Tech Spotlight – 5 Minute Bubble Bleed – Pinkbike

What are you doing? This is not a brake bleed, but rather a quick method to solve inconsistent lever feel such as pump-up and long lever throw. At BikeCo we consider this a “brake fluid service bubble bleed”, and we utilize this technique to improve brake system feel and performance throughout the lifespan of the…

Pinkbike and are excited to bring back the mechanical “how-to’s” with the new Tech Spotlight articles that you can expect to see on the last Tuesday of every month. Joe Binatena, BikeCo’s owner and world class mechanic, will help walk riders through a variety of tasks, from basic jobs to more advanced work that you’ll be able to read about in the future. We aim to provide readers with a gauge on difficulty and risk for these projects, and also encourage you to post any questions in the comment section below when it comes to things that need more explaining.

How To Perform a Bubble Bleed

What are you doing?

This is not a brake bleed, but rather a quick method to solve inconsistent lever feel such as pump-up and long lever throw. At BikeCo we consider this a “brake fluid service bubble bleed”, and we utilize this technique to improve brake system feel and performance throughout the lifespan of the system. Essentially, bubbles build up on the inside of the master cylinder bladder over time – think of bubbles in a two liter bottle on the plastic walls – creating performance issues, and this process will remove those bubbles and restore the performance of your brakes. What this won’t remedy is air deep in the brake line or caliper, contaminated fluid, or leaky fittings and sticky pistons. We are not here to articulate exactly how the air ends up inside a close system – simply that it does. If you are interested in the physics, there are many great articles around the talk about brake boil, fluid contamination, diffusion coefficient, absorption coefficient, etc from motorcycle, automotive and aerospace engineers.

BikeCo’s Joe Binatena began utilizing this technique while traveling as a mechanic with some of the world’s fastest gravity racers, and he would perform this service in between runs on brakes that saw extreme heat cycles on the race circuit. This service can also be done at the end of each race day on bikes that don’t see as much heat put through their brakes. Our personal bikes tend to have this service performed on them every month or so depending on how often they are ridden, and that experience indicates that these steps help to extend overall fluid service life, meaning we have to do full bleeds less often. Whether you have a hand-me-down rig or a dream build, consistent brake performance is essential to your riding enjoyment. Don’t tolerate poor performance because you see your bike as “so-so” or “not worth it”. Service like this stretches the life span of your components, allowing you to get the most out of your spent dollar and possibly increase the time to budget for your next purchase.

This is not a brake bleed.
Piston stick
Air in slave cylinder
Air deep in brake line
Leaky fittings
Contaminated fluid

This service MAY ADDRESS
Inconsistent lever feel – pump up, hard lever, long throw, etc.

This service takes an experienced mechanic five to ten minutes, meaning that with some practice you can have terrific brake performance with minimal pain and suffering. To illustrate this, watch Joe complete the entire service, while narrating, in less than 5 minutes

Tech Spotlight - Brake Fluid Service

Difficulty: medium – proper technique is vital to ensure your brakes function well
Risk: medium – it’s your brakes, so don’t mess it up

The tools you need for this service are very simple. We keep them in the shop, as well as a small vehicle tool box in case we need them at the trail head before a ride.

• Torx or Allen to remove the bleed screw
• A small bottle of the appropriate brake fluid (it goes without saying do not mix them, do not use the wrong fluid, etc – this will cause brake failure). If you are uncomfortable with the risk to your master cylinder bladder a Shimano bleed cup or equivalent may be used.
• Rag and Windex to clean up

Disclaimer: Brakes are critical to safety, so work with an expert service center it you are uncomfortable with this job.

Tech Spotlight - Brake Fluid Service

• Step 1 – Carefully remove the bleed screw and associated o-ring. Do not pump the brake lever at this point to avoid air ingress. Remember to treat all brake fluid as damaging to finishes, contaminating to brake pads and rotors, and not super good for your body.

• Step 2 – Gently screw the bottle into the bleed fitting – it’s only plastic, so it just needs to be finger tight. You may also need to trim the tip down to allow the nozzle to form a tight seal on the threaded port without bottoming out in the internals. The bottle should ideally will be able to easily support its own mass throughout the procedure, thereby freeing your hands for the other steps. If it doesn’t, having a friend lend a hand will be very helpful.

Tech Spotlight - Brake Fluid Service

• Step 3 – This service can be performed with the brake levers on or off the bars. For speed – which also equates to motivation to do the work – we leave the bike assembled. It is important that you understand the basics of your brake system’s layout. You will want to begin and end the service with the bleed port at the systems highest point.

With the bottle inserted as an additional reservoir, gently squeeze the brake lever. Your first few lever strokes should be with the port at the systems highest point to maximize the air release. Slowly rotate the handlebars side to side while pumping the brake lever a few times to insure that you have released any bubbles that maybe caught at different points in the system.

Tech Spotlight - Brake Fluid Service

• Step 4 – You have an additional option which helps eliminate the remainder of air trapped in the system, but there is a risk to understand here. Too much pressure in this step WILL cause your master cylinder bladder to fail, and this step requires a delicate touch to protect your brake system. This is NOT a hand strength exercise. The sealed bottle allows you to gently flush fluid into the bladder system compared to the open Shimano reservoir which does not provide this capacity. If you question your small motor skills, the open ended system has much less risk of damaging a bladder. Gently pushing a little fluid into the bladder releases the last bit of air, generally much smaller bubbles than your initial lever strokes, in the system. Think of it as softly inflating the bladder with fluid flow, which displaces the tiny air bubbles on the bladder walls which are sucked back into the bottle through the vacuum as you release the pressure.

Rotate the bar while softly pumping the system as you did with the lever procedure. Again, over inflating the bladder will cause it to fail. This is generally illustrated by brake fluid leaking out from the lever system upon completion. DO NOT pressurize your brake system! The goal is to fill the system without air bubbles. This step is designed to flush bubbles out, displacing the bubbles with fluid.

Tech Spotlight - Brake Fluid Service

• Step 5 – Return the bleed port to the highest point and remove the reservoir bottle. Refit the bleed screw while making sure any O-rings you may have removed are correctly replaced to insure your seal integrity.

Tech Spotlight - Brake Fluid Service

• Step 6 – Remember to thoroughly clean any excess fluid from your bike. Windex, or even water, and a clean rag get you handled. Your brake lever should offer greatly improved feel and consistency. If you correctly completed this service and lever feel has not improved, or worsened, you should complete a full brake service and bleed to identify the area of failure.

Tech Spotlight - Brake Fluid Service

IMPORTANT NOTE: This service will “top off” fluid during brake pad life. When replacing your brake pads you MUST remove the bleed port prior to resetting pistons to clear the new pads. Opening the system will minimize stress on the bladder system and may purge a bit of excess fluid.

Disclaimer: Many of our tech tips ARE NOT factory approved service techniques. Riders should evaluate their skills prior to attempting any published tips. Knowledge base is needed to define success or failure after any tip procedure – if you doubt your understanding of any system described in our tech spotlight work with a professional resource. All systems on a bicycle should be considered critical – riders risk injury or possibly death with improper service or application. Be safe and enjoy your riding.

We hope that you found tips in this that will help you better maintain your ride. We will look forward to future instalment of Tech Spotlight here on Pinkbike on the last Tuesday of each month. If you have an idea that you would like to see covered, please e-mail [email protected] to let us know what issues you’ve had that can’t seem to be solved and we will put Joe on the task.

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