that model number says yer machine is about 10 yrs old and has a variable speed tranny setup (not a hydro).
Those type trannies use TWO belts and need to be OEM (both purch as a set from MTD).

follow the link below to d/l a Troy-Bilt manual - it has directions/warnings for tranny belts changeout.

Also use this link for a owner's manual to that Briggs engine (copy/paste into yer browser's address bar):
http://manuals.mtdproducts.com/mtd/Safety.do?pid=138543&model_num=14BV809H063&serial_num=00000000000&filename=274794.pdf

Here is a link that might be useful: Troy-Bilt manual

Thanks. I replaced both belts last year. The short belt is like new still. Is it the norm for one of these variable speed trannies to eat the long belt every year?

I appreciate the links. I already have the pdfs for the chassis and engine.

It is correct that simply letting up on the drive pedal will not bring the tractor to an abrupt halt if you are making good speed. It will slow down if the vari-drive pulley is changing geometry properly, but it is not the same as appying the brake pedal.
If your tractor features the "cruise control" option, make sure that it is operating properly and not self-engaging.
If the tractor doesn't stop when you push the brake pedal, better check the brake caliper and puck.

I had an 800 series tractor this spring that strangely was wearing the front belt as well. In this case, there was a bolt used as guide that was installed in wrong hole in the idler bracket. You could see that the last installer just made an error. You have to have something such as that happening because the front belt should run 2 or 3 times as long as the rear. A belt gets nice and hot to chunk in 1 year.