There are--or were--two black flat rubber seals with an o-ring underneath each. The pinkish center piece looks like simply a bumper to protect the face of the piston against contact with the front of the compression tube.
After digging the mess out, it looks like the bumper was glued in place.
Let's see what's going on.
There's a huge blob of excess brass in the end of the tube.
Huge amount of overfill from brazing the front of the compression tube cap to the body. Tube is shot. I kinda doubt Crosman has any of these compression tubes laying around. It's been probably twenty years since the gun was in production. For most of us, this would be the end of the project. I think I've got an ace in the hole this time around. More on that later.
There's absolutely no good reason to remove the compression tube for normal maintenance. So don't bother with the following step: The compression tube is affixed to the breech block with two socket head cap screws. A 3/16" allen wrench didn't have enough swing room.
The bolts were extremely tight. I didn't think they were coming out without heat, but a longer handle and additional pressure did the trick.
An o-ring seals the compression tube to the breech.
Might as well finish this. A small set screw retains the bolt handle.
Like everything else on this gun, it's SAE. I didn't measure it, but I think it's a #8-32.
I expected the bolt handle to pull out after removing the set screw, but it was threaded into the bolt probe and has to be unscrewed. The probe is pretty long, as is the spring behind it.
Next time around I'll see what I can do to source another compression tube. I'd rather not return the gun as I haven't seen another in .20 cal. I don't like the piston seal design so I may retrofit a parachute design seal.
We'll see what happens.
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