Technical e-mail question.
I run a small auto repair shop with my father in Milford, CT. Over the years we have had a couple instances of front wheel drive vehicles with front disc brake and rear drum brakes, that we could not get a good brake pedal after extensive repairs. In the most recent case the vehicle came in with a contaminated brake system we replaced the brake pads, calipers, wheel cylinders hoses and master cylinder. All wheels bleed with good brake fluid pressure and free of air but we cannot get a good pedal. The cars stopped fine but the pedal is just about on the floor. We released the vehicles to the customers and asked them to return after the brake shoes seat in. We re-adjusted the rear brakes to no avail. There is no brake light on. We are tempted to adjust the master cylinder push rod to bring up the pedal. Can you tell me if we are missing something?
Dear Steve. As you know it is almost impossible to diagnose a brake system properly without removing the wheels and physically inspect the vehicle. It sounds like you have many of the bases covered.
What to do...
- I would not recommend adjusting the master cylinder push rod because if you figure out the low brake pedal condition and repair it the longer master cylinder push rod will now lock the brakes up.
- Ask the customer to drop off the car again.
- Turn the key to the on position to check that the brake warning light is operational if it is working we might assume that there is no serious imbalance in the hydraulic system.
- Be sure that the bleeder screws on the calipers and wheel cylinders are facing up. We have seen calipers and wheel cylinders installed with the bleeder on the bottom and if that is the case you will never be able to get the air out of the system resulting in a low and spongy brake pedal.
- Perform a pinch off test. Set line lock pliers on front and rear hoses. Remove one locking pliers at a time. Check for pedal drop.
- Down adjust emergency brake adjustment, adjust rear brakes and re-adjust rear brake shoes.
I hope one of these suggestions solves your problem. Good luck.
Dear Bill. I had the chance to inspect my customers vehicle again and used your procedures. And BINGO I found that the emergency brake was tightened too much prior to the vehicle coming into my shop. I loosened the cable and was able to get a much better adjustment on the rear brake shoes. Re-adjusted the parking brake cable and unbelievably have a high brake pedal. On behalf of my dad and my customer, I would like to thank you very much for taking the time to direct me in this process.
Steve. What happened in this case is the parking brake cable was adjusted and when you installed the new brake shoes the self adjuster was held wide open as though the emergency brake was on. The brake shoe was not seating against the anchor but on the adjuster. This being the case the wheel cylinder pistons had to travel further to engage the brake shoe against the brake drum. That extra travel needed caused the brake pedal to be low. Once the brake shoe was again against the anchor and adjusted properly the pistons in the cylinder had less to travel to stop the car resulting in the correct brake pedal height. I am glad that this worked out for you. All the best. Bill