Chevy & GMC Vans

My son has a 95 Monte Carlo and the brake peddle is very hard to push and the brakes don't grab well. I've replaced the front pads, rotors, rear wheel cylinders and shoes. (they needed to be done) I've also bleed the brakes but these things have only improved the brakes a little. Any suggestions on what to look at next? The brake booster seems to hold its vacuum overnight but I'm not sure if that tells me if its bad or not.

I suspect the Brake Booster. Disconnect the fat vacumn line and plug the intake manifold.

Is there any difference?

Thanks SoCalli, that was what I suspected but wanted to hear it from someone else. I hate throwing money at it only to get poorer with some bloody knuckles. I'm on a business trip for a couple of days but will try it when I get back.