Glamorgan ended day two on 126-6 at Colwyn Bay, 152 runs ahead of Sussex.
The home side were well placed on 60-1 at tea, with debutant Jack Murphy top scoring with 27, but lost five wickets for 35 in an hour.
Earlier in the day Glamorgan bowled Sussex out for 268 with Craig Meschede (4/61) and Ruaidhri Smith (3/64) the pick of the bowlers.
"I was so happy to get that single to get off the mark and get off a pair, then it took me one straight drive to get comfortable, it was nice to find my feet and get a few runs but a shame about the dismissal," said Murphy.
"We managed to see Robinson and Jordan off and we were going quite comfortably but both set batsmen getting out in a couple of overs proved crucial and they bowled well especially in bad light.
"Hopefully we can come back with Mesch (Craig Meschede) and Tom Cullen, our heroes from the first innings, and get a lead of 250 on this pitch with a few keeping low."