Yes, it's normal. Non-vented cap is the key. No other vent either. The cap can let air in but not out. On my 70 chevelle the preesure would actually deform the tank enought to where the filler neck would hit the bumper. Eventually broke the solder joint at the tank and started to leak. I created a vented cap after I replaced the tank.
Tom (I live in Phx, Az so the heat and pressure build can get nasty.)
68 & 69 Elkys and station wagons require a vented cap (PN 3931449). My records show that this part may still be available from GM. Only coupes, sedans and conv (rear fill cars) use the non-vented part.
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