Brake problem

I own a 2000 Astro LS. I'm having a problem with my break pedal. Every now and then when I press on the pedal to stop, it gets hard. I release it and press it again and its gone. I have no problem with the steering. Can anybody help me out here? I've gone to mechanics and they can't figure out what's going on since I have no problem with the steering.

Usually when the pedal gets hard, it's the power brake booster. The 2000 year model has the hydraulic booster which works off the power steering pump. Check your power steering fluid level. If it's a bit low, it might not effect steering but it will effect the booster. I've seen a '93 chevy truck with a teaspoon of fluid in the power steering resevroir steer just fine...well it did groan alot. If the fluid is low, there's a leak somewhere.
If the fluid isn't low and the problem is intermittant, the inside of a hose could have sloughed off and is clogging a line periodically, or the booster itself may be going out.