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Tech Questions Answered in CC's What's Your Problem?

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Also, a couple friends are telling me I'm dumb to put new pads on old, uncut rotors, but the rotors looked great, and as I said, the brakes on car they came from functioned perfectly up until several weeks prior to the swap. Should I have them turned again? If I get new rotors, do I have to have them turned? Will I need to perform some special break-in procedure for the new pads?

Jay Buckley, Bendix Friction Materials division, Honeywell Corp.: Your questions touch on fairly common trouble areas that cause even professional brake technicians to call our tech line regularly. All can probably be dealt with quickly.

First, the trouble you're having bleeding the rear brake circuit is probably due to the proportioning valve. Two kinds of proportioning valves can cause this situation, sometimes both in the same vehicle. The combination/metering/proportioning valve shifts when it detects a leak in the rear so brake fluid won't be continuously sent to a wide-open path of escape. Once this has happened, the valve must be re-centered until brake fluid pressure in the rear again reaches a normal level. You can usually use a spring-clip to re-center the free-floating valve.

Note that some newer vehicles (though not your Chevelle) use some form of load-sensing proportioning valve. Any time the rear wheels are hanging-as they usually do on a lift-this device (if present) will block brake fluid flow to the rear. Most of these valves have an override function for bleeding (described in the appropriate manual), but in some cases you may need to jack up a suspension member.

Your friends' comments about combining new pads with old rotors aren't necessarily correct. Some of the best rotors you can install are used ones, as is. When everything is working as it should, with quality pads and rotors, as the pads wear they condition the rotors, smoothing their working surfaces and embedding them with a microscopic layer of friction material. Using rotors like these (if they meet specs for minimum thickness and thickness variation, and if they're not badly scored) usually delivers superior initial stopping performance than using rotors with new surfaces. Obviously, as-is rotors must be used in pairs to avoid the tugging of the steering wheel one way or the other under braking.

All new brake pads require breaking in before they are fully effective-something now acknowledged in many newer vehicles' owner's manuals. Fresh rotor surfaces also need breaking in, so the combination of new pads and resurfaced rotors means the driver will definitely notice a (temporary) need for more pedal pressure as well.

The brakes will break in after only 100 to 200 miles of average driving. But hard braking that excessively heats the friction material, like panic stops, can slow the process or halt it entirely. If you want to accelerate the process, use the simple 30/30/30 break-in procedure: With moderate pedal pressure, bring the car to a gradual stop from 30 mph, 30 times, with at least 30 seconds between stops to let the brakes cool. It can be done in under a half hour.

Here's one other tip that may come in handy next time you're swapping brake components: Whenever removing or replacing a brake hydraulic component, use a pedal depressor (or suitable substitute) to slightly depress the brake pedal. This will close the ports in the master cylinder, preventing brake fluid from draining out. It will also greatly speed up the bleeding process.

Backward Holley
Brian Rock, via e-mail: I think it's great to have a decent carb article in the mag ("Holley Rehab," Apr. '05). I have been a hands-on carb guy for a long time, so I'm scratching my head at why Tim Moore has the main body installed backward on this Holley. What am I missing here?

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