I don't think there is anyway to avoid the extra \r\n that the monitoring daemon adds to the send string. It might have been viewed as a feature at some point, but obviously it creates problems with POST requests that have a body as RFC's state there shouldn't be any content past the content-length count.
I'd suggest opening a case with F5 and find out whether there is already a CR requesting to have this tweak removed.
To work around this issue, you could create an external monitor which references a perl or shell script. The shell script can open a connection using curl to send a POST request to the pool members. There are some good examples in the monitoring codeshare section.
Aaron
Nimbostratus
