I think you're on the right track and I have confirmed with microscope. I have removed one seat from a new primer cap. I knew there must be something on the top of the chamfer which you may suspect is a burr which is biting the viton tip and yes, you're right, there is. There is a slight bulging and microscopic burr around the top of the chamfer. This is due to the manufacturing process using manual press. The seat was machined using an advanced 7-axis CNC machine but the chamfer was done the final stage using manual press. During this chamfer press process with center guide, it pushed the top edge outward to create a slight bulge and microscopic burr. I will get back to the manufacturer to give them feedback about this finding and suggest them to improve the seat surface on our next order but at the moment I have to do all the mods myself on our remaining stocks.
Mowerfreak, You may need to modify your new primer cap to improve surface contact and sticking needle. You can follow this easy solution I did below.
My solution: Using a 3.7mm trimmer string rod with flat end for sanding the seat with 3.7mm sanding disc attached. . Punch-hole a 3.7mm 1000gritts sandpaper disc and glue it into one end of the string rod above. Use the rod with sandpaper disc on the tip on sanding the seat surface by spinning the rod by hand. No need to use drill or power tool to spin the rod. Use NormK autosol method for final sanding.
This really works great and shows a big improvement.
With unmodified seat, I can still observed needle sticking even when fitted with the new float lever. When quickly releasing the float, it took a second before the float drops. The sticking needle is holding the float for a second.
When modified and quickly releasing the float, the float will just drops instantaneous from zero to its gravitational fall.
Overall, there's no need to spend to source for special tool. There's no need to chamfer or grind the seat as it's already been perfectly chamfered at 70° for maximum surface contact with viton tip. Do not sand or modify the viton tip rubber. Only use the method above to remove the bulging and burr.
On the attached photos, the tip of the rod with sandpaper, a ring mark concentration on the center is very noticeable is due to the seat bulding in contact with the sandpaper disc.
