Compressor issue

This roll air compressor runs as quiet as a church mouse when initially started but refuses to start back up as it should once the start up switch detects that the pressure has dropped sufficiently.
When it’s running I can hear air leaking, discovered air leaking back out the air intake tube, difficult for me to even cover tube with my thumb.
Why might this be happening?
Someone on another forum suggested an unloader valve?
If this might be the issue how would I find this valve?
It will start fine every time when plugged directly into a power source but when an extension cord is used there is presumably too much of an amperage drop and presumably the electric motor just struggles with trying to spin it over because the cylinder is locked up ( once the air that’s escaping the intake tube lessens in pressure it will start on its own just fine )
If not an unloader valve malfunctioning than why would air be escaping out the intake tube.
Thanks
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Sounds like the starting capacitor(s) may be failing causing the hard starting...
 
As for the leak, take off the plastic cover on the starting switch.
I've had the diaphram inside there go bad, specifically "spit" the edge of the diaphram out the side
(even when tight).    
 
.