Sorry to hear about the troubles.
First off what is the 'landscape' of the install. How long is the run from the tank to the pump, Pump to the Heater. Is the pump oriented in an up tending direction, where is the pump, where is the heater. Was a razor knife used to cut the hard plastic fuel line or sidecutter pliers. What standpipe was used and how deep does it going into the tank. Is the fuel line firmly butted up to the input and out sides of what it is pushing fuel through. (Apologies if these were already covered in your build)
Second and generally speaking, I am confused on the fuel line descriptions .
a) Our install used the 4x1.25 for both intake and delivery side. (Heater under passenger seat)
b) There are a number of other folks in the forum that got the 4x1.25 fuel line in the espar kit from espar of Michigan and their systems are working fine
c) The description of a 4x1.25 really has got me confused "4x1.25mm fuel line (thats 4mm OD, 1.25mm wall thickness, 1.5mm ID)
c.1) I concur that it is 4mm OD. At least that is what I measure.
c.2) However, (There is that darn however;-) Using a collection of small drill bits to test the ID, I could not get a 1.47mm bit into the tube. If I try really hard I can get 1.37mm drill bit into the fuel line. (I don't have a 1.25mm bit handy.) From where I sit that would lead me to think that the tube ID is 1.25mm +/- some tolerance. (not the 1.5mm)
c.3) This is splitting hairs, literally and figuratively and it does cater to my overthinking these things but what is the ID supposed to be on the 4x1.25. I always assumed 1.25. Anyone else take some measurements?
d) This is rhetorical but do these new observations indicate that the manufacturer has updated guidance for the 'Benzin' heaters on the ID of the fuel lines for the input and output side of the pump?