Q: The brakes on my Mercury Grand Marquis are behaving badly. They're fine for about 2 miles, then they get sticky. I feel the brakes grab when I'm not pressing them, and they sort of drag the car to a stop. The farther I drive, the worse it gets. Eventually the car won't move because the brakes are locked up, hot, and steaming. If I let the car cool, it'll drive for another couple of miles. What's going on?
A: Clearly, you shouldn't be driving this car in traffic until the problem is fixed. In cases like this it pays to be very systematic about diagnosing the cause and testing any repairs. Start from the outermost brake corners and work upstream. The first thing to consider is that a caliper piston might be dragging, causing friction that cooks the brake fluid and results in high brake-line pressure, locking that corner. Drive around the block and see if you notice just one brake hotter than the others. Unfortunately, I doubt that it's a caliper. If all your brakes are locking up simultaneously, the problem is likely with your master cylinder.
The master cylinder is what transforms the mechanical force you develop by pushing the brake pedal into hydraulic pressure, which does the work of moving the caliper pistons (or brake cylinders in drum brakes). The system relies on a series of seals to maintain pressure, and those seals need to slide in the housing easily. If the seals in the master cylinder begin to break down, the piston running through it doesn't return properly and may maintain line pressure even if your foot isn't on the brake. If the line pressure is enough to press the pads into contact, all four brakes will generate heat and lock up. The only solution in this case is to replace the master cylinder—not a huge issue with your car because the master cylinder and vacuum brake booster sit right on the firewall. Still, all the brakes must be bled, and if you have ABS, the ABS distribution block may need to be cycled at a brake shop to get a proper bleed. The valving and mechanisms in the distribution block can trap air, and the only reliable way to get rid of it is to hook up the car to a specialized piece of hardware that will cycle the system according to maintenance requirements.
Because a master cylinder replacement is the most expensive repair in a brake system, you may want to consider throwing a few extra dollars toward getting new calipers and rotors all around. You'll have peace of mind knowing that brake problem won't come back any time soon.