Try lowering the gear by the normal system on a 757 when a bolt in the linkage "falls out" during retraction with the valves in the "up" position. Nothing happens!!
The emergency free fall may release the up locks but with the gear being powered up still nothing happens.
Even depressurising hydraulics didn't help. Three greens were regularly replaced by reds as the over centre locks were broken by minimal flow/pressure from the depressurised system.
Two hours later, cycling the speed brake on the approach to completely dissipate the "up" hydraulic pressure, and three greens were achieved for long enough.
Stop on the runway.. gear pins inserted... taxy to the stand...go home.. have a beer, a job well done.
Wish I had been there when it was suggested, to Boeings rep at London, by looking at the thread that the bolt had never had a lock-nut installed at manufacture!!