Well every time i start the truck the vacuum pump starts up , is this normal or am i chasing a vacuum leak?
How long does the vacuum stop in the reservoir for on your truck?
How long have you left the truck for, and started it up after a long period and the pump hasn't started?
I've fitted new knuckle seals and hub seals and vacuum solenoid.

 

I've had mine of the road for a few months and when I pulled a vacum line off the reservoir a few days ago there was a his of air rushing in.
It seems that mine holds vacum for months so it sounds like you got a leak somewhere.
Good luck, not going to be easy to trace.
Hope it aint behind the dash amongst the heater stuff.

 

When I switch my truck on I hear the vacuum pump come on. After a minute to so it cycles off and doesn't come back on again.

There is also a similar sound as the fuel pump builds up pressure. This also stops after a minute or so. I have a fuel pressure gauge and can see that the fuel pump switches off when the pressure in the fuel bowl gets to 60psi (4 bar on my metric gauge).

If your vacuum pump comes on and remains on for longer than a few minutes or 3, I think you're up for chasing a leak!

 

Nope sound like i'm okay , when i start up the pump cycles and turns off after a couple of minutes, but it does this every time i start up over night.

 

The tell tale sign of a vacuum system leak is that the heater flow will stay on demist no matter what the control is set to.