Jim’s Tech Talk
By Jim Langley
Last week I told you about attending a mechanics class on hydraulic disc brakes at the United Bicycle Institute #bikeschool in Ashland, Oregon. It was well worth the time and cost ($250). Here’s that story: https://www.roadbikerider.com/united-bicycle-institute/.
This week I’m sharing 10 tips about hydraulic discs and working on them that I learned at the seminar.
1. A cool tool for attaching disc hoses/lines to levers
If you look closely at the illustration here I hope you can see that there’s a nut with a wrench on it. That nut is what attaches and seals the hydraulic hose to the lever. Because the nut is so close to the dropped handlebar, it’s difficult to tighten it sufficiently with a torque wrench (usually used with a crow’s foot adapter) because there’s not much room to work.
The risk is that the crow’s foot might scratch or cut the handlebar. Which would be a very bad thing because scratches and cuts in aluminum and carbon often lead to breaking the bars.
We learned in the class about an awesome tool that solves the problem, which is shown in the second photo. It’s an 8mm ¼-drive socket. Notice that it has a slot in it. This allows slipping the tool over the hydraulic hose so that the socket stays aligned and cannot touch the handlebar. Here’s a link to the tool: https://amzn.to/3Ftow6N
2. Getting air out of syringe
When bleeding brakes, syringes are used to push hydraulic fluid through the lines. To do this you feed the syringe hose into the container of mineral or DOT fluid and pull its plunger to suck in the right amount of fluid.
At this point you want to get any air out of the syringe because if you don’t, you will push the air into your brakes – if you don’t know any better than to try to put the fluid in with air still in the syringe.
In all the directions and videos I’ve watched, the way that’s shown to get the air out is by inverting the syringe so that the hose is on top. A rag is placed over the end of the hose to catch any fluid that comes out. You then are supposed to push the plunger until all the air is out of the syringe.
This doesn’t work. Because it’s missing one critical step. Which is to first pull on the syringe plunger to draw 100% of the fluid back into the syringe. This will also ensure that 100% of the air rises to the top of the syringe. At this point you can easily push all the air, and only the air out of the syringe.
If you don’t do this first step and you try to get all the air out simply by pushing on the plunger, you will push a lot of the fluid out with the air and probably won’t get all the air out either.
3. Creating a vacuum without sucking air through the fittings
During the bleeding process there are times when you need to create a vacuum in the syringe and the instructions say to “pull on the syringe.” I always read this to mean that you pull on the plunger enough to move the plunger inside the syringe.
Actually, pulling on the plunger hard enough to move it is a huge mistake if all you need is a vacuum. Because the bleed fittings that attach to the calipers and levers are tiny and barely connected. If you pull that much on the plunger you suck air past the fittings and right into the lines, which is absolutely not what you want.
And, unless someone explains that what you’re doing is wrong, you will probably think you’re succeeding in removing air from the system, which would be a good thing – except that that’s not what’s going on! You are drawing air from outside into the brakes and you definitely do not want that. To do it correctly you just put a slight pressure on the plunger to create a vacuum and wait to see if any air bubbles come up into the hose and syringe.
4. Hydraulic hoses are high tech
Until this class I had no idea that hydraulic hoses, even when full with fluid, are significantly lighter than any metal cable and housing system. Also, quality hydraulic hoses can withstand pressures up to 1,800 psi. Also, the thing that makes disc brake fluid superior to brake cables for braking is that it cannot be compressed (brake cables can stretch).
5. Squeaking is normal with discs and you can create it yourself if you try
According to the UBI instructors disc brakes are susceptible to squeaking under certain conditions, such as when it’s raining or wet.
They said it’s normal to want to try to stop squeaking but that sometimes with discs you just have to accept that the brakes squeak under those certain conditions and not worry about it when it happens.
One of the techs mentioned a customer of his who would walk around dragging his discs in the bike shop to get them to squeak so he could complain about it. Which the tech pointed out was the perfect way to make them squeak worse because the customer was putting a glaze on the pads.
Switching brake pads can help stop squeaks. Resin and organic pads are less prone to making noise than metal pads.
6. Bedding in, burning in new disc brakes and what it does
All new disc brakes should be broken in with a process called bedding in or burning in. This transfers material from the new brake pads to the new rotors. On a microscopic level this treatment smooths imperfections in the surface of the rotor and it also helps align the pads with the rotor.
To bed in new brakes, you get up to speed and then apply the brakes to slow down. You don’t slam on the brakes and you don’t stop completely. The goal is not to get the rotors hot. You repeat this bedding in braking about 20 times.
7. Use the whole manufacturer’s system / don’t mix up parts
Whichever disc brake system you choose, the UBI instructors say to stick with it as a system. Do not mix and match parts. Absolutely do not use a brake fluid not made for your system. The best way to do this is to only use the fluid made by the brake manufacturer.
8. Follow the manufacturer’s bleeding procedure
Just like tip 7, team UBI said to follow the bleeding steps as specified by the brake manufacturer. They said that mechanics may have bled other hydraulic systems and then want to use “shortcuts” to save time or simplify the process.
But they said that bicycle disc brakes are unique and not the same as other systems (such as on cars and trucks), and taking shortcuts can and likely will lead to brake problems. Along the same lines they recommended only using the bleeding kit designed for the brakes and also the one made by that company.
9. You may not need to bleed the brakes
One of the best tips at the seminar was that bleeding may not be needed some or a lot of the time – even if you’re convinced it is (depending on your riding conditions). For example, worn out brake pads will feel like there’s not enough fluid in the lines to stop. Replacing the pads will restore the braking not bleeding.
If air or dirt gets in the fluid and the levers feel spongy that’s a sign that bleeding may be needed.
10. How to tell if rotors are worn out
This is an easy one but not everyone may know it. Rotors should have a minimum thickness written right on them (photo). So by measuring them you can tell if it’s time to replace them. Speaking of replacing rotors, the UBI techs said that if they run into a bent rotor they will try to straighten it (their favorite tool is Park’s Rotor Truing Fork https://amzn.to/45LAxiy).
However, after 10 minutes or so if they can’t straighten it, they will replace the rotor. Because rotors do a ton of work and are subjected to a lot of heat, wear and tear. So if it’s bent it might never become straight again and a new rotor is best to restore the braking fully.
I hope these tips help with any disc brake servicing you do.
Jim Langley is RBR’s Technical Editor. A pro mechanic & cycling writer for more than 40 years, he’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Tune in to Jim’s popular YouTube channel for wheel building & bike repair how-to’s. Jim’s also known for his cycling streak that ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.
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