Replace the entire valve assembly instead of the seals.
Blade may have a nick somewhere you cannot see.
Pretty much the same procedure as replacing the seals, remove 4 bolts, apart from tank pipe and and remove valve.
Reverse procedure to reinstall.
Do have to be very careful that you do not displace the seal when reinstalling as it can be a very tight fit going back in.
To make it easier, remove and replace the grey tank valve at the same time since they connect via a "Y" and removing both at the same time makes it easier to separate and reinstall.
If I remember correctly, the package recommended to use a dab of Vaseline/grease between the valve body and the seal. The grease will hold the seal in place, otherwise the seal simply falls out of place. I can say that I noticed the seals are easy to dislodge with out the grease so I used some Vaseline grease.
Found a video which may be helpful, at the 5:20 mark, you will see the black valve being reinstalled, although they did not mention or show adding the grease, they did add grease to the seals and that is visable around the 5:20 prt of the video.
VIDEO HEREHere is another replacement video, that actually mentions the grease..
HERE