Causes of a Low Brake Pedal
9/6/12
By Right Stuff
Our tech line often receives calls from customers experiencing a low, soft or spongy pedal. Sometimes they even say the pedal just goes straight to the floor. Whether using our products or those of another company, the first thought people generally have is that they have a bad master cylinder. Most of the time this is not the problem. Following these quick instructions will help identify and fix the problem quickly:
1. In the next 5 minutes, you will have identified whether the issue causing your low pedal is in the front of your brake system or rear. Nearly every vehicle has a rubber hose at the center of the rear axle that attaches to a block on the rear end. Simply clamp the center of this rubber hose tight enough to not allow fluid to pass, but not so tight that you damage your hose. If you have a braided stainless hose, you will have to plug the port in the hose rather than clamping to avoid damage. Once you have the hose clamped, start the car and see if you suddenly have a good pedal.
2 (a) If you have a good pedal with this hose clamped, then there is no question that your problem is somewhere aft of that rear flex hose. If you have drum brakes in the rear, continue bleeding and check your wheel cylinders. This issue is much more common with vehicles that have rear disc brakes. If you have rear disc brakes, there are two crucial things you will need to do to correct the problem. First is bleed the brakes the right way. Even if you have pulled 10 gallons of clear fluid through the rear calipers, you still have air!! You will need to remove the pins that mount the caliper to its mounting bracket, tilt the caliper back so that the bleeder is sitting at the very top of the piston (this does not always mean the bleeder has to be pointing straight up, it just has to be at the top of the piston). Bleed the brakes with the bleeder in this position before bolting the calipers back to their bracket. The second major issue only applies to calipers that have a built in parking brake. If you have a built in parking brake on your caliper, you absolutely have to use it. If you do not want to, you should exchange your calipers for some front calipers that do not have the e-brake built in. Often the caliper pistons can be adjusted out simply by ratcheting the e-brake mechanism until the caliper piston pushes the pad tight against the rotor. Other times the bracket on the side of the caliper has to be removed and the piston has to be adjusted manually by turning the hex bolt. When you set your e-brake, the lever should travel 1/2" or less from its resting position to lock the rear brakes. If it moves more, you still have adjusting to do. See the video "rear disc installation support" in our tech center to see the proper bleeding and caliper adjusting processes.
2 (b) If clamping the rear hose does not help you get a good pedal, then your issue is in the front. First thing to check is that your front caliper bleeders are pointing forward and up. The port that the bleeder screws into needs to be at the top of the caliper piston, so that an air pocket cannot sit in the piston above the bleeder. If the bleeders are not in the correct position, you probably have your calipers on backwards and can fix the problem by switching driver side caliper to passenger side and vice-versa. If you have your calipers on correctly and have bled all of the air out of the brakes, then the next step is to check to make sure the booster rod is the correct length (power vehicles only). The rod that comes out of the brake booster and enters the master cylinder needs to be almost making contact with the piston of the master cylinder, but not pre-loading it. You want as small a gap between the booster rod and the master cylinder piston as possible.
3. If you have exhausted the above options, your problem could be a bad master cylinder or you just have a tricky air pocket somewhere. Keep in mind that if you can "pump-up" the pedal at all, you still have air in the system.
Whether using our products or a different brand, we hope this helps you get the stopping power you need. If not, give us a call today!