"Empty Trash" Menu
You can add this missing feature in LXDE, emptying the trash can using a menu thanks to the file manager actions (it should be compatible with all versions of PCManFM except maybe Qt based builds, for now). 
For Lubuntu 15.04 and 15.10
Open a LXTerminal and use these five commands:
- sudo apt-get install trash-cli
- mkdir --parents $HOME/.local/share/file-manager/actions
wget -O $HOME/.local/share/file-manager/actions/ask-trash-empty.desktop https://raw.githubusercontent.com/NicolasBernaerts/ubuntu-scripts/master/lubuntu/trash-empty/ask-trash-empty.desktop
sudo wget -O /usr/local/bin/ask-trash-empty https://raw.githubusercontent.com/NicolasBernaerts/ubuntu-scripts/master/lubuntu/trash-empty/ask-trash-empty
- sudo chmod +x /usr/local/bin/ask-trash-empty
Re-login again and everything should work smoothly.
For Lubuntu 14.10, LTS and below
Even though these options haven't been tested, the options should work as suggested in the above paragraph.
Further Contacts
If this workaround stops working for any reason, please let us know at the appropriate mailing list (lubuntu-devel-discuss) or at #lubuntu-devel @ freenode.
This workaround was originally found at Bernaerts blog.