I just finished restoring my 1930s NSU bicycle. It's off to it's new owner......but, I can't get the seat lowered, or get it to move at all.

It's been sitting in WD40 for a couple days, but won't budge--any ideas?

What I did for mine was put an old crappy seat on it and then hit the seat with a mallet to rotate not push down the seat post and it should snap free.

Instead of wd-40 which is meant to remove squeaks, try something that is meant to unfreeze/unlock bolts, etc.

can take months to remove a post, but if you keep adding wd40 day after day it will eventually budge

From a suggestion I read [maybe] on here, I used a 50/50 mix of acetone/ATF and it worked on a rusted in stem after a good day's+ soak. The original test for this was in a mechanic's (not bicycle related) rag.

Their test found this mix worked better than WD40, PB Blaster, and Kroil.

Search is your friend. And Sheldon Brown has a good article on it as well. Odds of WD40 working are just about zero. There are various levels of attack, from simple to more involved. The last step if nothing else works is to cut it out (very carefully of course). I have one I have been trying to free up for several months, avoiding the cut it out step. But that's next. I'll do it after I finish my other bikes first.

I've told this story at least once before here so I'll try to summarize:

At the co0-op recently I could not budge a seized stem. After several cycles of lubricant and mallet a shop mentor started to cut the stem open with an angle grinder. Almost immediately into the cutting process the stem came loose. The moral of this story is that before you do anything really drastic try doing something to introduce some serious vibration.

I've been thinking that one of those new electric nailhammers might work well to vibrate a stem and seatpost loose, without harming the frameset or fork, plus you can direct the vibration and impact wherever you need it.......sort of a poor man's impact tool.....with no need for compressed air....... I think Ryobi and B&D makes them and can be checked out at your nearest big box hardware stores.

Chombi

I can attest from recent experience that dumping a can of Pepsi/Coke in the seat tube via the BB and letting it sit overnight works. I even used Diet...lol

Always had good results with PB Blaster. Forget about WD-40. I had a stuck SR Laprade seatpost in a Specialized Rockhopper Sport. In there very well. 24 hr. soak while inverted, followed with a big wrench and me standing on its end, forced a small turn. That's when I knew I was going to be successful. It has managed to break free BB's seatposts, suspension bolts and tie rods, where Liquid Wrench had failed.
Those in Europe who can't get PB Blaster can try Kroil. Heard this stuff is like magic.

This is going to sound a little random but I used Lemon Juice and the rubber mallet twist approach to get a frozen seatpost out of a Miyata 710 that appeared to have been sitting outside for at least a couple of years based on the rotted seat etc. I firgured the acid from the lemons would eat the rust and after a couple of hours apparently it did.

Though many people will suggest the soak and brute for method for removing seat posts or steering stems, you do so a great risk to the frame or fork set. I have destroyed a lovely old Torpado frame set once just because I used the above method. Now days, when I encounter a badly stuck stem or post, I cut them out and ensure that no damage will be transmitted to the frame.

This is how I do it...

https://www.mytenspeeds.com/My_TenSpe...troduction.htm

Hope this is a help.