LG G Pad 8.3 v50020a Custom recovery install (TWRP)

There are a lot of methos to unlock bootloader and intall custom recovery for old version of LG G Pad. I tried to those methods...but noting works

Only the below,works under v50020a, before doing this, you must root device.

http://forum.xda-developers.com/showthread.php?t=2698267

In my case, adb reboot recovery command does not works after intalling TWRP.
but I can go into custom recovery mode by key combination.

After installing , you can see below messages when it boot as recovery mode but nothing happens

Refer to below, end of this video show how to go into recovery mode by key combination after TWRP install.

http://www.youtube.com/watch?v=_tOtfF6gKkg