Lets remember what 14 psi means. While the system is under a vacuum, you have approx. 14 psi pushing down on the valve core to try to get into the system. 14 lbs per Square inch. How big is the area of the valve core? Pi R squared to find the area of a circle. The valve core is about 3/16" across. In simple terms about 1/36th of a square inch. So there's only between 1/3 & 1/2 lb of force on the valve. The spring can handle that. I thought about it too when drawing a vacuum on mine, & when it didn't leak, I just had to figure it out. Paul.

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1984 300CD red/blk 290,000 partial resto
1980 240D 4 sp Brn/palomino gone
1997 Mazda Miata STO edition wife's baby
1988 Lincoln Mark VII LSC next resto
1986 Subaru GL wagon daily
1993 Isuzu pickup field beater
2008 Honda Civic EX-L 5sp wife's new car
1978 Honda XL350
1974 Honda XL70 my Calif. ride in HS