Need HELP, continuing brake problem, think mechanic is telling me wrong info!

[Amelia21]

Took car in to look at brakes due to hissing sound coming from brake pedal and harder than usual brake pedal. Mechanic said needed new master cylinder and replaced it. Went to pick up the car, problem was in no way solved. Took it back to them, they said oops and that it was now the power brake booster, which they then replaced (free of charge thankfully). Went to pick up car again, hissing sound has lessened, but the brake pedal is EXTREMELY hard. Feel unsafe driving the car because it is so difficult to brake. Asked the shop about this, they said give it a few days and it would probably become easier to brake. Is this true? It seems counterintuitive, since I thought the purpose of the power brake booster was to make it easier for the driver to brake the car? If what they told me is not true, what else would be causing this problem? Feel like they are not competent? Want to go in to talk to them with a better grasp of the situation.

[s10blazerman4x4]

Have you checked for vacuum leaks?

[BlazerLT]

Take it back.

[GMMerlin]

The power booster was the cause of the hissing sound.
If the vehicle was properly roadtested after the repairs the noise and hard brake pedal would have been noted and the proper repairs would have been done.
It sounds like you have no power assist. the purpose of the booster is to multiply the effort you apply to the brake pedal to make it easier to brake.
Either the vacum hose to the booster is not installed or the new part is faulty.
Worse case is the mechanic that worked on your vehicle is incompitant and damaged something.