This all started with a fuel leak which developed while I was overseas – I came back to find a large pool of petrol under the car, still dripping. It had not been run for 2 months!
On pulling things apart I found that the hose which leads from the pump to the filter had rotted through at the pump connection. Probably it was the original hose (32 years old) or someone had replaced with the wrong type.
I decided while I had it apart that I would check a few things so took out the sump (expansion?) tank, to find it contained half a cup of mud-like dirt, – not rust. The metal surface inside is in perfect condition.
So the dirt is coming in from the main tank, I think it is dirt from the environment or in the fuel.
I am now going to fit a low pressure fuel filter in the line from the tank to the sump, it should catch all future muck.
I have been told that all piping in petrol systems should be high pressure (fuel injector) type.
They are less susceptible to rotting than low pressure types.