so my brakes are fading under heavy braking and i wanted to bleed them, but my mechanic told me that if when i pump the brake pedal it becomes stiff after 3-4 pumps means there's no air in the system, is that true??

You can compress air but you cannot compress liquid. If there is air in the system it will use the cars cars braking ability to compress the air before it will apply the brakes. Typically you will have a low squishy feeling brake pedal if there is air in it. What your mechanic is talking about is turning off the car and pumping up the brakes. If you turn it off then engine isn't going to make/add vacuum to the booster to give you power brakes, pump the pedal a few times and it will use up the vacuum in the booster and the pedal should get rock hard with almost no movement (till you start out and the booster gets vaccum again). Typically its used to make sure the pedal keeps the hard feel and its not loosing pressure when you pump it up and hold it.

Either way I'd go ahead and bleed them because if your car is like every single other J-body I've seen the brake fluid has never and will never be flushed unless they did a big brakes or SS lines where they had to.

Brake fluid absorbs moisture to keep the brakes system from rusting from the inside out, but it can only absorb so much, which is why typically your supposed to change it every 2-3 years.

I agree with Darkstars here. I bleed my brakes for a complete day when i did my big brake kits to get them to feel right. just bleed and bleed and bleed til they feel right again, and feel how Darkstars has described.