Slow toilet leak from tank to bowl

I read through this one prior post on same subject:
http://www.houserepairtalk.com/f33/toilet-tank-leaking-into-bowl-1332/

Replacing the flap (which seems to seat fully) did not solve the problem, so maybe I should replace the entire Fluidmaster apparatus. I'll purchase it to have it available for the plumber before I make the appointment.
What I'm wondering is could the slow leak be caused by a problem with the toilet tank itself (and not the innards)? I don't see any hairline cracks.
Since I turned off the water supply, tank water is only about half inch in the tank bottom. Should I put food color in the tank to see if it goes into the bowl, since the water's not high enough to reach the flapper?
I'm trying to rule out the need to replace the tank or even the whole toilet (Almond American Standard Cadet circa 1999).
What I'm asking is if there's any reason - other than the flush apparatus - for the tank to be leaking into the bowl?
My resources are very limited, and I'm trying to keep the plumber labor to a minimum so I don't want any shopping time or return trips. That's why I've already acquired the Kohler faucet kits to repair the kitchen and lavatory faucets - to have them ready for the plumber. The dis-attached bathtub drain stopper also needs to be re-attached so it works with the toggle handle.
Hopefully, I'll be able to get it all done within the $199 voucher from Angie's List.

Check this out and see if it helps:
http://www.fluidmaster.com/do-it-yourself/fix-it-zone

An overfilling top tank can cause water to drain into the toilet ... BUT, on older toilets, the "spud gasket" can get brittle and crack ... this allows water to bypass and drain from the tank into the toilet.

One very elderly lady had a toilet that had ghost flushes about every half-hour for months before her adult son called me. It was easy to fix. Just remove the top tank (bolts are on the underside) and pull it off. Replace any washers and upper gaskets, and the spud gasket (comes in a kit). Re-seat the tank, carefully tighten the bolts (over-tightening can crack the tank), fill and celebrate!

CallMeVilla said:

... One very elderly lady had a toilet that had ghost flushes about every half-hour for months ...

Click to expand...

Thanks so much! Perfect response and great diagram.
I forgot to mention the ghost flushes that have happened every 20-30 minutes - on the American Standard toilet. I just turned off the water supply until I had a guest here to use it. It just started happening on the Kohler toilet, too.
Is the 'spud' universal or manufacturer specific? Do I get it from a plumbing supply place or is it likely to be found at one of the big hardware/home improvement stores - do you know?
While I've got the tank off, do you think I should go ahead and replace the rest of the innards?

kok328,
Thanks so much for the Fluidmaster link! It's a great overall resource for toilet problems. I especially like the systematic approach for figuring out why the toilet is ghost flushing. I forgot that it does that when I posted. So now I have some stuff to do before the plumber comes - to figure out the problem. I really appreciate the quick response.

If you are bringing in a plumber already, I think you should go the works, all new parts. Just not that much money.

You don't need the plumber. DIY!

Well, he does have a $199 voucher to cover the cost of the service call.
Not everything will result in a DIY fix, sometimes it's just to help enlighten the poster so that they may be informed when calling in a contractor.
Rebuild 1 toilet & install 3 customer supplied fixtures, service call, misc. materials, tax, title, shipping & handling, your looking around $300 ? :2cents:

Rebuild 1 toilet & install 3 customer supplied fixtures, service call, misc. materials, tax, title, shipping & handling, your looking around $300 ?

Click to expand...

The OP has a coupon worth two hundred and a bill that you're guessing is about $300...sounds like a hundred dollars out of pocket. Besides the OP has already bought the parts and did most of the diagnosing. Tell 'ya what... I'll do it for $80

Kok, I will admit that it is smart to be an informed consumer, and I do use this site to further my knowledge...even on projects that i don't plan on tackling. But this one............