The hydraulic brake system must be bled any time one of the lines is disconnected or any time air enters the system. If a point in the system, such as a wheel cylinder or caliper brake line is the only point which was opened, the bleeder screws down stream in the hydraulic system are the only ones which must be bled. If however, the master cylinder fittings are opened, or if the reservoir is allowed to empty, air must be bled from the entire system. If the brake pedal feels spongy upon application, and goes almost to the floor but regains height when pumped, air has entered the system. It must be bled out. If no fittings were recently opened for service, check for leaks that would have allowed the entry of air and repair before bleeding the system.
As a general rule, once the master cylinder is bled, the system should be bled starting at the furthest wheel from the master cylinder and working towards the nearest wheel. Therefore, the correct bleeding sequence is: master cylinder, right rear wheel cylinder, left rear, right front and left front. Most master cylinder assemblies on Chevrolet full size vehicles are equipped with bleeder valves, bleed them first then go to the wheel cylinders or calipers. If the master cylinder is not equipped with bleeder valves, it may be bled by loosening and tightening the fittings.
This method of bleeding requires two people, one to depress the brake pedal and the other to open the bleeder screws.
- Clean the top of the master cylinder, remove the cover and fill the reservoirs with clean fluid. To prevent squirting fluid, and possibly damaging painted surfaces, install the cover during the procedure, but be sure to frequently check and top off the reservoirs with fresh fluid.
It may be necessary to hold in the metering valve pin during the bleeding procedure. The metering valve is usually located beneath the master cylinder and the pin is situated under the rubber boot on the end of the valve housing. This may be taped in or held by an assistant.
- Fill the master cylinder reservoir with fresh brake fluid.
- If the master cylinder contains any air, start bleeding at the cylinder assembly.
- Install a box-end wrench on the bleeder screw for the master cylinder assembly. Some master cylinders may not be equipped with a bleeder screw. If so, the air must be bled from the assembly by loosening and tightening the hydraulic pipe fittings. Start with the forward fitting, then bleed the cylinder from the rear fitting.
- Attach a length of small diameter, clear vinyl tubing to the bleeder screw. Submerge the other end of the rubber tubing in a glass jar partially filled with clean brake fluid. Make sure the rubber tube fits on the bleeder screw snugly or you may be squirted with brake fluid when the bleeder screw is opened.
While bleeding the system, do not let your friend release the brake pedal if the bleeder screw is still open. Should this occur, air may be drawn back into the system.
- Have your friend slowly depress the brake pedal and hold. As this is done, open the bleeder screw half a turn and allow the fluid to run through the tube. Close the bleeder screw, then have your friend fully release the pedal.
- Repeat the procedure until no bubbles appear in the jar. Check and refill the master cylinder reservoir.
- Repeat this procedure on the right rear, left rear, right front, and left front wheels, in that order. Periodically check and refill the master cylinder so it does not run dry. Remember, if the reservoir is allowed to empty of fluid during the procedure, air will be drawn into the system and bleeding procedure must be restarted at the master cylinder assembly.
Be very careful when bleeding wheel cylinders and brake calipers. The bleeder screws often rust in position and may easily break off if forced. Installing a new bleeder screw will often require removal of the component and may include overhaul or replacement of the wheel cylinder/caliper. To help prevent the possibility of breaking a bleeder screw, spray it with some penetrating oil before attempting to loosen it.
- If the brake warning light is on, depress the brake pedal firmly. If there is no air in the system, the light will go out.
- After bleeding, make sure that a firm pedal is achieved before attempting to move the vehicle.