Re: S40 Warped Disks
if vibration has occured from the moment that you had them replaced dealer I would check the following.
1. ABS activates under normal braking -- If you don't see the ABS warning light, I doubt that this is the problem.
2. Warped rotor, excessive run-out -- Since your had them replaced many times already, it's probably not the problem.
3. Swapping to a dissimilar pad -- The pads are designed to leave a VERY thin deposit on the rotor. If you change to a different pad, at some point the rotor will have deposits from both. If the coefficient of friction between the new pad is different with the two different pad deposits, you'll experience vibration until the old deposit is worn away (I've had this problem). Or worse yet, the different pad deposit can result in severe galling of the rotor, which I've also seen (but thankfully not on my car). Not sure the pads you have used.
4. Using a race only pad on the street -- Especially if the pad has not been properly bedded, you may have an uneven pad deposit on the rotor, which causes vibration as in #3 above. Can you determine exactly what pad compound your currently using, or confirm that it's a street pad?
5. Cracked rotor -- Especially if you now have drilled rotors, cracks can initiate at the holes, grow radially from hole to hole, and cause vibration especially when the rotor is very hot. The crack will tend to close when the rotor is cold, and the crack can occur on either side of the rotor. As a result, your rotors need to be removed from the car, and both sides must be carefully examined. Assuming that you have stock rotors I would doubt this to be the problem.
6. Rotor distorts under pressure -- I've never seen this, but others have reported this can occur if the wheel bolts are not properly torqued in the proper order when the wheel is mounted. I gather that you're not located in the U.S. by the Staffordshire UK tag under your name; but here, half of the mechanics I've seen either don't properly torque wheels or don't know how. If I didn't personally witness a mechanic torque my wheels, I'll redo it myself when I get home.
7. Bad wheel bearing -- This can cause vibration under braking loads. Since you've had a vibration problem with 3 sets of rotors now, perhaps this is the problem?
8. A SVT owner reported that his problem with vibration under braking was caused by worn tie rods. May be something to look into as well.
Just some thoughts and ideas as to why you are having the issues. Hope you can work it out. Let us know.