She charged to the line to beat the Lindsay Park-trained Hafawaa ($5.50) by a short half head with Olympic ($4.20 fav) another three-quarters of a length away.
Pin Sec is by champion Lonhro out of Listed winner Moderate and Hyeronimus said she travelled sweetly despite him taking her back in the field early.
“She jumped well and probably could have been closer but I brought her back to get her into a rhythm,” Hyeronimus said.
“The topweight (Bacchus) bumped me a little but she's a little sweetheart and nothing bothers her.
“I was able to get her back into her rhythm and she did the rest.”
Godolphin representative Darren Beadman said she was a filly with a lot of improvement to come.
“It was very pleasing. She overcame difficulties a couple of times,” he said.
“She's a lovely, scopey filly.
“She copped a few bumps but there and there are bigger things in her future.”