takeuchi tb105 only drives one direction

[moogles]

i am working on a mini excavator that will only go in reverse.sometimes when you push the sticks forward it will start to go but then slows to a crawl and stops. rasing the arm while trying to go forward seems to help but im not sure whats wrong. the only thing that i have done is checked the hydraulic fluid in the res witch seems overfull to me. i know engines but not Hydros. can someone please give me an idea where to start.