NO breaks at low speed

My 99 sierra's breaks are not working very well at low speed when I am comming to a stop. They act as if I have no power to them. The pedal is VERY hard to push down, but it will stop if I push hard enough.

Could this be a vacuum leak going to the booster?
Or somthing else?

Thanks,Eric

It could be a vacuum leak, or there may be an internal leak within the brake booster.