The left and right side mid/rear hyd levelers on the six point system are not connected to each other..
They are however in tandem mid/rear on each side
I rather doubt that both Hydac valves are faulty...there is one for each side...I’d suspect more likely you have air in the system ..system is self bleeding and can be bled easily
I would however check the manual override set screws for both of those Hydac valves..if the6 are not fully closed it will allow the hyd pressure to bleed off
The levelers are held retracted via 2000psi hyd fluid pressure
I also doubt that you have two internally leaking cylinders.... it would have to be one on each side to affect all four