bleeding automotive brakes since 1969.
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Old 02-05-23, 08:18 AM
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Originally Posted by DaveSSS
If mineral oil is so great cars and motorcycles would use it but almost none do. Water in the system drops to the calipers where it's most likely to boil.
The reason it's not used in motor vehicles isn't the water boiling, it's the mineral oil boiling. (590į)
Car and motorcycle brakes operate at a much higher temperature than bicycle brakes do.
In racing applications you very well might see glowing rotors. (mostly carbon brakes, but steel too.)
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Old 02-05-23, 08:28 AM
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Originally Posted by zsid
What are the consequences if I don't flush my hydraulic brakes?
I had a mountain bike with DOT brake fluid for 10 years. Never flushed the fluid, never even opened the system. When I sold it the front caliper was getting sticky and making the brake drag sometimes. I told the buyer and they fixed it.

I have a mountain bike now with mineral oil brakes. Same routine, never bled or even opened the system, works fine.

I never flushed the brakes on any car I had. I had a Blazer for 18 years and I put brakes on it when I got it including rebuilding the rear wheel cylinders. The next time I put brakes on it was at least 100,000 miles later (mostly highway) and the cylinders were nasty but I just rebuilt them again. Never opened the front calipers.

I don't recommend doing maintenance like I do, however. I tend to let things go, works for me.

Some car makers recommend brake fluid flush, some don't.
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Old 02-05-23, 09:07 AM
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Originally Posted by Spandex_fairy
I'm using Shimano/ mineral oil; looks like I'm good for some time then. The brakes were mushy from brand new. The service guy told me that's how it's supposed to feel. I didn't believe him so I took the bike home and flushed it myself (multiple times). It seems he was right because nothing changed. I'm using the short reach shimano 105 shifters that came on my 52cm bike. What a difference in pull stroke and force vs performance moto master cylinders, such as a brembo rcs19 or an Accosatto equivalent.
Maybe you should not -flush- the brakes, risking introducing air to the system. My tiagra hydro brakes was getting a little mushy after about one year. I only opened the top screw, put in the funnel with oil in it and and let out some air bubbles. Ive found pointing the front wheel down (like riding down a very steep hill) and gently tapping the brifter as you pump the lever easily lets the bubbles out. Airing out the system helped firm up the lever feel and it was a 10 min job for both brakes.

Last edited by Racing Dan; 02-05-23 at 09:13 AM.
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Old 02-05-23, 09:29 AM
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Ah yes, the beautiful simplicity of the Shimano hydro brake systems. Good, solid engineering is tough to beat which has a whole lot to do with why they are the big gun in the parts industry.
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Old 02-05-23, 09:51 AM
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Originally Posted by Polaris OBark
They should be much better than even the best dual-pivot rim brakes, but some of the cheaper SRAM/Avid brakes seldom live up to the promise. Pretty much any Shimano should "just work." However, new pads (possibly) and rotors (unless something is wrong, 1000 miles shouldn't be a problem with what you have) could go a long way to making it work better. If the brake levers feel squishy, or if there is some other obvious issue, a full bleed is warranted. It might be worth having a competent bike shop do this to begin with, so that you know everything is done correctly, and you will get an idea for how they are capable of performing.

I have no experience, but I think Red AXS HRD should work well, and nearly all SRAM uses DOT fluid, which is corrosive and harder to work with (I personally avoid SRAM brakes because of this).
Thanks. It is probably quicker to learn it myself. The pads are kind of shot already. I'm going to replace them and maybe do the fluid at the same time. (One problem I have not been able to solve yet is the caliper slowly moves on the fork despite having the specified torque and then this is a squealing pain to my ears), It can't be much different than bleeding the brakes on my car.
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