Middlebrook and Hall put Northants in control in Cardiff
Middlebrook top-scored with the County making 292 all out in their first innings - opening up an overall lead of 158.
And in their second innings sorry Glamorgan were reduced to 71 for four before finally ending the day on 96 for four, still 62 behind.
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Hide AdNorthants had resumed their first innings on 145 for five, a lead of 11 after only 17.1 overs were possible on the second day.
Andrew Hall and Middlebrook batted for the first 90 minutes of the third morning taking the visitors past 200 to register a batting point.
But the sixth wicket pair, who added 60, were separated when Hall drove Mike Reed straight to Ben Wright at point. Hall went for 32 with the County at 202 for six.
Northants had reached 218 for six by lunch - a lead of 84 - boosted by a pull for six over backward square by Middlebrook off Graham Wagg.
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Hide AdMiddlebrook cleared the rope again after lunch striking slow left armer Dean Cosker for his second six as he moved into the 40s.
Glamorgan took the second new ball and Michael Hogan broke through almost immediately to have David Murphy caught at slip with Northans 266 for seven.
After a rain break Wagg claimed the last three wickets in the space of 13 balls as the County were dismissed for 292 - a lead of 158.
First he bowled David Willey before trapping Stephen Crook lbw for eight and then having Middlebrook at slip by Allenby.
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Hide AdGlamorgan openers Ben Wright and Will Bragg survived until tea reaching 21 for nought in seven overs. Bragg batted with Cosker as a runner after not fielding for most of the day.
The openers batted well until both perished in the space of only five balls.
Bragg was trapped leg before to Willey before Wright edged a rising Hall delivery to Trent Copeland at first slip.
And Australian overseas Marcus North failed for a second time edging a good Crook delivery behind as Glamorgan slumped to 69 for three.
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Hide AdThat became 73 for four as Stewart Walters fell to a sharp catch by wicket-keeper Murphy standing up to Hall. Four wickets fell for the addition of only nine runs.
Jim Allenby and Murray Goodwin, who scored three from 41 balls, took Glamorgan to the close without any further scares.
Play is due to resume today at 11am.