430 case skid steer hydrosatic problems

I recenly bought a case 430 skid steer[2007] with pilot controls and has 3200hrs on it. I'm having trouble with hydosatic drive, when i'm in low speed it doesn't want to drive in forward or reverse and when i'm in high speed everything works fine. I'm able to run with it in high but very hard to dig with, would like to get low speed working. This problem is just on the left side, right side works good. Is there any cade mechanic's out there that could help, thanks in advance...ps i cut open hydraulic filter and it was clean. I want to check charge psi and pilot psi but not sure where to t in or know what and where to adjust? I'm in the dark no repair manual, please help asap!