I have a 94 740i which I have had for 4 years now. A couple a years ago I replaced the front rotors because they were warped (violent shaking under braking at 50+mph). Thought it was odd because the previous owner had replaced them less than 20,000 miles before I purchased the car. I decided to go with cross drilled from Bavarian Autosport to help them stay cool since I had read that 740's rotors are too narrow for such a heavy car. Within a few months (1-2,000 miles) they started to warp and continued to get progressively worse. I had them turned and the same thing again in a few months. So I put on new slotted rotors from Bavarian Autosport and now they are warping after just about 6 weeks or less!:eyecrazy. In the past I had this happen to an old Mercedes, I had just replaced a rotor and a stuck caliper and took it for a test ride and came back and I could smell a burning smell. It turned out that the flex hose to the caliper was bad and was not letting the pressure off after I applied the brakes. It that case the rotor was blue! The rotors on my 740 are not discolored but with a dial indicator they are warped. I happened to be listening to Click and Clack on the radio yesterday (a very funny car problem show on NPR) and a guy called with the same problem and they suggested a bad master cylindar and or power booster. I think it would be unlikely the flex hoses on both sides of the car are bad at the same time.

Generally, Euro brakes don't seem to like to be turned. Losing any mass off the rotors seem to promote warpage.

I had one set warp on my 3er after about 5000 miles; the best cause I could discern was that I didn't clean them throughly. I bought a new set, used a full can of cleaner and a bunch of rags to de-oil them, and they've been fine for the last 20k or so.

Tom and Ray are funny, but I don't think their advice is always very good. You can easily tell if the calipers are partially engaged just by spinning the wheels while jacked; the pads will naturally make a small amount of friction just because they always are in contact with the rotor, but it should take very little effort to turn the wheel.

I've never seen a master cylinder or caliper failure on any Euro car that has had its fluid changed every two years or so.

What is the condition of the control arms/bushings and shocks? It only takes a bit of wear in those parts to produce noticeable vibration under braking.

While warped rotors are possible, in most cases vibration under braking will be from uneven pad deposits or worn suspension components. One common problem is a failure to clean corrosion from the hub where the rotor seats when changing rotors. That can result in a slight rotor run out that will cause vibration. Uneven pad deposits are caused by the use of pads that aren't compatible with one's driving style/condition. Low dust pads are a common culprit.

The best brakes will be when using plain OE rotors (as in from your dealer) coupled with OE or an HPS type pad.

I don't think it's the suspension because the problem gets progressively worse and disappears 100% with new rotors.

I also am pretty diligent about cleaning the new rotors prior to installation.

The machine shope also checked the run out prior to turning and they were way out.

Get some good brand blanks, cross drilled rotors are just for looks.

Do you bed in your brake pads? I would imagine 740s get their brakes pretty hot if you drive it hard, and if you leave your foot on the brake pedal on hot rotors too long it will leave pad deposits and this will cause the vibration(rotors won't actually warp). So it could just be your braking style.

Check for binding brakes after you have been for a long drive. Jack the front up and spin the wheels, they should move quite freely obviously.

Also check to make sure the rear brakes are engaging properly....

Use blank rotors.

Never turn them. Use garnet paper to clean deposits.

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