Power(less) Brakes?!?

[chewed 71'er]

Is there a method for checking your power brake booster? My brakes are gradually losing thier ability to stop. Normally the truck seems to brake okay. But in a fast, hurried stop, the truck wants to keep rolling (not much, but noticeably). Though not desirable, shouldn't you be able lock up all four wheels in a hard braking? I've recently replaced the master cylinder and proportioning valve (they were leaking),and throughly bled the system per the Chevy manual. The brake pedal is solid, not spongy, and doesn't need to be pumped. Also, the truck stops straight, doesn't pull to either side at all. There's plenty of lining on the brake shoes and pads. It just seems I'm not getting much assistance from the booster (which will hold vacuum overnight).Replaced the vacuum hose, no difference. Just hope it's something simple.