cub 129 transmission

I just got this machine, serial number crosses to being built in June of '72, have a problem with it.
When I got it home and tried to start it, pedal locked down, and in Neutral, it wanted to lurch forward while cranking. It was bad enough so as to "fight" the starter/gen, I wound up having to pull the driveshaft at which time the engine started right up... It has the manual relief valves and they appear to be free/neither is stuck. When I got it the tunnel cover was off and in a box, as the flapper to depress those pins was broke. which brings up another question; I got a replacement flapper, and there is no spring pressure to hold them down to push/tow. I have to hold the lever to hold the relief valves in a released position. Is this right?

Not sure if I am confused or you are but those release valves are not there to provide a neutral for starting. They provide a release so you can push the tractor around without the engine running. They do not have to be held down to work, you depress them with the lever and then they reset when you start the engine, this is going on inside, not the pin you see projecting from the top.

The transmission is controlled by the "trunnion" located to the left side. You control the trunion with the lever or pedal and the trunnion controls the transmission. Part of its function is a cam that is operated by a link from the brake pedal that places the operator in neutral when you push the brake. Several things can cause the situation you have. One is wear in the linkage, two is wear in the cam or pin that operates it, three is the two springs up top that actually operate the transmission compress and the seats go bad,lastly is somebody adjusted things that didnt know what they were doing.
The repair and adjustment procedure is well documented and too long winded to type here. Google "Cub Cadet trunnion repair and adjustment to get lots of information and you tube clips on the repair.

Put the rearend up on jack stands and take off the rear fender/seat to get to the hydro linkage. The linkage may simply be out of adjustment. Looks like there is even a tutorial available for the 129:

https://www.youtube.com/watch?v=phz_VqTZFRI

If it does not adjust correctly, or jerks forward or backward the spring assembly the top of the trunnion may need attention. Like Butch said, lots of info out there.

yeah I understand what those release valves are for. But I have also seen untold many posts about them being "stuck" and the tractor not moving at all in one direction or the other, this one does the opposite, it wants to go go go.... so I was wondering if they might be stuck "the other way"
I'll have to check out that trunnion repair thing that you mention.

arrgh I see no "edit" feature.
I wanted to add that the hydro lever on the dash is stripped out, I have to twist it like you would a tiller handle on an outboard to get it to bite and the linkage to move. but I don't think that is the whole answer to my issue. When I hit the pedal, I can see the linkage "center" itself with the tunnel cover off. and it lurches even with the pedal locked down.