Surging rpms when braking

I have a 2007 Chev Colorado PU (automatic) and whenever I use the brake to come to a stop my rpms surge from 500 to 1000 - both visualy on the tach and you can feel it also. When the truck is stoped it returns to noral (750). accelerates fine - shifts fine - fuel economy is a bit down.

Have your mechanic check the power brake booster.
My best guess is that you have a vacuum leak in the booster.

Remove and cap the vacuum line from the brake booster and step on the brake. If there is no surging the booster has a leaking control valve. There are no DIY repairs for boosters. Any failure requires replacement.