How to Bleed Brakes

 

How to Bleed Brakes

 

 

 Another “Back to the Basics” piece. As you probably know, anytime you disconnect a brake line air will be introduced into the brake hydraulic system. Air, unlike brake fluid, will compress. That means no pedal or a spongy one at best. The brakes must be bled to get all air out of the lines. If a master cylinder is replaced, it must be “bench bled” before reinstalling it. More on that later. Iwon’t get into the silicone fluid debate. From what I’ve read and heard, I recommend staying with good old std DOT 3 fluid to avoid problems.

For bleeding the lines, there are pressure bleeder machines and vacuum pumps available that make it easier but most people are stuck with manual bleeding. If bleeding all four wheels, the recommended order is: right (pass) rear, left rear, right front, left front. If your system has front disc and rear drum and has a delay valve like pre-’71 Chevelles, the delay valve must be open during the bleeding. You can do that by wedging a piece of wood between the plunger and booster.   To bleed the brakes, remove the tires/wheels as necessary and get the proper fitting wrenches for the zerk fittings on the wheel cylinders or calipers. Also get a length of tubing (clear vinyl is best) that will fit snugly on the rounded part of the zerk and long enough to reach the floor. Place the open end of the tube in a clear glass or plastic jar filled about ¼ full of new brake fluid.  The end MUST stay submerged in the fluid at all times. Top off the master cylinder and install the cap. . Have an assistant pump the brake pedal then hold it down.   While holding the tube on the zerk fitting, open the zerk  about ¼ turn and watch for fluid and air bubbles to exit the tube into the fluid in the jar. When it stops flowing, close the zerk by turning clockwise. Repeat this at each wheel until no more air bubbles come out, just fluid. Make sure you check the fluid level in the master cylinder often and keep the level of fluid in it from reaching the bottom. If the fluid level gets low enough to allow air in through a port in the bottom of the MC, you must start over. You can gravity bleed brakes by just opening the zerk and letting the air bubbles work their way to the jar but it is time consuming. It works much better on the front brakes than rear.

As mentioned earlier, if replacing the master cylinder you must “bench bleed” it before reinstalling it. To do this, you must basically use short brake lines that run from the line connections back into the MC fluid reservoir to “short circuit” the system. Some new and rebuilt MC’s come with a kit or you can make one by getting some short lines and cutting off one end. Put a short section of rubber tubing that fits snugly on the line and clamp it on with a hose clamp. Run the tubing into the reservoir so the ends are on the bottom and temporarily wire in place. Secure the MC in a vise and fill it with new fluid. Use a screw driver or rod to pump the MC til air bubbles stop coming out of the lines. Remove the bleeder lines, install the cap, install the MC on the car, and connect the brake lines. Now bleed all four wheels as described above. Happy bleeding!