66-71 C80CK Hi/Low lever jumps out of gear when forward

I have a 66-71 580 CK Loader/Backhoe that I drug home and I dont know its previous sitiuation as I bought in barn and I rralize it was not running for long time but so far I had to have the 188 Diesel fuel pump rebuilt , replaced 2 bent pushrods , newer radiator, revamped the stupid fuel filter setup with a electric fuel pump amd inline filter , now it runs and starts great. Got it fired up and had to add alot of HYdraulic fluid to get loader and hoe working with some seal leaks that I will get fixed ..Its a 4 speed geared trans with HI?LOW ...So I tried to drive it but would not move so it has the shuttle with a dipstick and I added fluid to it and actually added way too much but that did not help and it would not move ..I did notice when I bought it the HI/Low shifter on the tractor had a bungie cord holding it forward and I suspected trouble with it when I bought it and so .....it will drive move when I pull the Hi/Low lever towards me and I feel it go in gear real good but when I try to put it forward it does not feel like its going in all the way in so I try to hold it in but it jumps back out at me when I let the clutch out ??? By the way is forward the LOW and pulling it back towards me the HIgh range as I suspect the low range would be the one used the most....anyways what do you think is wrong and how do I fix it ? where do I start?