I'm unable to get the PV gas tank sender unit seal on the top of the tank to seal. I replaced the gas tank with a new one and ordered a new gasket for the sender unit which screws down with about 6 screws. But fuel leaks out from around the gasket seal. So I tried putting Blue Hylomar sealant onto the gasket. No cigar! Just can't stop the leak. Anybody have this problem and what did you do? Help!

What type of gasket are you using?

Is it the same material as the original? (You did keep the original)

Maybe you can fashion a gasket from a material similar to the original, it might swell and seal better.

A double gasket might work - this is not a situation where pressure is an issue.

You might check the flatness of the tank hole and the sender flange. Out of the box, you could use the top of a large jar that has been checked for flatness itself.

FYI, I was about to sell what Volvo is now selling as a gasket for the 122 and 544 fuel sending units to the OP, but realized that the rubber gasket Volvo now sells won't work as the rubber is now much too thin to seal the original style of VDO sender. The VDO sending units have a lip around the perimeter of the mounting flange and require a much thicker gasket.

What Volvo now sells for a gasket might work if 2 are used, or if an aftermarket sender like what VP sells is used. The VP aftermarket sender has a flat mounting flange.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

My thoughts are that the mounting surface is the first place to look. After mounting the sending unit, see if you can run a feeler gauge or equivalent between the two surfaces. Perhaps it is slightly warped and not clearly visible. Perhaps a few gentle taps with a hammer can fix the issue.

Perhaps Permatex PermaShield # 85420?