Since both pumps in question are 4 years old, let me ask a few questions here...
Any and all quick disconnects on your system have rubber O rings to seal them, has anyone bothered to clean the dirt that deposits around them and the surfaces they sit on, then lube it up with some silicone pool lube?
This also goes for your Pump lid and O ring seal on it.
If that does not work, have you tried removing all teh air out of your sand filter?
I just found a great way to do that recently, place filter on rinse setting, hold your discharge hose above top of filter and fill it with the garden hose, it fills up your filter quickly.
Also, has anyone bothered checking the basket in the sand filter, many people don't ever bother taking the selector valve body off their filter, when they do, the basket's full of leaves because somewhere along the line, someone ran the system without a skimmer or pump basket thinking they can discharge the leaves by vacuuming to waste and pugging the hidden basket. (top mount valve unit - not sure about others)