Clientless mode with 401 and 403

I am attempting to use basic authentication, HTTP 401, in clientless mode with F5. When prompted for username / password through the http client, only valid requests should go forward. Requests with an invalid username / password combo, should redirect to a 403.

This is how my iRule looks like:

when HTTP_REQUEST  {
 insert clientless mode
    set apmsessionid [HTTP::cookie value MRHSession]
    if { [HTTP::cookie exists "MRHSession"] } { set apmstatus [ACCESS::session exists -state_allow $apmsessionid]} else {set apmstatus 0}
    if {!($apmstatus)} {
         Insert Clientless-mode header to start APM in clientless mode
        if { [catch {HTTP::header insert "clientless-mode" 1} ] } {log local0. "[IP::client_addr]:[TCP::client_port] : TCL error on HTTP header insert clientless-mode : URL : [HTTP::host][HTTP::path] - Headers : [HTTP::request]"}
    }
}

when ACCESS_ACL_ALLOWED {
    HTTP::header remove username
    HTTP::header remove domain
    HTTP::header remove roles

    HTTP::header insert username [ACCESS::session data get session.custom.login]
    HTTP::header insert domain [ACCESS::session data get session.custom.domain]
    HTTP::header insert roles [ACCESS::session data get session.custom.roles]
}

when ACCESS_POLICY_COMPLETED {
    Authentication request for non bowser user-agent session denied
   if { ([ACCESS::policy result] equals "deny") } {
      ACCESS::respond 401 noserver WWW-Authenticate "Basic realm=\"My Web Services Authentication\"" Connection close
      ACCESS::session remove
      return
    } else {
        set allRoles ""
         grab the session data containing the groups associated with the member
        set s [ACCESS::session data get "session.ad.last.attr.memberOf"]
        set matched [split [string map [list {| CN=} \0] $s] \0]
        foreach match $matched {
            if { $match != "" } { 
                puts [string range $match 0 [expr [string first , $match] - 1]]
                set queryarray [split $match ","]
                set firstvalue [lindex $queryarray 0]
                set cleaneddata [string toupper [join $firstvalue _]]
                if { $allRoles eq ""} {
                    append allRoles "ROLE_$cleaneddata"
                } else {
                    append allRoles ",ROLE_$cleaneddata"
                }
            }
        }
        puts $allRoles

         grab the session data containing the username associated with the member
        ACCESS::session data set session.custom.login [ACCESS::session data get "session.logon.last.logonname"]
         grab the session data containing the domaiin associated with the member
        ACCESS::session data set session.custom.domain [ACCESS::session data get "session.ad.last.actualdomain"]
         grab the session data containing the roles associated with the member
        ACCESS::session data set session.custom.roles $allRoles
    }
}

If I set the line:

ACCESS::respond 401 noserver WWW-Authenticate "Basic realm=\"My Web Services Authentication\"" Connection close

equal to

ACCESS::respond 403

I get a blank page and no initial prompt for the login with an HTTP client.

Here is how my APM looks like:

What am I doing wrong?