iRule to send http status 400 code when pool is offline for a web service call

Scratch that first irule. It was untested and has flaws. I agree with Jie about the no-cache. Give this one a try.

when HTTP_REQUEST {  
    if { [active_members [LB::server pool]] == 0 } { 
        HTTP::respond 400 content "  
          
         
            Bad Request page
         
         
            400 - The request could not be understood by the server due to malformed syntax.
         
       
        " Cache-Control No-Cache Pragma No-Cache"
    } 
 }