case 1845c hydrostatic testing

I am working on an 1845c that Won't move and I could sure use some help on troubleshooting it. It's full of hyd oil and it has 100 psi supply pressure coming into large center hose on the right side of the hydrostatic pumps. How do I test the charge pump pressure?

Does anyone have info on testing the hydrostatic system or is there a place I can find it.