Why does front left brake not release on 1993 chevrolet 2500?

When a caliper smokes it's usually because the slider pins are corroded or somehow seized. The prevents the caliper from releasing pressure on the brake pads. So it's not about the piston, it's about the slide pins

Try checkn ur air.pressure or frnt end aligment ,replace hoses

replace the brake hose that goes to the caliper on that side. Some older hose collapse on the inside and make em slower to release

Did you bench bleed the MC before installing it? If not, that's the problem. MUST be bench bled or you'll always have air. If brakes lights are on, the brake switch is out of adjustment OR (since you replaced the MC), the push rod is out of adjustment

frist thing to check is for residual pressure, apply the brakes and let off, loosen the bleeders there should be no pressure left, if there is suspect the hoses , if not the brake fluid may be contaminated and the seals have swollen, if the fluid was contaminated the only fix is to replace all hydraulic parts that contain rubber

Check the brake light switch - it's mounted in front of the brake pedal shaft so that it compresses when the pedal is pushed. Work it manually (push, release) and see if the lights come on. If they do, try a small shot of lubricant (silicone, WD40, etc.)on the plunger to loosen it and restore normal operation. If that doesn't work, a replacement switch is inexpensive and easy to install.

repalce flex line, re bleed that side.

go here for more help.

Did you lube caliper slides?

http://groups.yahoo.com/group/villagerquest/

1. Raise and support the vehicle.
2. Remove or disconnect the following:
   
   Tire and wheel assembly Caliper and rotor Wheel speed sensor and brake hose mounting bracket bolt from the steering knuckle Electrical connection for the wheel speed sensor Front drive halfshaft assembly on four wheel drive models Hub and bearing assembly mounting bolts Hub and bearing assembly O-ring seal from the steering knuckle bore (2500 series)
   
3. Clean and inspect the O-ring seal

To install:

1. Clean all corrosion or contaminates from the steering knuckle bore and the hub and bearing assembly.
2. Install the O-ring to the steering knuckle (2500 series).
3. Lubricate the steering knuckle bore with wheel bearing grease or the equivalent.
4. Install or connect the following:
   
   Hub and bearing assembly Hub and bearing assembly mounting bolts. Tighten the bolts to 133 ft. lbs. (180 Nm). Front drive halfshaft assembly on four wheel drive models Electrical connection for the wheel speed sensor Wheel speed sensor and brake hose mounting bracket bolt to the steering knuckle. Tighten to 106 inch lbs. (12 Nm). Rotor Tire and wheel assembly.

You can go to autozone.com and register for free online repair manual with everything you need.

Does this vehicle have 4 wheel antilock brakes?

to do a complete job. you need to bleed all 4 wheels, starting from Right Rear. Right front. Left Rear. Left Front.