Ok..I did a complete flush, its summer and why not. changed the thermostat and put on a new cap on the expansion tank.. truck now runs 7 degrees cooler.makes me very happy. except now when I go to the truck in the morning or after it sits long enough to completely cool down the top tube from the thermostat to the T joint is sucked dry. no shape flat as a pancake. This is the only tube that this happens to. I have started the truck and watched it slowly come up to temp and the tube slowly expands back to normal shape and all is well. I and my mechanic are both stumped. The cap is new from atlantic british. the number on the bottom is 140. any thoughts. as always MOST comments are appreciated.

 

Assuming you have properly bled the system and the coolant level, when stone cold, is all the way to the cap, then I would suggest you re-install your old cap and see if that makes a change.

 

Mike, about as certain I can get about the system being bleed properly and the level is at cap height.. unfortunatly the old cap got thrown away.... I was going to do a double check today on the bleed. and then get another cap this week. any reason I should not drive.....