Hey all,

It's me again. I have a 88 740 NA, as i am sure you all know ;o) Ever since i have had the car the brakes have squeeled and the rotors have been what i can only describe as warped when they want to be (i.e. it is perfectly smooth to stop sometimes, other times it is jerk jerk jerk). I took my right wheel off the other day, and looked at the discs......i have an 88 model with no abs.....but i have bendix calipers and 262mm vented discs. The reason why this confuses me, is b/c my car isn't referenced in the brake section of the faq.......I pulled out my trust stanley tape measure (everyone cringes!), and i swear that my discs are more than 22mm thick. I swear it. Even if the tape measure is a little out, then the discs must be nearly brand new (even though they don't look it), they have a ridge on them from the edge (normal wear), and i swear when they were installed they HAD to have been thicker than 22mm........they are genuine volvo ones.......could this be a reason for warped discs (i.e. do rotors that are too thick cause a problem?). I really wanna get this sorta as i plan on a BIG service soon (clutch, universals, discs (if i need to :o( ), brake lines, etc.) I really need to know why my discs look so thick, and if they really were larger than they were meant to be, can i complain to volvo? (i imagine they wouldn't care less, but worth a try for some new discs). When i spin my front wheels while the car is off the ground the rotors appear to definately be warped......but, how come sometimes it is smooth to stop? The pads are new, about 6 months old.....if i have to replace these too, then so be it!

Thanks in advance all!

/// I don't have your model. What you describe is not possible, unless...

If the rotors are warped then the brakes will pulse all the time unless you also have a frozen caliper that sometimes refuses to operate. Then you would have no brakes on that wheel while it is frozen. (This is a sliding caliper, right, one big piston?)

It is not possible to look at the rotor and decide if it is warped. It is put on a lathe and rotated. Alternately a radial run out dial is mounted on the caliper (magnet) and the wheel of the dial placed against the front rotor surface. You can read out the warpage as the difference in the high and low measurement.

Putting a micrometer on at several points tells you the rotor thickness. The minimum thickness allowed is cast or stamped in the rotor. Usually this is at the point where the wheel lug cap meets the flat rotor surface at the rear of the rotor. Sometimes sandpaper is necessary to read the numbers.

Warping is caused by heat. Heat is caused by a caliper failure. The usual case is that the caliper fails to relax to the position before the brakes were applied. Brakes on that wheel are then on, or partially on, all the time until pad material wears off enough to clear the rotor. This can be caused by several problems both inside or outside the hydraulic system. (Rust, pitted pins, over sized bushings, worn pins etc.) If you service the warped rotors but fail to repair the cause of the caliper failure and heat, you will warp the repaired rotors in short order. (very few miles)

Be careful, brakes must be right all the time.

Aaron, I don’t think the discs were too bid to start with, if they were, then they would have had to install some worn pads for then to fit. I would have the discs machined to try and get rid of the warp. Does the brake pedal move up and down when your are having the jerk jerk jerk braking? It could be the brake hose pipes that are going bad. Good luck