I have a Grundfos Scala2 supplying water to a small domestic household. The water source is a large rainwater tank. 

Over the last few months the pump has been running more and more often, unneccessarily. 

I know that a common cause of this problem is that the internal (air) pressure tank loses pressure and needs to be recharged. There's a schrader valve on the pump to allow this. 

When I measured the pressure I found that there was no pressure at all, cool I've found the problem then. But the weird thing is that it takes very little air to get the internal tank up to pressure, and being a schrader valve the full volume of air I pumped in just escapes when the pump is disconnected from the valve with a loud and very brief PSHHHT. This is identical to the loss of a very small amount of air just like a bike or car tyre when you remove the line, but because the tank is so small it's ALL the air escaping. 

This seems odd - how do you pressurise a tank so small that all the air escapes due to the design of the valve you use to pressurise it?

Also a note on the behaviour of the pump - it turns on when we run water and turns off when we stop. Good, that's what it's meant to do. But then 5 minutes later it runs again. It starts slow, gradually increases in speed, slows down and stops in a cycle that takes about 10 seconds. It repeats this start stop cycle for 5 to 20 minutes. 

There are no leaks anywhere. 

I haven't even found a plumber locally who sells or services these pumps - the only one who had any familiarity at all said they have to get their parts for this pump from Singapore and don't have any in stock. 

Grundfos NZ haven given very little support so far. How does this pump seem so common but so unsupported?

Does anyone have experience with this pump who can offer advice?