4WD working but light flashing

So, I wanted to restore the 4WD functionality back from manual hubs to vacuum lockers.

Replaced the solenoid/switch
All pipeworkHubs1 kingpin bearings/seal, other side was done 12 month ago.

I jacked the car and tested both sides, the wheels are locking when 4WD engaged, however the 4WD light continually flashes as though I have a vacuum leak. 

What could it be, to my understanding there isn't anything left, with perhaps the exception of the pipe to the cylinder under the wing?

Tia
Danny

There are 2 seals in the hub that the vacuum system relies on. One is where the CV joint goes through the back of the hub spindle; this one affects the hub un-locking. The other is behind the wheel bearing and affects the hub locking. They can develop a slight leak that is enough stop the vacuum switch registering, but not enough to stop the hubs working.

All wiring looks good, control box all good too.

Interesting to know. I'm not overly technical but I can do things if I'm replicating a video or someone. I changed the kingpin bearings and both seals ( one at the back and the one inside) I take it these are the two you're referring to or is there another somewhere else?

The seals you replaced when doing the kingpins are not part of the vacuum system. The seals I'm talking about are in the hub. You need to remove the wheel bearing to acces one, and the whole hub to access the other.
 

You can check them by sucking on a hose attached to the ports on the hub. You should feel resistance. If air flows freely then there is a leak. Infact the same test can be done on any part of the system.

Thanks Busta. Will do more research on this. Do you know of any guide/video on replacing these seals?

A wheel bearing change video will show how to get to the seal behind. To do the CV one you can either remove the king pins like you did befor and get to it that way or go this route which is much quicker if everything comes undone: