I experience the SAME thing in my '56 skyhawk.  Seems pretty common.

     Dave

Does anyone know if putting vented caps on both tanks will solve this, and is it legal?

To check if your tank is draining.  One simple approach would be to select the tank of interest from the inside and pull the fuel strainer open and drain fuel into a clean bucket.  If you can drain a few quarts of fuel I would say it would be OK.  (If you like science experiments).... You can do the other tank and compare flow rates if you suspect a restriction of somekind.  I would do this also with the vent caps on the tanks loose so venting problems can be eliminated in this part of the experiment.  Then do it again with the caps on tight.  If you notice a significant difference venting could be problem.

The assumption here is that the fuel tank selector valve is good.  To gain some confidence in it, try selecting the fuel selector valve to off and pull the strainer til fuel stops flowing.  Then select the tank of interest.  Time and measure the quantity of fuel and then do the other other tank. 

One last caution about selecting off.  After owning my plane for a short time, did just that.  Then I noticed an unisolable leak from the valve.  What a mess.  of course it was a sunday. So I rigged a 5 gallon drip pan and overflow into a 10 gallon gas can.  Had to wait til the maintenance came in on Monday to deal with the mess.

The selector valves are not frequently turned to the Off position and there o-ring seal was aged so it lost it's resilience.  This resulted in the inability to seal once the selector switch was moved to a new spot on the old o-ring.  So do this test when you are prepared to deal with a leak.

my two cents

Bob,

What was the outcome?

JimJ

When I tried turning it back to both it started to leak faster.  I found the selector valve position that gave me the minimum leak - drops per minute.  For a while I kept my empty, plastic collection rig tied to my tie downs, just in case.  Someone ran over it when I was gone and that ended that.  Fortunately, after a new o-ring was installed the problem was eliminated.

What approval data are people useing that decide to modify their certified fuel systems?  I would be very cautious about putting a vented cap in the low pressure area of the wing. (which is where most caps are)

Thanks Ernie--where's a good place to order a vented fuel cap?

HIgh-wing Cessna's are notorious for one wing seeming to drain somewhat faster than the other.  Don't let this bother you. 
  In most cases it is only due to tank venting anomalies and/or siphoning from one side to the other. 
  Example: The right cap is vented the left is not (or the left underwing fuel vent services both tanks via a cross-over vent line.)  This results is slightly different vent pressures, but more likely the truth is that both tanks feed the engine equally, but the cross-over line also siphons fuel from the right tank over to the left tank for various reasons.*  The result is the right tank READS lower than the left because the left tank was somewhat replenished by the right tank via the crossover line.  After the right tank fuel level drops below the ability to supply the crossover vent line with fuel, both tanks will continue to feed the engine equally and both fuel levels will drop at similar rates.

*-The various reasons are: The right tank cap is vented while the left is not, so a slight suction can exist to siphon fuel from right to left.  Or the right rudder applied during climb (also happens to coincide with the highest fuel consumption period early in the flight) causes the right tank to keep it's fuel pushed up against the crossover line thereby allowing a siphon action to resupply the left tank with fuel from the right tank, etc. etc.
  In any case, notice that after a while both tanks feed the engine just fine.  Keep notice on your time in flight vs the fuel consumption rate of your aircraft, and you'll see that both tanks actually feed the engine just fine, and that this is not a real problem.

  Dual vented caps may or may not solve the problem but there is no danger of fuel sucking out of the cap vents because the caps are in a low pressure area.  The vent valves are one-way valves.  They let air in, but don't let fuel out. 
  (However, a cap's gasket can become dry and cracked, and can leak fuel out past the cap due to the low-pressure area.  So inspect and keep your fuel cap gaskets in good condition.)

George,
Thanks for your wisdom, esp. re. using the "off" position.  However, my concern re. uneven tank draining is that if it takes 1-2 hours of flying before the tanks decide to drain evenly, I am, at that point, about 70-80 lbs. heavier on one side than on the other, creating an "out-of rig" flying scenario.  And, if I fly a long distance without adding fuel, I will be flying on only one tank towards the end (i.e. Cessna POH warnings  re. landings on one tank).  I think I will install a vented cap on the other tank to see if this helps the draining.
Thanks again for your response!
Bob

Daniel...
I think that WAS supposed to be Viola...the musical instrument...
like in the scene of a movie when one opens a door and has a
startling surprise..the background music just happens.  In this case it was the sound of a viola...it's like a big fat violin...

I glad to say I haven't seen "walla" in print for awhile.. ;p

Michael

George,
That would work but I don't trust ANY fuel gauges.  I ordered a vented fuel cap yesterday from Hill Aircraft in ATL (Cessnaparts.com) and received it today!  I will let everyone know if this solves the problem.  Thanks to all.
Bob

Cessna created a Service Information Letter for 1979 and Later 180, 182s.
Subject: Fuel System Venting Adjustment.  (Uneven Fuel flow)

Letter number - SE81-8

This letter  gives instructions to adjust vents if the fuel tanks levels are greater than 5 gallons apart.

If you can find a copy of this SE - it might be your answer..

I will enter it (word for word) on this site later today for everyone, if it can't be found on the web.  It was Cessna's method of   adjusting of the vents to solve uneven draining.  Jack