Toto model 703 self flushes

I have three Toto model 703 toilets in my home. in the one on our main floor , I recently replaced the fill valve and float mechanism with an aftermarket unit from the local plumbing supply store ( not a Home Depot or Lowes). While the toilet shuts off now when the water gets to the correct level, I find an almost continual leakage of water into the toilet bowl itself. Every so often the toiler will auto flush.

I've also replaced the flapper and added a seat for the new flapper which came with the kit.

Any ideas as to what might be causing this to happen?

Thanks.

Make sure that there is no tubing placed down the overflow tube. That would siphon water from the tank into the bowl. The tubing should "daylight" above the overflow tube.
The original fill valve had no tubing for the purpose of refilling the bowl.

Also make sure you have at least a little slack in the chain. I had yet another recurrence last week of a chain issue. We once again had a toilet "leaking" in our home and the fill valve just very slowly running. Upon initial inspection, everything seemed in order, but I knew to check the chain and, sure enough, I noticed that the chain was tight, rather than having even a bit of slack like I had previously set it. I realized that the flush handle had rotated just enough to be pulling the flapper ever-so-slightly off the seat. Tightened the flush handle in its new position (because the rotation seemed by design to be able to go no further; remember, they are reverse-threaded) and added two links to the chain, and -- voila! -- problem solved.

Given that you replaced the flapper, a chain length issue might be indicated. (Also, make sure the converse isn't true, that the chain isn't bunching under the lip of the flapper because it's too long.) Recognize, too, that sometimes putting the lid back on can alter the trip lever position. Probably not indicated in your toilet, but worth a mention.

Finally, and maybe this is obvious, but make sure that the water level you set is below the top of the overflow riser and you're not getting running over the top of the riser. From your description, again it doesn't sound like the issue, but worth a mention.

Nine times out of ten, it's what Terry mentions -- a siphon caused by the refill tube being just stuck down the overflow riser rather than being daylighted on a clip above it. So that's obviously where to start.

Come back if none of this works. And if none of this works, also tell us what kit you used so we can think of other possibilities.

Good luck!

It's rare that a new flapper valve seat is required on a modern toilet. And, if you didn't use an adjustable or the OEM flapper valve, the one you have is probably not using the designed amount of water. Korky makes the flappers for most of Totos toilets, and those area available in many places: Lowes, Ace, etc...

What can happen on a toilet is that the valve seat nut (on a two-piece) may not be properly snug, but this usually doesn't happen on a toilet later one, it would happen from new and just needs to be snugged up. Also, some debris might have lodged on the valve seat, or there's a small casting bump. If it is smooth all the way around and the flapper isn't sealing, it's either the wrong flapper or it's installed improperly. A universal flapper fitting over the overflow tube with a ring often gets distorted. Those that use the holes to go around the lugs usually are okay unless you try to use both the ring and the lugs.