Originally Posted by Capt.Bill11
Maybe the cap is weak. Or you are trying to keep the expansion tank to full.
Lehman's seem to like to run with the fluid level in the tank about an inch or so from the top. If you over fill it. it just expands it out till it finds it's happy level. Hence the term expansion tank.
I would not jump right to a head gasket issue.
Absolutely right. Mine does exactly the same. If I only fill to about that inch below the neck, as bill says, no loss. When the cap fell to bits recently I thought I might have trouble sourcing a new 4lb pressure one here in Oz, so emailed American Diesel, and they were ready to ship me the recycling type of neck adaptor and cap, but it was going to cost about $100 all up. Then I found a replacement cap locally for $12, so thought what the heck, and left it at that. I only have to add coolant about once a year anyway.