Thanks for help.
I checked the https://support.f5.com/kb/en-us/solutions/public/3000/200/sol3224.html solution before, but I don't think that's it.
I used GET / HTTP/1.1\r\nConnection: Close\r\nHost: \r\n, GET / HTTP/1.0\r\nConnection: Close\r\nHost: \r\n and many others.
I tested it with nc (really good idea) and everything seems to be responding well, but I still have the same problem.
It seems that my LTMs are checking only for 80 lines of received code (my file had 363 and I was checking a string on the very bottom).
Everything before line 80 is making the monitor go up, after line 80 including line 81 is making the monitor to go down.
I'm fine with it.
I can pick something from line 27, but did you have any similar experience with it before?