WCCP return paket

Hi all

I'm working on a WCCP Optimization with Riverbed. 

The WCCP it self works and the init paket from the client to the server are forwarded to the Riverbed (WAN Optimization) and send to the server. All good at this point.

But, the return paket are sending directly to the client. But, Riverbed need this paket as well, because there are exchange information between the local and the remote Riverbed.

In Cisco example configuration you need to activate the WCCP on the outgoing and incoming ACL.

How can I solve this issue with a Fortigate?

Thanks in advance.