My 87 Syncro GL 2.1 WBX releases way too much pressure, I believe, when the fuel filler cap is opened. It may be my imagination, but the problem seems to get worse the longer I drive and the emptier the tank is. When I first had the van, before any work was done, the fuel sender was leaking badly allowing any air (and fuel!) to escape readily. 

The engine/ trans and fuel tank were removed and all fuel lines, grommets, rollover vents, seals were replaced along with fuel filler neck, fuel sender, and tank straps prior to reinstalling refurbished engine last year. Only the cap and tank itself are original. The tank insulation was omitted during reinstall. 

Any suggestions on what could be wrong?

Thanks in advance, Steve

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