How did you determine the MC was bad? I suspect you still have air in the system. How much fluid did you go through bleeding the system.

 

Possible Rubber hoses ballooning under pressure

Rear Brakes not adjusted correct Parking Brake in good working condition ??

Sucking Air in one of the Master Cylinder connections

 

 

Throwing parts at a problem is a poor way of fixing it. Relies on total luck and money.

So I ask again. How much brake fluid did you use to bleed the system. A small bottle, a large bottle, 2 large bottles.

 

It can't be the booster unless your engine runs like crap when you hit the brakes or you have to hit the brakes REALLY hard to stop. A soft pedal will be what the others have mentioned, air in the system, improper rear shoe adjustment, hose ballooning or a combination of those things.

 

 

That the 32 OZ bottle or 64OZ bottle ??

 

The more detail you give us the better we can help. But if you don't help us by answering the questions then how are we supposed to help you?

 

 

 

You probably still have air in your lines.

Did you bench bleed the MC first?

 

 

 

That is because the brake booster is helping you push the pedal when the engine is running.
try tapping on the side of the master cylinder during the bleeding process, you will likely see air bubbles come up into the reservoir when you do.
If you start the bleeding process this way (getting all of the air out of the master cylinder that you can) it will make the whole process go faster and you will have better results.