Tech Dept. (all brands of Import Cars) Tech # 3
Problem- After installation of brake pads, the brake pedal will pump up but, it will not stay up. Meaning that if you take your foot off the pedal and then step down on it a few seconds later, The pedal goes to the floor or almost to the floor. After pumping the pedal several times it will pump back up again but as soon as it is let up for a short time the problem reappears. If this is the symptom you have, read on.
First- Jack the car up again (If you are a Do It Yourselfer don't forget to put jack stands under it.) If your an experienced mechanic I shouldn't have to remind you. Check for loose play in the wheel bearings and then look very closely at the disc (rotor) and it's relation to the caliper while some one pumps the brake pedal. If you note any movement of the disc (rotor) in relation to the caliper in the amount of the thickness of a nickel or more then this can be your problem and it must be corrected before you proceed to the next test. If this car has a rigid type caliper [A piston on both sides of the disc (rotor)]. Then the problem is most likely a stuck piston. If it is a free floating or pivoting type caliper then the problem is most likely a sticking caliper. These two types usually can be freed up with a little cleaning and lube. A rigid type with a stuck piston usually requires a caliper overhaul or replacement. I would not recommend a DIY to attempt this unless you were an experienced brake DIY. If any movement was noted it must be corrected before you proceed to "SECOND". If you found movement and corrected it and it solved you pedal problem, fine! your finished. However, if you didn't find any movement or the correction of the movement didn't solve the pedal problem then proceed.
Second- Now that you are sure there is little to no movement of the disc to caliper when the pedal is pumped. Clamp off each brake flex line one at a time with flex line clamps. (Available at any tool truck and most auto parts stores). You experienced wrenches already have some (Small needle nose vice grips with two pieces of fuel line slipped over the jaws) By clamping off each line and checking the pedal you can tell how many wheels and which wheels are causing the problem. Of course you should have a very firm pedal that stays up with all the wheels clamped off. If you do have a good hard pedal with all the lines clamped off, you know you have an operating master cylinder and no air in the system or at least none down to your clamps. Release one clamp at a time and recheck the pedal. You should be able to quickly arrive at which wheel or wheels are at fault. If these are the wheels that you just installed new pads on then proceed. Remove one brake pad from each faulted wheel if it is a Free floating or Pivoting caliper. If it is a Rigid caliper remove both pads. Now install the old worn pads in place of the new ones you removed. (Don't worry your not going to drive the car like this) Now, check the brake fluid level. Do not fill it FULL but do not let it get empty while you are pumping the pedal back up. Remember that the caliper pistons displace a lot of fluid when they move so you don't want to get air into the master cylinder when you pump the pedal to get the caliper pistons out to meet the thin old pads you just put in. Once you have pumped the pedal up and you are sure you didn't let the fluid get too low, NOW the important part. Remove the OLD pads and VERY CAREFULLY pry the piston back into the caliper, BUT only enough so that you have to force the new pads in. If you pry the piston in so the pads can be slipped in, you must repeat the process with the old pads. The OBJECTIVE is to force the new pads in so hard that they must be bumped in with a hammer handle or such and the end result is that you can hardly turn the wheel hub by hand. You don't even need to fully install the new pads. Meaning that if you get them in at least 3/4 the way in, that's OK because you are still not going to drive the car.
Third- Now you have the disc (rotor) hard to rotate by hand and the new pads in or most of the way in. Now, check your brake pedal. If the pedal is firm and it stays up after you let it sit for a minute or two you have proved what we were doing has corrected the problem. Bump the pads all the way in and button every thing up and install the wheels. DO NOT DRIVE THE CAR YET! If you have a large parking lot available then drive the car in circles briskly and in both directions. If a parking lot is not available, make harsh "S" turns down the street staying in low gear and being careful to avoid obstacles: Children, parked cars, oncoming trucks, telephone poles, and of coarse "The Fuzz" as they will be certain you are drunk. Less than a block should do it. Jack the car up and see if the wheels now spin free and that you have not lost your pedal. If the problem is solved and you want to know what all these strange procedures were all about read "CAUSE".
Cause- Almost all Import cars have a square cut seal in a groove inside the caliper which the piston slides into. The piston does not actually slide in this seal when the brakes are applied. When you apply the brakes the master cylinder applies pressure to the fluid. Since fluid does not compress, that same pressure is applied to the caliper piston. The pressure is amplified due to the increase in square inch area of the caliper piston. The seal is rather tight. So, as the piston is forced toward the pad and disc (rotor) it flexes and deforms the seal. When the brake pedal is released and the pressure is relieved, the deflected seal pulls the caliper piston away from the pad. This is a normal part of the design. However, sometimes when the piston is moved, like when pads are installed, a dry part of the piston is now in the seal and when the brakes are applied, the seal OVER flexes and draws the piston too far back when the pedal is released. When I had you force the new pads in, that deflected the seal in the wrong direction which then held the piston against the pad, making the wheel hard to turn by hand. At that point if the problem had gone away, it proved "Watson" that was the problem. The reason for the circles in the parking lot or "S" turns was that when in a hard turn the wheel bearings give enough to allow the disc (rotor) to force the pad over against the piston to push it into the caliper just enough to relieve the pressure, so driving the car would not overheat and glaze the surface of the pads, yet not move the piston in so far as to cause the piston travel to cause the problem to return.
Notes- This can be a real hassle on a car that uses the caliper piston with the parking brake. However, if you have this problem, think of the alternative. I have seen many shops, DIYs and even dealerships replace calipers to correct this.
Short Story- One time I received a call on a Hot Line. A parts store had given a third set of rebuilt calipers to a mechanic and asked if I would talk to the mechanic to see if I could determine the cause. Talking to the mechanic indicated he clearly had the symptoms. So, I went through the whole test procedure as listed above. When I finished the mechanic said: "That's the biggest pile of happy horse shi* I've ever heard. To start with, this is a front wheel drive car and they don't have wheel bearings". He then slammed the phone down.