When I engage the blades they work and you can here it when I start cutting grass there is a sqeaking sound and the blades slow down and will not cut very well. I have got new blades and still have the problem. What is wrong?

What type of mower is it? It it a tractor with belt driven blades? If so, its possible the spindles the blades attach to have failing bearings. If you took the belt off and disengage the brake brakes do they spin freely?

I think Smoothy is in the right area but I think your belts are slipping.