ther is a very simpe problem that could be happening.
you could have air in the brake lines.

you will need a person to help you.

so bleed the lines. open your master cylendar, your also going to need to take off each one of your tires, at the same time or independently is your choice.

behind the caliper you will find a little valve, if you open this it allows brake flued to leave the lines.

while the car is off pump the brake 5-10 times and open the valve, brake fluid should sqirt out, it may spay to start, that would be the air mixed in as well, then close the valve.

continue this until it spurts a steady stream.

then check the master cyendar and fill it if necisary.

do this to all of the wheels and brakes should be very firm.