Cros 362 Piston Help - What is this?

[WrongSide]

Was having a nice shooting session with my 362 when accuracy started getting worse.  I cleaned the barrel (I use Ballistol), resumed shooting session and accuracy came back for 5 shots, then it started spraying a 1.5" group (usually get 1/2" groups at this distance).  That's when I noticed there was a crack in the black band (gasket?) on the piston located next to the pump arm linkage connection.  I've had this 362 since February and been very pleased with it's accuracy until today. 

What is this black band? It looks like hard plastic based on the way it broke.  I have not done anything with the piston, valve, pump arm or linkage (all stock components that I've never disassembled).  If it is a gasket, does anybody have a part number for this?  Am I not using enough lube on the 3 pivot points for this airgun?  There is a nice oily sheen on the linkage arm pivot points with the pump arm and the piston. 

[WrongSide]

Here is a closer view of the broken item.

[WrongSide]

Here is a pic of the cup seal end of the piston.  I noticed there seems to be some black sludge.  Do I need to disassemble the 362 and clean the piston? 

[WrongSide]

Here is a pic of the cup seal end of the piston.  I noticed there seems to be some black sludge a little bit up the piston, but the piston looks good.  Do I need to disassemble the 362 and clean the piston?

[Back_Roads]

 That is how it was made, some sort of split ring spacer, you may have loose barrel breach screw etc ..

[Back_Roads]

 There is considerable  gunk like buildup, perhaps a break down and clean the pump and tube would not be a bad idea.

[Howdy9]

Don't own one of those guns. That black slundge looks like maybe something is scraping or rubbing on the split ring, leaving pieces of material in the tube. I agree with above suggestion. A good cleaning and inspection (check screws ect.) would be a good idea. Might be something blocking air flow through the breach seal, affecting power output.

[Spooner]

Like so many of the 362's many of us own... Freddy Krueger lives inside. Razor sharp edges on the slot edges. When you take it apart, deburr the pump tube from front to rear. In your picture, I can see metal shavings. When you remove the piston, be careful to do it slowly. If the burr is sharp enough, it can damage it.

With accuracy, the little grub screw in the top of the barrel band on mine kept coming loose. It could be that as well. I ended up using plastic safe surface curing loctite to keep mine from working loose.

Good luck.