Sounds like you have some mis-matched parts. The brake parts need to be matched together just like engine guys match cam, compression and heads.

You didn't tell us anything about the parts so it is impossible to help you, but my guess is that your rear calipers aren't the correct size to work with your front calipers. And/or, nothing is correctly sized to work with your master cylinder.

The brake system is a hydraulic system. Piston sizes need to match properly at all four corners to give you the correct brake force at each tire for your vehicle's weight distribution. If you mess up the brake force by using calipers that are too small or too big, then the car doesn't stop very well.