In our regular tech Q&A series, we put your burning cycling questions to industry experts. This time, Shimano explains the logic behind its one-way bleed system.
I would really like to know why the brake fluid change is usually (and in most recommendations) done by pushing the new oil into the caliper. In my perception, the denser oil and possible residue from seals, for example, would tend to be in the lowest part. Therefore, it would be more logical to pull the old oil from the caliper, of course, removing the bubbles first and leaving oil available for the levers.
Sidnei, via email
To get a definitive answer to Sidnei's question, we spoke to Shimano. Julian Thrasher, head of training at Shimano’s UK distributor Madison, explained that many miss an important first step in the process.
He pointed out that “best practice is to flush the old mineral oil from the brake system first”. This flushing stage removes contaminated oil and residue, so there’s very little old fluid left in the system before fresh oil is added.
As for why the new oil is pushed into the caliper rather than pulled out, Julian said it’s because Shimano’s calipers follow a “one-way bleeding” design.
This means the fresh oil needs to be pushed through carefully to ensure “the least amount of air in the brake”.
In other words, the system is designed for oil to flow in one direction during bleeding, and pushing fresh mineral oil through after draining helps flush out air and contaminants effectively.
- Read more: How to bleed Shimano disc brakes
5 more tips for the perfect hydraulic disc brake bleed
Whether it’s your first time or you’ve done this a hundred times before, these simple pointers from BikeRadar's workshop gurus will help you avoid the usual headaches and get your brakes feeling spot-on.
Take your time setting up. Rushing the setup is one of the most common causes of a poor bleed. Ensure the lever bleed port is level and the caliper is at the lowest point in the system. Clean everything with isopropyl alcohol before you start and lay out your tools in advance.
Eliminate bubbles before you begin. Before you even attach the syringe, hold it upright and tap to release any air bubbles. Then, slowly, push the plunger to remove them. Introducing air at this stage can ruin an otherwise perfect bleed.
Use genuine mineral oil and keep systems separate. Shimano systems are designed specifically for mineral oil – never substitute with DOT fluid. Also, avoid using bleed kits or syringes that have been used with a different type of brake fluid. Cross-contamination can damage seals and compromise performance.
Watch for discoloured fluid. Old brake fluid tends to be darker. If the fluid coming into the funnel is brown or murky, keep flushing until it runs clear. Dirty fluid suggests contamination or degradation – either way, it's not what you want in your braking system.
Don’t forget lever angle changes. Shimano’s one-way bleed method relies on shifting the lever into different ‘burp positions’ to move trapped air. Skipping this step can leave small bubbles behind, especially near the master cylinder. Check the Shimano tech docs to find the correct angles for your brake model.
Do you have a cycling question you want answered? Every month, we’ll select the most interesting questions and bring you an expert-level response from our unparalleled access to the greatest minds in cycling, both on the BikeRadar staff and across the industry. Send your questions to podcast@bikeradar.com
