Testing the sump pump I discovered that the pump did not function.
The sump pump that we have is less the 6 yrs old, but we discovered
during testing that it was not working.

Testing: poured a 5 gallon bucket of water into the sump.

1.) The pump did nothing.
Took stick and slapped the float switch. After 3 knocks the pump
kicked on.

2.) Then I discovered that the pump was spinning, but failed to pump
any water.
Started to remove pump by loosening the hose clamps, when I herd it
suck some air.
Hearing this, I plugged the pump back in and it kicked on and emptied
the sump down past the float. (not entirely empty)

3.) Poured another bucket of water in and this time it behaved
correctly.

Why did the pump act this way ? Do I need to replace the float switch
or the pump ? why did the air lock keep the pump from functioning ?

Pump: Zoeller M53 - 1/3 HP Cast Iron Submersible
(at least that’s what it looks like, tag is too crusty to read).
The float switch is not what is pictured on the Zoeller site, its a
black cylinder about 2.5 in dia. and 2.5 in high mounted on an arm
next to the pump.

Thanks