I had an 02 polo and the brakes were spongy. I seeked help but was told by a few people that it was normal... Just my 2 pennies
Plus check for any fluid leaks at each wheel and around the master cylinder, including the carpet in the footwell etc.Resevoir should be between min and max indicators on the side of the container.
my girlfriends polo has spongey brakes too!! It is a bit older than yours (1999) but i changed all the discs and pads and bled the system and they were still the same, ive decided that polos just have crap brakes!