First off those brass ball valves are not for hydraulic service. I am assuming you are using them to block off the lines running to the dozer circuit?
Those brass valves are typically rated for 300 PSI, while the hydraulic system is running at a minimum of 800 - 1000 psi.
Another issue I see is the placement of the filter. Typically the filter is placed between the valve reservoir on the low pressure return line, it appears that it is located on the pump suction line which can lead to cavation and seal failure.
I am thinking that the line you plugged might be the relief valve return port on the unit.
I think you will have some time trying to track that style of control valve mounted on the bottom of the unit, unless someone has one hidden away that still has good spools and body.
Here are some valves rated for hydraulic service.
http://www.surpluscenter.com/item.asp?UID=2008082219450524&item=9-7282-12&catname=hydraulic
http://www.surpluscenter.com/item.asp?UID=2008082219450524&item=9-7281-12&catname=hydraulic