Q: I have changed the back drums and brake pads recently on a 2001 Chevrolet Cavalier and have had nothing but problems braking. I changed the drums and pads a second time as the mechanic thought the drums might be oval. The problem continues. If you do not apply much pressure to the braking pedal, everything seems OK. If you brake suddenly, the brakes in the back grab and it can cause you to skid on a wet surface or on slush and ice. I have recently had the braking solution changed as the garage thought the solution might be contaminated. It has cost lots of money and time, and the problem is still not resolved. The garage tells me that all Cavaliers have that problem. Can you help, as I have tried several garages? They have loosened the adjustments on the back brakes several times but, since they are self-adjusting, the problem keeps coming back.

A: The problem is most likely the brake proportioning valve, says Yvan Charlebois, a mechanic with more than 30 years of experience.

"Its function is to distribute the appropriate amount of fluid pressure to the front and rear brakes."

He says that much more pressure is required for the front brakes, as when stopping the vehicle's weight is shifted to the front wheels (nose dip), and the rear lifts up because there is less weight on the rear wheels.

"A defective proportional valve can allow for excessive fluid pressure to the rear brakes, resulting in wheel lock," Charlebois said.

The valve is located below the brake master cylinder, and you can find it by following the brake lines out of the master to the valve below. There might even be an electrical switch attached to it, Charlebois said.

"The cost of repair is not that much and hopefully the one on your vehicle is not badly corroded," which will add to the labour needed and require a repair of the brake lines.

"With winter coming and icy road conditions, have this issue corrected soon. It is not very safe under any conditions," Charlebois said.

Pierre Brunet, co-owner of Attach Performance in Pierrefonds and an experienced mechanic with more than 30 years in the business, has another possible cause.

He said that the only way to eliminate the brake lockup is to put original GM brake shoes in the back; something he admits can be costly.

Once they are on, you must make sure there is no groove in the resting point in the back plate.

"If so, it must be refilled and ground smooth," Brunet said. "The adjustment has to be correct also because if it is too loose, it will lock. So it is better to be tight than loose."

Brunet added that the front brake must also be in great shape or the entire weight load will be transferred to the back and they will lock again.

"The slider must work properly and the pads must be of very good quality," Brunet said. "And don't forget that when they engineer a car's brake system, they start with the pads and its friction, and then move on to the hydraulic pressure. If you change the friction, then everything changes."

Q: My car, a Subaru Legacy Brighton Wagon 2001, has a knocking sound in front of the engine when it's cold, which ceases when the engine has been warmed up. Could this be a "timing belt tensioner" problem? This started at 50,000 kilometres. The present mileage is approximately 78,000 km.

A: Brunet said that the problem sounds more like a piston slap.

"As it warms up, the piston expands more than the cylinder and gets tighter, hence the noise going away," Brunet said. However, he added that the only way to be sure is to have a mechanic listen to the noise for a proper diagnosis.

http://www.montrealgazette.com/Cavalier+rear+brakes+handle+pressure/3967894/story.html