Q One cold morning I found a crack about three inches long in the lower right corner of my Explorer's windshield. It started from a small pit and has been inching along so it's now reaching the windshield's center. At what point do I need to have the glass replaced?

ANNIE BRYANT   
   San Jose, California  

A You should do it now. Pits in auto glass, often caused by flying debris, can seem almost alive when cracks begin to snake out from them like tentacles. Helped along by fluctuating temperatures, they extend their reach either while no one is looking or too slowly for the human eye to notice. Over time, the result is an impressive crack that weakens the glass enough to make it a safety hazard. Having a pit fixed as soon as it appears may ward off the process, but once a crack starts creeping across the glass, there's no good way to discourage it.

Q Sometimes, while waiting at a light with my foot on the brake, I feel the pedal sink down slightly. Am I imagining this or is it really happening?

RAYMOND WONG   
   Milpitas, California  

A It really happens. A slowly sinking pedal is often a sign that the hydraulic brake system leaks either externally or, more often, internally. If your brake fluid level remains constant, the leak is internal.

When you step on the brake pedal, a hydraulic system amplifies your foot pressure and transmits it to the brakes. Fluid is distributed from a reservoir and a piston inside the master cylinder pushes this fluid toward the brakes. Seals prevent it from leaking past the piston and flowing back toward the reservoir. When the seals wear, they let fluid by, and your pedal begins to sink ominously toward the floor as less and less fluid remains on the brake side of the piston.

This can be dangerous, of course: You never know when the seals might give up the ghost and let a lot of fluid flow by, as your pedal rapidly approaches the floor without much effect on the car's speed. Whenever there's a change in the way the brakes are working, it's a good idea to have them inspected. Brakes should also be checked at least as often as every other oil change or when the tires are rotated.

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Please send car problems to Your Car, VIA, 150 Van Ness Ave., San Francisco, CA 94102. Send email to viamail@csaa.com. Questions will be answered only in the magazine.