Small Engines (Lawn Mowers, etc.): peerless transmission, walk behind mower, transmission model

Question
I have an old circa 1995 troy bilt 33" wide cut walk behind mower.
When the blades are engaged the transmission does not disengage in forward or reverse. If it is just started it disengages or the bladed are not engaged the transmission disengaged when you let go of the handle. We have tried adjusting the linkages and installed a new brake and disc and pads. Is this an internal tranny problem and if so what is malfunctioning in the transmission?

Answer
What is the model number of the Troy bilt machine or what is the Peerless transmission model number?

I can use either number but I need at least one of the model numbers so I can look up what you have.

This link has information on how to identify your transmission:

http://www.wfmfiles.com/download/Tecumseh-Peerless_Motion_Drive_System_-_Transmi(691218).pdf

Eric