Hello,
Just been through a similar scenerio. Only my right gear was collapsing on rollout when landing. Found that the air pressure dropped 30 or so psi during flight. Retracts alone held pressure and also held pressure with small leakage when retracted while on the ground. Tested each line separately and eventually found a brass T for airlines was scored from using a hobby knife to remove tubing thereby causing a leak. I would recommend testing each line separately to find the leak using a half split method of troubleshooting.
John