John Deere 510d Brakes

I have a John Deere 510d backhoe on which the brakes have quit working. It appears that no hydraulic fluid is getting to the brake valve, as I took the two lines that supply fluid to the brake valve from the transmission loose and they did not have any fluid in them. All the other hydraulics work fine.

Any ideas regarding what may be causing this?

Thanks!

John Deere 510d Brakes