Q Can you discuss the pros and cons of drum brakes vs. disc brakes, in terms of functionality and cost? It seems to me that with current manufacturing technology, disc brakes would be standard. I'm surprised that manufacturers still use drum brakes.

A In terms of functionality and performance -- the only perspective I deem important -- disc brakes win hands down. Look at brake systems as small engines. The engine under the hood converts energy in fuel into heat and mechanically converts heat into work to move the vehicle. Each of the four brakes converts energy of motion back into heat, slowing the vehicle.

By the very nature of a rotating disc being squeezed by hydraulically applied brake pads, the rate of energy conversion of motion into heat in a disc-brake system is significantly higher than drum brakes. There's far more surface area on the two sides of the rotor than on the inside of a brake drum. In addition, because the disc/rotor is open to airflow, heat dissipation is far greater, as is the resistance to fade from overheating friction material.

So why still use drum brakes? Front brakes do most of the braking on motor vehicles. Load is transferred onto the front tires as the brakes are applied, increasing the grip or traction of the front tires. This allows carmakers to engineer more brake capability into the front brakes than in the rear. So, on some light trucks and front-wheel-drive vehicles, the more-efficient disc brakes in the front do most of the stopping while the less-efficient drum brakes are still adequate as rear brakes.

From a cost perspective, I suspect the additional hardware -- including calipers, mounting brackets and rotor/hub/wheel bearing assemblies -- are more costly to manufacture than simple drums, backing plates, wheel cylinders and brake shoes.

Q The timing belt and water pump were replaced on my '97 Mazda 626 at 105,000 miles, and now at idle the engine clatters like a diesel engine. Above 1,100 to 1,200 rpm, it goes away. Before this, the engine was always very quiet. Any clue about what might be causing this?

A Engine clatter has several possible causes: carbon "knock" due to deposits on the tops of pistons, an issue with the camshaft friction gear/drive gear, low oil pressure or excess clearance between the hydraulic lash adjusters (HLAs) and cam lobes. A mechanic's stethoscope can help pinpoint where the noise is coming from, and if it's from the valve train on top of the cylinder head, a simple measurement of clearances between HLA and each cam lobe with a feeler gauge may identify a bad HLA. I'd also suggest a mechanical check of oil pressure at idle, because low oil pressure could allow the HLAs to collapse.

Q I have a 2003 Chevy S-10, purchased new. It developed an oil leak from the timing chain cover at 37,000 miles; it leaks less than a quart between oil changes. In spite of the low mileage, the dealership will discount the $1,700 repair by only 10 percent. I retired just before the economic meltdown, and avoiding this kind of expenditure is a real priority. Are there any oil leak sealers that would help?

A I doubt it, but feel free to try. So, you own a seven-year-old vehicle that leaks less than one quart of oil every 3,000 to 5,000 miles? Tell me again what the problem is? My vehicles would have to leak a lot more oil than that before I'd be concerned. Topping up the oil and putting a drip pan under the engine seems like the most cost-effective fix right now.