I presume you mean a pump head locking clip which holds the pump head to the canister body.
Here is a link to the exploded parts list for a Eheim Pro 2 canister (all parts the same on 2026/2028 except for the actual canister body).
Eheim Parts - 2026 - Professional II Canister
Eheim Parts - 2028 - Professional II Canister
The pump head locking clip is part #7444450 - EZ clip $9.58
Eheim Direct part prices are terrible so try to always purchase somewhere else. Once you find the Eheim parts number just do a search on it to locate sellers.
You can purchase this EZ clip for much less ($4.18) at BigAlsOnline.com for much less and they have a very good selection of Eheim canister parts:
Big Al's - Search Results for 7444450 EZ clip
Your Eheim Pro 2 should have a large round suction button on the top of the pump head which is designed to start a siphon flow. You press it down and when it pops back up it is suppose to start water flowing. The trouble is over time these buttons get really sticky as their lubricant dries out and get harder to press down and hardly pop back up and if so do so slowly that its does not siphon.
On a properly lubricated Pro 2 siphon button it should press down easily and pop right back up.
If your siphon button is not working correctly you have these options:
1) Manually start a siphon on the output tube with you mouth or siphon bulb. Keep the output tube low and run it into a bucket until it flows good.
2) Replace the standard Eheim intake tube with their more expensive modular intake tube which opens at the top and allows water to be poured down the input hose. There is a similar modular spray bar which works very well.
Eheim Installation Set 1 - Suction Side - 594
3) Disassemble the pump head and re-lube the siphon button O ring with good silicone lubricant (possibly replacing it as well). Once it is relubed it will pop up and down with little or no effort.
O ring silicone lubricant (which can also be used on hose attachments).
Lifegard Aquatics Silicone Lubricant
I attached a nice PDF write up someone did on how to replace the O ring on a Eheim Pro 2 canister. You can purchase replacement O rings online as well.
