New manual fuel pump stopped working after 10 miles

Hi I have a 1983 Golf cabriolet.
Recently  I have changed over a lot of things due to age and not wanting to  be stranded

eg Clutch, starter, water pump, thermostat, alternator, hoses, wiring loom for alternator, shocks and bump stops and  manual fuel  pump found on the front of the engine.

All  these things were original  35 years old  and as the car is doing a long trup this summer I did not want to be stuck in France.

I fitted a MEYLE German manual fuel pump and all was well and did a few local short  journeys. On the  3rd journey, then at mileage total  10 miles, the car stopped so abruptly I though it was ignition related and so pulled in and checked the spark - lovely and juicy and fat.

The fuel filter on cranking ( visible  type )  was dry, so I removed the pipes off the  pump to confirm  nothing being pumped.

The wife came and got me  and then  I returned with the old pump which I had not thrown away and was keeping as a spare.

On starting to remove the pump, as I loosened it, there was  a pump action  and fuel spurted.  On removing the pump and activating the cam lever,  the remnant fuel spurted out   in a good stream and  vacuum was  created when putting finger over the inlet.
I took it back to the supplier and they said it was  fine and so I was sent on my way.

This is no good as it should work on the car and is sold as a fuel pump for Mk1 Golf and Beetle   The cam lever is very slightly longer  but I cannot see how this would affect its operation  other than maybe the piston on the engine  hitting it earlier and never letting the lever return. However, the point of impact is under the lever, on looking at the old  pump and is shown by the black dot.

Does anyone have an idea why it should work for 10 miles  and then  fall off the piston  or suddenly not be able to "return" allowing the one-way valve to work ?

I'm lost on it  as there is only one way to fit this and it would not have worked for 10 miles had a fitted it wrongly.

Thanks
Ted

I have posted picture of shiny pump x 2    The black dot is drawn on by me and represents  the point of impact on the piston   I saw on the old pump when I removed it. Obviously the  old pump is on the engine  at this point so I cannot show it  

We had an issue once with fuelling and suspected pump, however it was rot in the fuel filler neck blocking pick up filter in tank. This caused a vacuum in the pipe between the tank and the pump, pointing to the pump not working, basically evry 10 or so miles it would run out of fuel and by removing pipe from pump would allow bits to drop off filter and we could drive on.

It may be worth checking filter in tank as I cannot see why new pump would work off the car but not again, it could be a coincidence this has happened with new pump or maybe it has better pulling power lifting bits off bottom of the tank. 

HI thanks for this./ Is there another filter on the tank piclup pipe?.   I did not know this.

I put the  new pump back on  again, and took the car out with the  old pump beside me and it drove fine and we are  now in France.

I do not know why this happened and it is a mystery, don't  forget it was when I removed the pump that the pump action worked  when I broke down.  
Thanks for your input
T