I've read several of the posts on vented gas caps but still have a question.
As many of you know I just finished installing a new engine. The builder had a rubber gas line from the pump to the carb (not good). After driving it a few miles today I started having idle issues. Got it home, parked it and tried restarting and just cranked for a long time till it started up again. I figued vapor lock or fuel percolation. So I decided to install the stock pump and connect the return line and use the hard line to the carb. I also put a thicker carb gasket on.
When I disconnected the gas line from the pump, gas squirted everywhere with pressure. I quickly clamped it off then opened the gas cap. Wooosh.. a lot of vacuum there. I have the vented gas cap (repo). My question is shouldnt a vented gas cap not allow a vacuum to build up like it did? Or will reconnecting the return line cure the problem?
Thoughts?
Thanks!

Ditto. Exact same question. I'd love to hear the answer.

A vented cap should never allow pressure or suction to build. In addition, many cars had either a vent port and tube in the filler neck or under the trunk drop that would allow internal and external pressure to equalize. My '69 has this, as well as its original "VENTED" cap. In later years, these tank vents were routed to charcoal evap canisters, but mine is just an open tube.

If you replaced a tank or filler tube equipped with a vent with one without a vent, you will have issues.

I wonder if I have a defective cap then? Been using the non vented style (picture on the right) and never had any pressure build up issues. It might be because the seal is worn or missing and infact acted as a vented cap. I bought the new vented design (pic on he left) and now have issues with tank pressure building up.
If I blow into the vented cap, air will not flow, but if suction is applied, air will flow. This indicates pressure would build in the tank but not allow a vacuum to build.
I've got the vapor return hooked up now and still had pressure build up in the tank. I might just use the old "non-vented" cap for now.

a vented cap lets air IN (as gas gets used up), not out...I have had a rubber line from pump to carb (with a metal filter inline) ever since I bought it 10 yrs ago...driven almost 30,000 miles , no problem....
but IF I park my car with FULL tank in a hot garage 95F plus , I get a problem , blew the gas tank sending unit gasket once...I just dont fill before parking now...but a small drilled hole in the gas cap might would prevent that...I most often get a "woosh" when opening the gas cap...

BTW: I have CORRECTED vapor lock problems before by installing a rubber line from the pump to the carb and due to that, it is my preferred plumbing method for that section of line. The rubber section resists getting heated up by engine parts.

Although I do put a braded SS cover over that line for protection.

Ditto that, but I used SS covered nitrile line with AN fittings and haven't had any problems.

thats how its supposed to work.....does your non vented maybe have a pin hole drilled?