Dean was at the top of the leaderboard with a third Yorkshire player – his Lindrick club-mate Jonathan Thomson – overnight.
However, while Thomson found reverse gear with a 76, Walker Cup candidate Brown (Bedale) shot a three-under-par 69.
He began with a birdie and completed his outward nine with three in a row. He gave one shot back at 10, birdied 14 and then bogeyed 15 for his 210 aggregate (71 70 69).
Dean (72 68 70) showed laudable composure by not allowing a double bogey 7 at the second to knock him off kilter.
He birdied the fifth and ninth to be out in level, and had five birdies coming home – including four in a row from the 14th, as well as three bogeys.
Thomson (71 69 76) birdied the fourth, but then had a three-hole stumble, shedding shots at five, six and seven.
He had two birdies and three bogeys on his inward nine, leaving him tied 11th, six shots back of the leading Yorkshire pair.
But he is capable of a sub-par blitz, as he showed with a six-under-par 66 on his way to sixth in the recent Brabazon Trophy and when six under through 10 holes in a 7&5 singles win against Cheshire last weekend.
So Yorkshire could yet have three players challenging for the title on the final day of the competition.