I found out recently that if you get a reverser light on the ground, when you cycle the reverse thrust to try and clear it, you need to leave the reverser unlocked for around 30 seconds, then keep it closed for 30 seconds. Apparently you need to give the vales etc time in order for them to reach their full open/closed position.
The main reason not to cycle the reversers too fast is that the hydraulic fuses may trip, and then nothing is going to run until the fuse(s) are reset. On the NG you have to cycle the reversers 5 times (full open/close cycle) to clear the engine accessory unit fault history and hence get rid of the reverser light.
