PreCheck at PHL is a mess. I fly out of terminal F and they have no PC lane at any hour or day of the week. They do give you the option of "PC Lite", as another poster called it, whereby they put a TSA sticker on your boarding pass (if paper), which lets you keep shoes on and go through the regular metal detector instead of the full body scanner.
A few days ago I was at terminal A around 7pm, and PreCheck was closed there too!