Magura RT8 brake How-To
Written by: Greg Kopecky
Added: Fri Mar 29 2013
What’s the big deal with the RT8? It’s hydraulic. Meaning – there is no brake cable to actuate the caliper; it uses fluid to do the job (similar to your car). They are also the norm on modern mountain bikes. Hydraulic brakes bring a few key advantages to bicycles:
1. The system is closed to the outside environment. There are no brake cables to get dirty and sticky over time.
2. They tend to offer better modulation and feel. In layman’s terms, there is a larger zone between “zero braking” and full wheel lockup. That is generally accepted as a good thing.
3. They have the potential to be light weight.
4. For triathlon – and this is a biggie – they open up a potential world of possibility for aerodynamic brake design. If you want to place the brake in an odd location, cable brakes have a tendency to lose their ‘feel’. A hydraulic line doesn’t ‘care’ where it has to go and there is no cable to get kinked.
All of those things sound great to me. However, I always ask two questions: 1) Does it work? 2) Do the benefits outweigh the costs?
This in mind, I wanted to get my hands on the system and install it on a bike. The RT8 mounts to any standard frame or fork with a single brake bolt. I wanted to approach this as though I was a consumer wanting to upgrade an existing bike with an aftermarket RT8 brake. In my experience, this is always the more difficult path; stock bikes normally have some measure of installation done for you ahead of time (i.e. internal cables routed, etc). How difficult is the RT8 starting from scratch? Let’s find out.
Also note that the brakes do NOT include pads, pad holders, or any pad hardware. You must source this on your own. Magura representatives told me that they are working on their own design for this, which will be included in the future. I was unaware of the lack of any pad hardware when I received the brakes, so I resorted to rummaging around in my parts cabinet. I ended up finding enough SRAM Force pad hardware to spare, along with Zipp’s now-discontinued Carbon/Carbon pads (which, incidentally, is still one of my favorite pads for alloy rims).
Magura says that you should always trim the line from the caliper end; do not trim from the lever end. To remove the line from the caliper, unthread the bolt with an 8mm open-ended wrench:
Next, install the caliper on to the bike. Don’t go crazy tightening it down – you will have to remove it again later. This step is only to help cut the line at proper length.
This is also the step where I realized that I was going to have to improvise a little bit:
However long you cut the line is up to you. If you install the RT8’s on an existing bike (that fits you), cut them as you normally would. If you still have potential fit adjustments to make, my advice is to leave the lines a little bit long. It is always easier to trim them later than to install a completely new line.
Once your final cut has been made, it’s time to install the line fittings that we cut off earlier. First, remove the caliper. You should reuse the 8mm bolt, but do NOT reuse the olive. Magura supplies a couple spare olives with the brakes specifically for this purpose. Their design – like many hydraulic bicycle brakes – relies on this one-time-use item to be crushed when you tighten the bolt, making a tight seal.
Now it’s time to bleed the brake. Why bleed brakes? The process of bleeding removes any air bubbles from the system. Unlike liquid, air can be compressed. If you have air bubbles in the line, your lever will feel mushy as you squeeze it – you are compressing the bubbles instead of moving fluid. Bleeding is extra important on the RT8 system because it is completely closed. There is no fluid/air reservoir to catch bubbles (unlike most mountain bike brakes). The master cylinder pushes directly on fluid, which moves the caliper piston.
Before you begin bleeding, Magura recommends that you tilt the bike down slightly to position the brake levers in a way that avoids air bubbles. If your base bar tilts up like my Profile-Design T2, you should tilt the bar down more than a base bar with flat grips.
My work stand attaches to the bike at the fork and bottom bracket, so there is no real way to angle the bike. What to do? Improvise:
Take out your first syringe, stick the hose in your fluid bottle, and fill it up to about the 25ml mark. Then, turn the syringe upside down to let the air bubbles rise, and push them out. You’ll want to wrap the end of the tube in a paper towel to catch any fluid that sprays out.
While we could proceed with the bleed from here, I like to do one extra step – I ‘bleed’ the syringe. Put your finger over the end of the tube and pull down on the plunger. You’re creating vacuum. What this does is make any tiny hidden air bubbles make themselves known:
Next, push the barb fitting in to the end of the hose:
Now, remove the bleed port screw on the caliper with a 3mm allen key (not shown), thread the bleed syringe in, and squeeze the top of the caliper to ensure the pads are fully opened:
You can also open the brake quick release, shown on the right (the red dot shows its point of origin):
Now we need to attach the top syringe to catch fluid at the lever. Remove the EBT screw from the top of your brake lever with a T25 Torx.
The supplied hose almost fit in to the bleed port, but not quite. I happened to have some ¼” vinyl tubing handy, which was more flexible than the stock tubing. I cut a piece of this, smashed one end down with pliers, and managed to jam it in to the lever’s bleed port. It was difficult, but actually sealed quite well.
I later found a third set of instructions online in PDF form, which include a graphic for what you’re actually supposed to do with the cheaper bleed kit:
Regardless of whether you use the syringe, the Pro bleed kit, or my jury-rigged bleed kit, the next step is the same – start to push fluid from the bottom syringe:
Most mountain bike brake systems rely on a more complicated bleed procedure – often with vacuum created at both the lever and caliper. The nice thing about the RT8 system is that the inner diameter of the hose is quite large – 2.5mm, vs about 1.5mm for most MTB systems. This means there is a much smaller chance of air bubbles getting caught in the lines – so you don’t need as much vacuum on the line for bleeding.
Once the air bubbles stop, you can disconnect the top syringe. Note that if you do not have the Pro bleed kit, there is a 100% chance that you will spill fluid while removing the syringe. Be prepared with paper towels and do not wear nice shoes. My fashion-forward Crocs fit the job perfectly.
With the top syringe out, replace the EBT port screw with your T25 Torx. Then remove the bottom syringe and replace the 3mm bleed port screw.
To test the system, squeeze the lever. You should see the caliper move immediately. If there is any lag time, the bleed was not done properly.
The 2.5mm adjuster works well, but I found a potential problem when using a narrow rim. My front rim, for example, is a 20.5mm Shimano C24 clincher. It’s not super wide, but also not as narrow as the 19mm rims of yesteryear. As you bring the pads close to the rim, the caliper spring tension goes down. I found that when I had the lever throw where I wanted it, the spring was quite soft – resulting in a slow return. Magura says that this can be remedied by using one or more small washers between the caliper arms and brake pads (similar to what other modern TT brakes do). The only problem is that none are included with the brake. I didn’t have any in my parts bin, so I’m left with a brake that feels softer than I’d like.
As far as hydraulic brakes go, the design is very simple. That being said, I’m guessing this installation and bleed is more complicated than about 80% of triathletes want to deal with. That’s not a deal-breaker, however, as many bike shops have the tools and experience to install these brakes. If you are a retailer, I advise that you invest in the Pro bleed kit, as it will make your life easier. The bonus is that once the brakes are set up, they’re essentially maintenance free. Magura guarantees the fluid for at least five years. You still likely want to lubricate the pivots every once in a while, but that’s no biggie.
My only real beef with the brakes is the fact that they don’t include pads, pad holders, or washers for spacing them in. With a wider rim on the bike, I found the lever feel to be much improved. Magura says that the lever itself was designed with aerodynamics in mind, which makes sense for the application. I personally would like to see a lever that is 1-2cm longer, and sweeps away from the rider towards the end of its length. The current lever is completely flat, and does not suit my preferences for a secure feel (i.e. on rough pavement). With my base bar, I can only reach the lever with my index finger. Of course, the reach and shape of your base bar highly affects the reach to your brake lever; other bars may improve this part of the experience.
I have a feeling this isn’t the last we’ve seen of hydraulic road rim brakes. For all of you P5 owners out there, do you like the feel and function of your RT8s? If you do not own a P5, would you consider upgrading your existing bike to RT8 brakes?
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