Adjusting seat post on Jamis Coda sport

I found this bike under a tarp at the place I just moved to. Tires are flat, chain and sprockets pretty rusty but I should be able to take care of that. Gear shifting and brakes work fine. Before I put in the elbow grease, this is a 17" frame and I am 5'5". I'd like to push the seat down a bit, but it's not budging. I opened (and actually removed the clamp) and put WD40 around the outer rim where the seat tube goes in. No improvement. How can I tell if there is room to push down?