Had same problems a few years ago did everything to try to get this ball type valve to shut off .. solution was to go back & fit the original SU shut off valve the type with a fine pointed needle that is mounted in a fluted plastic guide .. my problems were then solved.
I had the same problem, would be OK for a while and then start leaking. Tried a few different needle and seats but same story. I suspect crap coming from the tank, even after I had tried to clean it out.
Put the filter in, and no more leaky needle and seats.
Need to find a more period correct filter now.
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Get a very long inner speedo drive cable and a drill and stick it down the steel fuel line to help clean it out . disconnect both ends of fuel line first and flush out fuel
As said, kunifer doesn't "rust". The crap will be coming from the tank. Unless ofcourse it's a brand new tank, then it could just be a bad dose of supermarket fuel. Either way blow through the line with both ends disconnected and fit an in line filter as shown on the previous reply. I wouldn't go to the bother of replacing the fuel lines if they are new.
I have seen "stuff" precipitate out of the fuel itself, and, collect in the fuel bowls.....
Also, the "red dust" quite commonly found in the bottoms of float bowls, is pretty normal, and, nothing to be overly worried about....It is very fine, and, actually goes right through most filters......It isn't large enough particles to stick needle valves open.......
How old is your fuel line......If it is original to the car, it may well be rusted internally. if it is newish/recently restored, not very likely that it would be corroded....Those two depressions right at the bottom of the float bowls, are actually debris/sediment traps.....
Since you apparently have a good filter before the pump, I would put another see through one just before the carb, and, monitor that for debris for a few tank fulls.......
