The use of MAX Gain at high altitude is good operating procedure with some radars. This technique used with appropriate tilt control will sometimes reveal a tiny echo inside 40 miles that is the top of large CB particularly over the Pacific where we used it on the 737-200. Works over Vietnam where there are huge CB at times. In the normal AUTO position, chances are there will be no returns from CB tops. On MAX gain chances are an echo is visible.
The Pilots Guide for Collins WXR 700X Weather Radar (October 1994 edition) Page 44 states in part:
"The ability to adjust the Gain Control during weather detection modes provides some advantages in weather detection at the shorter ranges...specifically aircraft operating at high altitudes run a greater risk of encountering low reflectivity targets such as ice crystals. Adjusting the Gain to provide maximum detection capability will aid in locating areas of low reflectivity targets.
Remember that by increasing the system Gain, the target will appear to be more reflective than normal weather detection conditions would indicate."
In other words very tall CB tops.