Mk3 Transit fuel vapour vent valve

[SIMPLE]

Hi all, I have a 1988 2 litre petrol camper. I think I have a problem with the fuel vapour vent valve which releases pressure in the fuel tank in hot weather etc. I've tried blowing through it to no avail, but it might be beyond my puff level. Can anybody suggest how to test it or can tell me what the blowoff pressure should be? I can get a new one from Ford, at a price, but testing by substitution can be waste of money and my name isn't Royal Mint.
Cheers Simon.