Do you have to bleed the system when you change your brake pads?

Answer 1

NO. You should not open the hydraulic system to replace your pads. Any time you open you hydraulic system, you should bleed, but that is not a requirement of a pad change. "Bleeding" the brakes is terminology hailing back to earlier hydraulic braking systems such as the drum/shoe brake. It simply means adding hydraulic fluid to the reservoir on the engine firewall and pumping the brake pedal to work the fluid through the master cylinder and down the feed lines to the wheels, where existing fluid in the lines can be squirted out through "bleed screws" at each wheel. The purpose of this is a) to get fresh fluid in the system and b) to get air bubbles out of the system. If there are any air bubbles in the system they will cause loss of transfer of braking pressure from the pedal. In other words, the pedal will be squishy and often go to the floor, while little or no pressure is extended to the brake components. In current disc brake systems, you may not need to bleed the brakes with a simple pad change. However, if your pads are completely worn down you will find the caliper pistons have had to travel a significant distance out of their well in order to maintain contact with the shims on the backs of the pads. The new pad replacements will have up to 3/8" each of new material on them, requiring that the pistons be pushed back into their wells to accommodate the increased thickness. Pushing the pistons back requires the fluid behind them to also be pushed back up the lines. Although in pre-ABS brake systems you can merely take the top off the fluid reservoir and if necessary remove some of the potential back-flow as the pistons are pushed back into their wells, in ABS brakes there are valves which prevent this and hence it is suggested that you open the bleeder screws in the calipers and let the fluid come out of there. This will reduce the overall amount of fluid in the brake lines, of course, and it will be necessary to check the amount of loss by inspecting the level in the reservoir, and bringing it back up between the min/max indicator as needed. Whenever you open the bleeder screws in a brake system it is always a possibility that air bubbles can enter. Thus it is wise not to open the screws so much that fluid runs out by gravity rather than by pressure at the piston or the pedal. Also, make sure that you re-tighten the bleeder screw once you find that the caliper piston is retracted enough to slide the caliper over the newly installed brake pad/shim assembly.