2003 GMC Sonoma Brake booster

I have a 2003 GMC Sonoma. 4.3. I have good brakes, but when I shut the engine down over night, I loose the brake pedal. It goes about half way down. When I start it up, it can take a little while for the brake pedal to come back up. There's no fluid lose, so I presume I have a vacumm leak somewhere. I did do a vacumm test, and with the engine at normal temperature my guage reads 21 in Hg. If I rev the engine to 2500 rpms. The reading is 25 in Hg. And when I quickly release the pedal it drops back down to 21 in Hg. When I shut the engine off, it quickly drops to 2 in Hg and stays there. Any suggestions? Thank you!