How To: Bleed Brake Lines And Calipers

1 The easiest way to refill a dry system is to force-bleed it from the bottom. You could try a squirt bottle, a large syringe or even the tool you use to suck oil out of a fork while setting the oil height. Brake systems are not compatible with oil, so any tool that has been used before must be clinically cleaned with brake cleaner before you begin. We bought a pump-type oil can for less than five dollars at Harbor Freight (www.harborfreightusa.com/Dirtrider), then labeled it for brake fluid. It will be used only for brake systems.

2 You'll also need a couple of feet of clean carburetor vent hose. Cut a section and force it over the oil can's outlet. We like to safety-wire the hose to the can. If the hose were to blow off from pressure, it will make a nasty mess. Worse, brake fluid can eat paint, so check your surroundings before you begin.

3 Whenever possible, but especially when filling a system, start with a new, sealed unopened container of brake fluid. Brake fluid absorbs water from the air, and the more water it contains, the less heat-stable it is. Note that our oil can is painted. The paint is beginning to be affected by the fluid, but the inside of the can is not painted. Just make sure no paint flakes get inside the can.

5 If you're working with the front brake, loosen the caliper bolts. You might want to leave them in a couple of threads until you crack the bolt that holds the line to the caliper. If you're changing the brake line, take the wheel off and, making sure the reservoir does not run dry, pump the brake pads out until they are almost touching. That will get the maximum amount of brake fluid in the caliper.

6 Loosen and remove the lower brake line fitting quickly. We were changing out the caliper to return this bike to stock, so it wasn't important for us, but if you are changing the line only, hold the caliper with the point the hose attaches up. After you remove the line, set the caliper down with the fitting hole up to prevent fluid loss. If the line has a banjo fitting, it will have copper or aluminum washers on both sides of the fitting. They should be replaced with new ones.

7 If you're changing the brake line, attach it at the master cylinder. Fill the cylinder, and keep it filled while letting the fluid gravity-bleed into the line. When you start getting fluid out the bottom, bolt it to the caliper and mount it back on the fork. Spreading the brake pads will push enough fluid back up the line to help with the bleeding. Put a wrench on the bleeder fitting, and attach the filled oil can's line, but don't allow so much air in the line like we have here.

8 Wrap the master cylinder with paper towels, open the bleeder valve and use the pump trigger to force brake fluid up through the line. This works if you are replacing the caliper or the line or both. When you have fluid without bubbles in the reservoir, shut the bleeder valve and remove the oil can hose.

9 Hopefully you have resistance at the lever at this point. We didn't. If you don't, play with the brake lever. Pull and release it rapidly, and pull the lever in and let it snap out. Use a screwdriver handle to lightly tap the side of the reservoir. Push the brake line down so it bows farther down toward the caliper, so there is no part of the loop above the master cylinder.

10 When you snap the brake lever out, you will see bubbles like these rise in the fluid. Keep pumping and flicking the lever until you no longer see bubbles in the fluid. Sometimes tapping lightly on the brake line will help as well.

11 Even when we stopped getting bubbles up top, there still wasn't a firm lever. Use more of the carburetor vent hose (Moose Racing or Sudco) to make a loop like this one. Use two zip-ties to hold the loop. Put a wrench on the bleeder valve, force the hose over the nipple and run the other end into a container. We used a plastic water bottle with a hole drilled in the cap. Have a helper squeeze and hold the brake, then crack the bleeder valve and immediately lock it back down.

12 As soon as we cracked the bleeder, air came out, and we got a fairly solid lever. The pump-type oil can actually added bubbles to the fluid as we pumped due to cavitation, so we had our helper repeatedly pump and hold the lever and keep the fluid level up. Each time he would hold the lever, we would crack the bleeder. As soon as the fluid is all the way around the loop in the hose, no air can back into the caliper. We opened the bleeder valve, and simply pumped the fluid through with the lever. We kept the reservoir filled, and stopped when we no longer saw even small bubbles exiting the caliper into the hose loop.