1993 Mazda 626 Pedal very hard and brakes sensitive

HARD BRAKE PEDAL Lack of power assist may be due to low engine vacuum, a leaky vacuum hose or a defective booster. Sometimes a faulty check valve will allow vacuum to bleed out of the booster causing a hard pedal when the brakes are applied. This condition can be diagnosed by starting the engine (to build vacuum), shutting it off, waiting four or five minutes, then trying the brakes to see if there is power assist. No assist means a new check valve is needed.

A quick way to check the vacuum booster is to pump the brake pedal several times with the engine off to bleed off any vacuum that may still be in the unit. Then hold your foot on the pedal and start the engine. If the booster is working, the amount of effort required to hold the pedal should drop and the pedal itself may depress slightly. If nothing happens and the vacuum connections to the booster unit are okay, a new booster is needed (the vacuum hose should be replaced, too).

So the symptoms would not point to a faulty master cylinder, correct if there's a problem with MC your foot will be sinking to the floor-

The caliper piston could be binding in its bore or the brake hose to it collapsed inside acting like a one way check valve preventing the fluid to go back into the MC.

DRAGGING BRAKES May create a steering pull and/or increased fuel consumption. Caused by weak or broken retracting springs on drum brakes, a jammed or corroded caliper piston, a floating caliper with badly corroded mounting pins or bushings (uneven pad wear between the inner and outer pads is a clue here), overextended drum brake self-adjusters or a sticky or frozen emergency brake cable.

Was the pedal like it is now, before the front brakes where done? If not check the pads and calipers again, if one of the pads arent seated in the mount correctly, you will have the symtoms described! Sometimes the wear indicators get cuaght under the mount hardware too.

If some put in ANY fluid other than brake fluid, it will destroy ALL rubber parts, these include, caliper seals and boots, flex hoses, master cylinder's rubber parts, combination valve. Then all steel lines need to be cleaned with alcohol, or brakkleen and compressed air. VER EXPENSIVE! Hold the shop that worked on it responsible! You may need to get a lawer. ABS Pumps motors and the like are even more expensive than the new brake parts will be!