Q I have a 2000 Honda Civic with more than 194,000 miles on it that runs like a well-tuned top. The brakes work, but after using them for a while they turn mushy, and after sitting at a stoplight they sink to the floor. I have no problem stopping, and the brakes have been bled.

A If the brake pedal slowly sinks to the floor as you sit at a stop with your foot lightly on the pedal, yet stays solid without sinking if you pump the pedal once and hold firm pressure, replace the master cylinder. This symptom is called a master cylinder bypass leak. The cup seals are allowing brake fluid to leak past them when only light pressure is applied to the pedal. When firm pressure is applied, the seals expand properly into the bore of the master cylinder and hold firmly.

Q I have a 2004 Jeep Grand Cherokee with 120,000 miles. Recently I was changing the brake pads and I noticed a lot of thick black grease on the caliper, pads and inside of the left rear rotor. I couldn't find anything that I thought could be the source of the grease. I did notice just behind the wheel well toward the bumper there is some sort of pump that was also covered in grease, but I don't know exactly what part I was looking at. Do you have any idea what may be the source of this grease, and should I be concerned?

A The left rear axle seal is leaking differential gear oil. The pump you see is the evaporative emission system leak detection pump, and it's covered with gear oil from the axle seal leak.

Q My wife's '03 Saturn Vue with 35,000 miles seems to have a not-too-loud rumble when accelerating to 60 miles per hour. It occurs intermittently a few minutes at a time and stops doing it when decelerating to 55. It seems to be powertrain-related. Never had any problems before, nothing but 3,000-mile oil changes.

A If the Saturn is AWD, my Alldata automotive database pulled up service bulletin 04-04-20-001 dated February 2004 that identifies a "drone noise between 50 to 75 mph" that could be caused by a lack of dampening of the rear differential assembly. A new rear mount bushing was issued to address this.

Other possible causes include a slipping torque converter clutch, wheel bearing, front-drive axle constant-velocity (CV) joint or even a tire problem. Swap the tires front to back for a test drive to determine whether this influences the rumble.

Q I have an '05 F-150 with 47,000 miles. I get a shudder from 0 to 20 mph when I accelerate. It is more noticeable when I start out on an incline. It's much worse when I pull my boat but totally disappears after 20 mph. Ideas?

A Ask the Ford dealer to check the rear-axle pinion angle, as in service bulletin 06-19-16 dated October 2006. Lowering the axle pinion with a special shim/wedge kit may eliminate the shudder.

Q We have a 2003 Dodge Ram 1500 truck. Just recently, the air conditioning blows hot when idling or in park. The A/C blows really cold when the vehicle is in motion. What could be causing the problem?

A The A/C pressure transducer on the high-pressure side of the system supplies a signal to the powertrain control module (PCM) that influences A/C operation. Low refrigerant level, high coolant temperatures and very low ambient temperatures can prevent the PCM from allowing the A/C compressor to engage. Buy a gauge or have a shop check A/C pressures to see if the system needs recharging.