Water pump loses pressure on second activation

[vbr666]

I have a weird problem with my car washing water pump (mini wash).
When I first activate it, it works fine, but after I release the button to close the water flow and reactivate it, it loses more than half of the pressure so the water flow is very low.
This happens in 99% of cases after you first let the water flow and then release button.

But it is best described on this video:

http://drop.io/5otiaxl/asset/my-movie-xvid-mp3-avi

What could cause such a behavior?

I've replaced this pistol, because I thought it is the one to blame, but even with another pistol, problem is the same.

The pump isn't under warranty, it's old and can't be taken to the service as it's too expensive, I've already had to take it apart and replace ball bearings which have been broken.

Another thing, when you remove the pistol (or leave it on, but just remove it's button for activation of the water flow, so that water constantly flows), the water flow is always at maximum, as it should be, then the pump works normally, without these variations in water flow.

I can be more specific if necessary, you ask what you think you need to know about it.

Thanks for the answers in advance!

 

[BigInch]

Cavitating from multiple liquid column separations in the hose.

Try a shorter (10 ft max) and larger diameter hose pumping from a 55 gal drum.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/