I bought this 740GL wagon 7000 miles ago with 120K on it. I got all the receipts (Volvo maintained since new). For the last 35K, he had Volvo put new rotors and pads ($250) every 5K because of front end shimmy. I drive mostly highway(Last car my brakes went 65K)and rung up 5K in 2 months and the shimmy while braking started for me. Independent Volvo mech. said runout too much?, machined the rotors and lubricated caliper pins.I had him check front end and was told all looks OK. All was fine for 1K. Now shimmies on braking again. Had new tires put on when I bought it and everything balanced out. What are these guys not checking? I also notice a slight clunk and 1/2" pedal drop when applying light brake pressure backing down slight inclined driveway. Any correlation?

My '93 945 does the exact same thing.

My clunk is accompanied by a slight mechanical whirring under the hood. Thus I've always attributed it to the ABS system's self-check, which according to the manual occurs shortly after starting and at 4 mph and which may be felt in the brake pedal.

My brakes also have always pulsed about 5-10K after servicing. Brake pulsations are probably the 3rd or 4th most popular topic on the Brickboard, and I have always read with interest the many theories. But this thread has some of the best ones yet!

When I first got into Volvos I had a similar problem on my 744 Turbo. I suspected warping on my Brembos. I went through three sets and finally discovered it wasn't warping but hot-spotting. The harder the metal the less friction the pads create. With a hot spot, the rotor is hardened at that point and the pad slides past that spot and grips at the point at which the hardening ends. The resulting feel in the pedal is a pulsating or shimmy. I ended up changing pad brands and haven't had the problem since on my 745 T wagon even with non drilled rotors.

What brand pads did you find best?

I used to buy cheap east asian pads from my Canadian import parts supplier. Now I use Metalmasters but only after ensuring that the guide or slide points on the pad are filed square and clean of burrs. I meant to add in my first posting that I was only able to use the pads that were on the car at that time by cross drilling the rotors, hence the reference to drilled rotors at the end.

This "Hot Spotting" you refer to is exactly why many rotors cannot be turned successfully. After turning a rotor with Hot Spots, the rotor does fine initially in service. But the difference in hardness of the rotor surface causes uneven wear, and brake shimmy comes with a vengeance in a couple of months.

What brand of rotors are you uning now?

Still using Brembos.

If you've gone thru several sets of rotors and pads, and the caliper pins are well lubed, and the car still develops warped rotor symptoms, I'd ask your mechanic to check the runout on the wheel hub itself. (What you see when the rotor is removed.) Could be dead wheel bearings, could be a damaged hub from a set of rotors being removed with a sledgehammer.

If the runout on the hub is too much, then it will be exagerated when you bolt the larger disc to it, and you'll eat rotors...

-Paul Demeo

1990 780T

Have you replaced the caliper hoses? They do deteriorate internally and can act as a one way valve, causing drag and overheating of the rotor. Also, resurfacing rotors makes them thinner and more easily warped.

In my experience, I question if the replacement brake pads are too thick and constantly are dragging on the rotors, thereby overheating the rotors and adding to the problem.

Have had the same problem. I go for aftermarket rotors as they are much less expensive and work every bit as well.

Hope his helps some,

dick

Brian,

Have the wheel lugs properly torqued? The alloy wheels should get no more than 65 foot pounds. They should be hand torqued to those specs evenly in a star pattern with a torque wrench. Putting and air gun to them and running them up to 100 + foot pounds can warp rotors in short order. Also I'd check the condition of the brake calipers,and tie rod ends, ball joints and control arm/radius rod bushings.

The shimmy is probably warped rotors - maybe you have a binding caliper or somesuch? The clunk could be the load compensator valve which is designed to come into play to redistribute braking effort as the car is loaded and it's just possible the the slope is enough to shift the weight of the car forward enough to have an effect on this.

Rich.