John Deere 300 b backhoe select hydraulic problem

[Tx Jim]

If front hyd pump is good & capable of building 2250 psi if supplied sufficient charge oil by trans pump then it's my guess the 300 hyd system has an internal leak in either the low or high pressure circuits or worse scenario BOTH circuits. The only method to know is by """performing very good hydraulic diagnostic procedures""'. Good Luck, Jim