I had a leak at the rear exhaust cam seal (lots of oil coming out from behind the camshaft position sensor). It was easy to replace. You take off the sensor and then you can dig out the seal with a dental pick or similar. My seal was original and very brittle, no longer at all pliable, maybe explaining the leak. I had to make little notches in it so the pick could grab it out. I guess it takes a little patience so you don't scratch the bore or camshaft. Putting in the new one, have every surface clean (may have to use a soft pick to remove traces of old seal). Use a piece of plastic pipe, wood or similar soft material as a jig to get it in evenly. It takes some moderate force to get it all the way in. The new seal I got from a dealer seemed more substantial and was for sure much more pliable than the old one. No leak so far.

On a previous Volvo (240) I had a couple of camshaft seals go. I had not realized to keep an eye on the crankcase vent (rebreathing) system, flame trap, etc., to avoid pressure buildup. This seems to be a common issue on the 850s too.

Yes there is a seal in there and yes you probably damaged it while you were in there, it would be one heck of a coincidence otherwise. I have not tried to do one without the special seal installing tool but it could probably be done without it although they did come up with a pretty trick little seal installing tool to do it with. Have you looked through Bay 13 to see if there is a step by step on it over there? Otherwise from the sound of things you managed to get the CMP in and out now you need to do that again plus change out the seal.

Mark