It probably has air in the system. There is a clutch master cylinder reservoir on the driver's side firewall. Remove the cap, pull out the rubber baffle, and check to see if you have fluid. If your fluid is full, read the next paragraph. If it's empty, fill it about 2/3rds of the way with DOT 3 or DOT 4 brake fluid, then bleed your clutch system. The M5OD clutch isn't hard to bleed, do a Google search if you don't know how. Now the question is; why did the fluid get low enough to air-lock the system? The slave cylinder on the M5OD is a known weak point, and is prone to leaking. To add insult to injury, it's inside the bell housing so you have to drop the transmission to replace it.
Antother possibility is the clutch master cylinder bushing that attaches the master cyl. shaft to the pedal assembly. Look up under the dash above the gas pedal where the master cyl. rod comes through the firewall. That shaft connects to the end of the pedal assembly on the "nipple". There is a plastic bushig that has little "fingers" that slips on the nipple, then the female end of the master cyl. shaft pops over it. These bushings break, and then the master cyl. is not actuated by the pedal. You can buy the bushing cheap at an auto parts store. If the end of the master cyl is washed out and won't hold the bushing, you'll need to replace the clutch master cylinder.
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2012 F-150 Screw 4X4, 5.0, XLT Plus, Off-Road package, 3.73/e-locking diff. A.R.E shell with extras. Ranch Hand grille guard. Tinted Windows. Line-X bed liner.