Troy bilt 1998 model 13AJ609G766 , automatic transmission, 18.5 Horsepower, 42" deck. Deck is not installed as yet. When releasing the foot brake pedal about half up, the engine stops. This is what I have done so far, checked the alignment of the brake and pto switches. Checked for corrosion and/or loose connections. Checked for tightness of the switches' mounting. The engine stops under all of the following conditions and the parking brake foot lever is released to about half way up: the parking brake "button" is released, the pto lever is engaged and/or unengaged, sitting in the seat and not sitting in the seat [ the seat is not raised], this model has the dual spring type of seat switch, when the shift lever is in the forward position. With the seat down and using a pair of needle nose pliers and squeezing the springs so that they make contact, the engine continues to run.

Hi,
I think you have answered your own question probably. The machine must detect a presence on the seat when operated. The manual is here:
PDF Manual Web Archive (http://manuals.mtdproducts.com/mtd/Public.do)
Just enter the model number in the top space and just put any digit in for the serial number; I use a "1" personally.
Peace,
Clarke

Everything you listed leads to problem being the seat safety switch. If the contacts do not touch each other when you are on the seat you need to adjust them so that they do touch.

I have the same problem! My solution is to take it back! The PTO will not engage and after its fixed the problem will keep occurring! It's a Troy Bilt thing! There should have been a recall on this!

Oh... The seat safety switch is not the problem. Releasing the break cuts the engine! Electronic PTO is the problem.