1985 s 12a. I have noticed that when my gas cap is shut like many other people I have a good amount of pressure build up in my gas tank. My issue is the check valve under the tank is leaking. This only happens withe the fuel is nice and tight. Any suggestions to why this is , and have others had this problem

 

Welcome to the forum.

Here's the test procedure for the check and cut valve, from the FSM for an 83. You should grab your manual from Foxed.ca - Mazda RX-7 Manuals

edit: should have mentioned that if charcoal canister is plugged up, then you could build up a lot of pressure...that might be why your check valve started leaking.

 

Found this: https://www.ebay.com/itm/NEW-OUT-OF-...-/183022854384

 

The rubber line going to your check valve is too short and barely on the nipple. This problem isn't why you're having the problem, but it's sure not helping. Check valve might be bad so I'd look into what Toruki posted for testing its operation.

But the #1 cause of pressure or vacuum in the tank is a non-functional vent system. The pressure that builds up in the tank from evaporation is supposed to vent into the charcoal can under your hood. There's a line under the car specifically for that. And when the car is running and sucking fuel that line breathes the other way, to let air into the tank so it doesn't get a vacuum condition.

Too much pressure will push its way out, like through the poorly connected hose in your pic. Vacuum in the tank will make your fuel pump work much harder than its supposed to and might cause fuel starvation issues.

 

Also might be a good idea in general to replace the lines and hose clamps.

The furthest left hose clamp closest to the upper trailing arm mount is pressing into the hose adjacent to it.

 

it also looks like the hose with the green on it is the leak, or it could be

 

Yep, all good comments. Your hoses all look bad and the pressure your seeing in the tank is what I and Toroki mentioned. Your plumbing needs fixed.

 

Thanks everyone for the detaled information and godlike fast response I greatly appreciate it