Only rain, which removed 31 overs from the day, robbed Brown of passing his career-best 187 he made against Gloucestershire in 1995.
The stoppages forced the hand of the skipper Adam Hollioake to declare on 581 for 7, the third highest championship score made by any county this summer. By the close, Surrey still led by 427, although Northants had eased their frail nerves somewhat by totting up 154 for 1.
Brown, unbeaten on 63 overnight, was fresh from a double hundred on Sunday but initially played second fiddle to Ben Hollioake whose 49 include an almighty six into a neighbouring garden.
The pair added 104 before Hollioake mistimed a drive from Scott Boswell's first ball of the day, but Boswell was still one of four Northants bowlers who went for over 100 runs as their attack laboured increasingly in vain.
Brown's innings was laced with 24 fours and two sixes, both from the bowling of the off-spinner Jeremy Snape, and it came from just 182 balls.
His seventh-wicket stand of 120 in 26 overs with Jonathan Batty saw the young wicketkeeper score just 23 before he fell victim to Tony Penberthy's second run-out of the innings. The all-rounder also claimed a stinging return catch to remove Chris Lewis.
Lewis provided a lively start to the Northants reply, clean-bowling David Roberts in the second over but his five-over spell saw him no-balled seven times and he came off with figures of one for 51
But then Alan Fordham, playing his first championship match for a year, pulled Northants round thanks to an unbroken stand of 134 with Russell Warren.
Both players struck valuable half-centuries, Fordham hitting 12 boundaries, while Warren produced the most eye-catching shot of the final session when he square cut a ball from Lewis for six.
Rana Qayyum made Sussex pay for fielding an inexperienced side by hitting a magnificent 75 not out as Pakistan A made 306 all out after being put in at Hove.
Qayyum smacked 12 fours and a six and enjoyed a sixth-wicket stand of 93 with Azhar Mahmood (63) and a seventh-wicket partnership of 71 with Javed Qadeer. In reply, Sussex were two without loss when rain ended play early.
The home side included nine uncapped players, eight of them under 25, and their decision to send the tourists in soon came into question as opener Ali Naqvi smashed 46 out of the first 58 runs
Pakistan, playing in their first four-day fixture of the tour and seeking a second successive win after beating Somerset, slumped to 94 for 5 before Qayyum and Mahmood revived the innings with an entertaining partnership.
Mahmood had struck 10 fours and a six off only 74 balls when he attempted to drive Robin Martin-Jenkins and the wicket-keeper Shaun Humphries held a low catch.Reuse content