Sorry to bring this to life again, but I wold be very interested to know if the diaphragm replacement solved your problem of the pressurised fuel tank. I have the same problem on my 1988 90, only difference being that my tank sprung a leak and I can assure you that the breathers and/or fuel cap are not the problem. I'm going to try and get a new diaphragm this week and see what happens once it is replaced.