You need a dial indicator to measure the rotor run-out. You cannot see the warpage by eye....We are talking about thousands of an inch that make the difference between a warped rotor that pulsates or one that doesn't.
Cantamination of the brake fluid is not just my theory, it's a well known fact. Brake fluid is hydroscopic which means it attracts and absorbs moisture from the air while sitting in the brake fluid reservoir. Since the water is heavier than the fluid it will move to the bottom of the reservoir and eventually get pumped through the master cylinder and into the brake lines. The heavier water will seek the lowest part of the brake system which is the calipers and/or wheel cylinder (drum brakes). That moisture/water will cause erradic brake operation, as well as begin to attack the metal parts of the caliper and piston. Most calipers use a cast iron or steel bore sleeve, and the pistons are often a steel or aluminum alloy or even plastic. Also, your brake lines are steel and will rust on the outside and the inside.
All you need to do is attach a hose the the bleed valve and the other end into a glass jar of clean, fresh brake fluid. Open the bleed screw and have someone pump the brake pedal and now look at the color of the fluid comming out of your caliper. If you have not flushed the fluid in 2 years or more, you will see what that moisture has done to your brake fluid. It will likely be a very dark reddish brown color like rust with bits and globs of debris, etc.
If the pistons where pushed back into the caliper bores when new brake pads were installed, the crud was pushed back up into the ABS system. If that crud damages the ABS system, you are in for some very expensive repairs.
The contamination can also cause pitting of the caliper piston bores behind the pistons which often don't show up unit the brake pads are replaced and the pistons are pushed back into the bores. This puts the piston back over the pitted areas of the bore and leads to sticking and damage to the piston seals and o-rings.
That's why it is wise to open the bleed screw when you push the pistons back in to install new brake pads. T?his allows you to seek the condition of the fluid, as well as prevenenting contaminated fluid from being pushed back into the ABS unit. It also make the piston easier to push back in. In fact I push the piston back in with my fingers and I can feel if there is any significant damage to the piston bores...Yes, you can actually feel any binding or grinding if the piston bore is pitted..
Of course this means you will need to bleed the brakes after installing the new brake pads, but I find that after going though a set of pads, it's time to flush the fluid anyway so It is always part of my brake job.
I go one step further and replace the calipers with rebuilt calipers at every brake job. It's cheap insurance and most auto part stores will ovver 50K mile and even lifetime warranties. I keep my receipt and take the used calipers back in and get free replacements for my 2nd and subsequent brake jobs, so it's not as expensive as it sounds. The most I have paid for a rebuild OEM caliper is $59 each and some I have gotten for as little as $19 each. Some might think that is overkill, but I insist on good brakes and do not compromise on having top notch brakes on my vehicles.
...Rich
