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Shop Talk | Bleeding Brakes & Seasonal Service

I have the discussion all the time about bleeding hydraulic disc brakes. It goes something along the lines of seasonal maintenance. Hydraulic brakes operate solely on the function of the continuity of the hydraulic fluid. Most bicycle brake manufacturers employ DOT 5.1, 4, or 3, or some variation of mil-spec mineral oil.

This oil, whether labeled hydraulic fluid or mineral oil or some variation thereabouts, gets old. It breaks down over time with use from a variety of factors. Here are a few links about it. One has a simple explanation, the other will require your attention for scientific study. Bernell (simple explanation) and HyproFiltration (long version). If you've ever wondered why brake fluid comes out black, especially mineral oil, the HyproFiltration link explains why.

The main idea is that you understand the fluid goes bad and needs to be flushed or replaced annually. Oil degradation can actually damage your hydraulic systems. The delicate rubber o-rings that are used in brake lever master-cylinders and slave-pistons at the brake calipers can begin to equally break down if this fluid isn't flushed or bled. Additives break down, oxidize, and even high temperatures erode the purity of your hydraulic fluid. No matter the oil, no matter the function, it should be flushed, or bled, at least annually on a bike.

Brake systems on bikes are microscopic compared to their motorcycle and automotive counter-parts. They already have a miniscule amount of fluid and therefore less reserve as fluid deteriorates.

I often bleed Shimano brake systems with this result, black oil excreted from the caliper, line, and lever. Ideally, it should never look this bad. This kind of fluid can eventually break down the internal workings of your brake system.

The Point of No Return

So, a few seasons go by, you haven't bled your brakes. They start to feel sticky. Your mechanic tries to bleed them. With new fluid in them, they still don't feel right. After repeating the process, you and your mechanic decide that it's best to replace the brake. I've worked in shops where I kept new brakes on hand to remedy bad brakes, simply because I knew what I was dealing with. It saved a lot of time and a handful of cash.

Old Hayes Mag Brakes (Also, the Nine model)

Do you have a pair of Hayes Mag brakes? They were about the first DOT based brake system available for mountain bikes. They didn't use a vacuum bleed like we use on most DOT systems today. Microscopic air bubbles permeated the fluid. As long as the fluid was flushed regularly, they continued to work well despite the air bubbles degrading the oil. However, it's rare to find one of these brakes today that still functions because they were not serviced timely enough in most cases.

Shimano Mineral Oil going in, and honestly should come out that color or clear, but ideally not black.

Care for Your Brakes

If you haven't had your brakes bled for a calendar year, have it done. If your mechanic recommends it, usually it's a safe bet to go for it. This is something that you can learn to do on your own bike, but I totally understand having someone seasoned in the skill of brake service do the work.

Whether your brakes are made by Hayes, SRAM, Avid, Shimano, Tektro, TRP, Hope, or one of the many other manufacturers, they all recommend fluid bleed at least once a year. This simple maintenance can mean the difference between brakes that feel sticky over time and brakes that feel smooth and resistance free for every ride.

Relevant Video:

Happy Trails

-- Insight --