Anyone had any luck getting those locked gas caps off without the key?

There's a serial number on the cap, based on what I've seen on others it usually matches the key.

Shotgun?

Have you tried a locksmith? We've had good luck with a local one who seems to recognize any key ever made, and can whip up a replacement from a serial number even for some pretty oddball stuff.

Hammer and screwdriver....

In reply to Turboeric :

Not yet.  This is on the Carryall at my Dad's.  I'm trying to figure out what tools I'll need to take it off when I get back up there.

I do need to figure out the locks for the doors and ignition.  So a locksmith might be in order.

Those keys are pretty generic. My lockout kit has a double ended tryout key that seems to work on most of them.

Go through your keyring and try everything that fits. One of them will probably open it.

Violence.  It's not always the answer but it is in this case.

A locksmith is the delicate answer. The brute force solution is the screw tipped slide hammer to pop the lock cylinder out.

Shotgun?

...someone had to say it

They are not super secure. I used to have one and regularly got into it without the key. It ended when i popped the "handle" part off, exposing the threaded portion that could then easily be unthreaded and removed.

I drilled one about 40 years ago.

John Welsh said:

Shotgun?

...someone had to say it

I was going to say hand grinder.

I highly doubt they're particularly secure, they probably rake open fairly easily.  No personal experience, but form what I can tell looking at Lockpicking Lawyer videos on youtube, that's something that takes $15 in tools and very little skill.

If it is a short filler tube like this Roadster one.

And you have  access to the back side like this. 

You can pull off the rubber tube and use a Phillips Straight edge to undo the lock

.

Drill out the cylinder and turn the mechanism with a screw driver?  

Or maybe screw a dent puller into the keyhole and yank out the lock? 

In reply to MyMiatas :

This might be a solution.  The tank itself appears to be a fabbed together setup, I'd bet that there's a rubber line like that on there somewhere.  Whether I can get to it or not...

Stampie said:

Violence.  It's not always the answer but it is in this case.

Is ALWAYS an answer. Almost NEVER worth the consequences 

Can you try a sliced tennis ball, or compressed air and a hose, and seal off against the top surface and pressurize the key slot to move the internal locks?

HotNotch said:

Can you try a sliced tennis ball, or compressed air and a hose, and seal off against the top surface and pressurize the key slot to move the internal locks?

This is such an off the wall solution I think I'm gonna try it no matter what.

I'm gonna bet that the air will leak past the internal structure of the tumbler.

I drilled out the barrel when I got my '61 Chevy (and none of the keys that came with it fit anything on the truck). 

It doesn't take long, it's pretty soft metal.

(Also had to drill out the ignition barrel to get it out as well).

You can rake them open in about 1 minute with a bent paperclip or a jewelry screwdriver.

Or if you want to pick up a new skillset, get a set of lock picks and have at it.

Stampie said:

Violence.  It's not always the answer but it is in this case.

I see that you left your violence on the ground.

The one time I was in the OP's situation, I just jammed a straight edged screwdriver in the key slot and turned it hard. It got the cap off pretty easily.