Brett,
There's been a couple of threads regarding fuel caps.
You might click on the search button and enter "fuel caps" and/ or "monarch"
The BAD NEWS...Monarch seems to be no longer in business, as the founder died in '02.  Their website is gone.  I found their old phone number in an old General Aviation News web page
"For more information or to place an order: Monarch-Air.com or 541-459-2056."
That was cut and pasted.
I think Cessna makes a new/better cap.  I don't know about other alternatives.
Good luck]
Michael

Brett,

Your description of the problem is not making sense to me. Perhaps I'm reading it wrong.  If the right tank is not venting compared to the left tank then it has a low pressure relative to the atmosphere and other tank, i.e. a vacuum.  The vacuum is created as the engine draws fuel out of the tank with nothing taking its place but the vapor pressure of the fuel.  There is no air available to take up the space creating the vacuum.  If that's the case then the fuel would not be sucked out, but air or fuel from the left tank sucked in.  Help me understand what problem you are trying to solve.

The fuel caps on my 182 were bad.  Strange things seemed to be happening.  The left tank seemed to use fuel faster and the right tank seemed to have more fuel upon landing than it did on take off.  Also my GPH incrased by about 15-20%.  I had gas stains coming from the right fuel cap.  This was occuring in flight.  The gaskets were cracked and, apparantly, did not seal well.
Apparantly the lower pressure on the top of the with and the improper seal allowed for enough pressure difference that the air in the tank was sucked out and the "rubber" tank bladder would collapse.  Then fuel would leave the tank.  With the collapsing bladder, the level (not volume) of fuel would rise, the float on the fuel sending unit would rise, indicating that the tank was now full.  This was all very confusing before realizing what was actually happening.
This problem isn't going to occure in the 150/2's or 170/2's with metal tanks.
This is also why the 182's are required to have that (unsettling for passengers) placard referring to properly seated fuel caps, loss of fuel, erroneous fuel readings and possible death....or something like that.
One day after a fairly long flight with the above symptoms and a GPH of about 16 instead of 10-12, I found a flyer (I think it was an advisory circular) in the mail from the FAA that explained this C-182 problem.  It suddenly made sense.  I suddenly got new caps.  All that fuel stuff went back to normal.
Get new caps, fly happy!!
Michael