I recently had the fuel cells changed and the fuel vent moved from the top of the cabin to behind the left wing strut. Now when I take off the fuel caps when the airplane is warm there is pressure in the tank. How is this pressure supposed to be relieved? The service manual mentions making sure the hole and the hinge are on top when installing the check valve in the tank. Is the hole mentioned a relief for positive pressure? Perhaps the used check valve I had installed has a plugged vent hole?

Years ago I picked up a G1 from a big shop after it had sustained some hanger damage. The job was very well done except for one rocket surgeon who installed the vent to one of the water methanol tanks backwards. It sucked instead of blowed (blew?), anyway that caused the bladder to collapse, easily fixed and only slightly related to your story. The point being vent lines should be an open tube into the tank and they need to be installed correctly.
As I remember your vents were moved to help prevent ice accumulation during IFR flights which could render the fuel tank inoperative.
This was incidental to the flashlight being left in the fuel tank which a month later caused us to have clogged fuel filters which is a whole different case of beer.

I replaced the left bladder and complied with the fuel vent AD on my 56 and do have a little positive pressure when opening the fuel cap (Monarch caps). This AD and bladder change was done about 300 hrs ago. Has not been an issue yet.

This is timely, I have just been through virtually my entire fuel system troubleshooting an issue. When we departed for JC back in June it wouldn't run on the right tank. I think I have it solved, I have isolated the baggage tank and not a hiccup since.

To your question, do you have the original fuel caps? Best I can tell, it seems as though they are set up to vent in a decent amount of air and out a slight amount. I have the fuel vent relocated as well, and the check valve seems to do the same - lots of air in, a very small quantity of air out. Suppose it is possible that on a really hot day the fuel might vaporize faster than the 3 vents can expel, but given my recent experience I'd want to check to confirm that all 3 vent sources are functioning properly. It's no fun having your vacation plans screwed up due to a MX issue.

Flynengr

According to some, the exact location of the vent behind the strut is also crucial--otherwise it will pressurize the tank. Last year at OSH, I received a chart from Butch Hartwig of Monarch that details that requirement, although my IA tells me that it isn't as crucial as their chart seems to indicate. I can't find that they have it on either of their websites.

There are also one or more valves between the tanks to prevent backflow, which can get clogged.

The other thing that is crucial about some Cessnas with only one vent on the left is that the fuel caps have to be correct. The left one isn't vented and the right one is. Reverse them and it's hard to draw from the right tank--either the gas won't flow or the tank will collapse.

Cary

My right bladder would collapse to fill the left side and then proceed to dump around 20 gallons an hour through the vent- forward into the airstream against the strut. I lowered the vent to about 1/4" below the recommended Cessna location, and the problem went away, and the fuel system now has an even fuel feed (within a couple gallons after burning a full set of tanks, compared to >15 gallons difference before).

A guy with the same year 182 as mine ('62) had a check valve at the single left side vent go bad/stick and had positive pressure problems. It was located right where the vent hose goes into the bladder. He removed the ball and housing, and it got rid of his substantial pressurization problem. My bladders don't have a check valve, although the vent will dump a gallon or two over the course of a couple of hours with full tanks in rough air. Perhaps you have a check valve problem?

I have to park pretty level with fuller tanks, something that others don't seem to have a need to do- perhaps because they have a check valve.