Ask a Mechanic: Bleeding Hydraulic Brakes

Welcome to our Ask a Mechanic column where our expert mechanic Daniel Slusser answers your bike maintenance questions. If you have a question for Daniel, please post it on our Facebook Wall or e-mail Daniel directly at 

Mineral Oil in Avid Brake

Don’t make the same mistake this guy did.

Today, rather than share an answer a reader’s question, I thought I would share a slightly amusing, yet serious problem we came across in the shop. After bleeding a customer’s brakes we noticed a strange substance flowing into the bleed syringes. It turns out that the slightly milky looking fluid we found floating at the top of the syringe, just under the plunger, is mineral oil. In fact, this oil is of the drug store laxative variety, rather than the thinner type used for brakes (which is typically dyed pink or blue for easy identification). All of the fluid in this customer’s Avid brake should have been yellow DOT fluid. You might notice the words printed on the syringe, “BLEED KIT DOT FLUID ONLY.” Similar words are printed on the brake lever’s boot, “! ONLY DOT 5.1; DOT 4.”

DOT 5.1 and 4 only

The words on the lever boot are not merely decorative.

Why bring this up? Because it is better to ask a question, even if you are afraid of the answer, than to make a serious error on a maintenance item that could negatively affect your safety. If you have a question, check out the tech docs link that we have on most product pages, right under the product image. These documents come from the manufacturer and contain the manufacturer’s manuals and specifications. For example, check out the tech docs on this bleed kit. We also have a large library of how to videos that are viewable in our Learning Center or on the Art’s YouTube Channel. Many of these videos are also available in links next to the tech docs on relevant product pages. If you don’t find the answer in either of those places, email me and I will help you out.

Daniel Slusser is a professional bicycle mechanic with over ten years of experience. He holds a bachelor’s degree in business administration from HSU and a master’s degree in history from Cal Poly. When he is not riding, wrenching, or writing he enjoys spending time with his wife and two children.