Brake problem - if i brake abruptly, my brakes will grab then pulse and grab again...

[$teady$upreme]

What's the problem?

If i brake applying even pressure, brakes work fine.

If i brake abruptly, such as even pressure then brake harder, my brakes will go grab initially then pulse (nothing) then grab again.

Can you guys please help me out, i feel very unsafe.

PS. Pedal doesn't sink to the floor

[$teady$upreme]

anyone?