Beeman R1 cocking problem?

[Odysseus]

I recently acquired a Beeman R1 rifle through a lot of guns our shop purchased. When I got this rifle it wouldn't cock. The only initial information I found was it could have a broken mainspring. After dis-assembly and reassembly I wasn't able to find any glaring faults with the gun. The mainspring is slightly canted at one end but otherwise intact. After a few days of tinkering, I came up with this. With the gun in the firing position there is about a 4mm gap between the end of the slot on the piston and the cocking arm shoe. I initially thought this to be normal because when you first break the barrel to load, the shoe comes into contact with the piston. I inserted a spacer between the piston and shoe and what do you know, the gun cocks fine. Now part 2 of the problem, when I fired the rifle (spacer removed), the cocking arm came out of the shoe. I'm thinking there must be something amiss with the arm/shoe/piston setup but I've never worked on one of these rifles to know what it's supposed to look like and how it should function. I included a picture of the suspect area. Any help would be appreciated. 

[Mark 611]

get a new cocking shoe! their about $5  sounds like the old shoe has stretched and is probably ready to break IMO

[Odysseus]

Awesome, thanks for the quick response. After I posted I was scouring google for pictures and came across a random picture of an over lubed shoe out of an R1, it looks significantly flatter than the one in this gun, I'd say you're absolutely correct. Off to get some new parts for this thing then I'll update with how it turned out. Thanks again.