Irritating petcock problem

When I got my bike the PO was using the petcock as a gravity feed only - the vacuum port was sealed off and was running the bike on Prim.
I bought a repair kit to return it to its proper function, but before I could fit it I bought a new ( old stock ) petcock to save me the trouble.
I fitted the new petcock, tried it out by sucking on the vacuum tube and everything seemed fine. Ran the bike, got a mile down the road and started to stall due to lack of petrol, quickly turned on the Prim and the bike picked up and ran, sweet as a nut.
So, as I had a repair kit, I fitted it so that I could check everything and tried again, as before it all checked out, but a mile down the road, same again stalling until turning it on to Prim.
I am using for the vacuum tube, the same tube as one that works with a Morgan carbtune.
When its idling fuel seems to be going through the filter, so........

Could the tube collapse and fail when the bike picks up speed ?
What type of tube are you using ?
Any other thoughts ?

On a happier note - the bike is running like a dream - mainly thanks to all the great info I have been able to get from this site.

 

the vac tube can collapse under serious suck, I used a bit of the tube that came with the vac gauge, works fine.

 

Wiz is right; it has to be actual VACUUM line or it might collapse. Also might want to check for any other possible source of a vacuum LEAK on the cylinder the petcock is plugged into, or move it to another cylinder, it really doesn't matter. I keep hearing the petcock is supposed to be on #3, my BOOKS say #3, but on both my original 550 Secas it's plugged into #2. Go figure. Also take a good close look at your vaccum line to be sure you don't have an undetected split or crack in it.