I'd start changing lines. Could be the supply to the ram, but I'd bet your restriction is in the return. Also have you tried to disconnect the rear steering valve and cylinder to isolate the front system?
I've seen several hydroboost brake setups act up when using a restrictive single return. The pump quickly out flows the return and causes either the hydrobooster or the power steering to quit.
On your set up since the rear is in line first it will always work, while the front is struggling. You may need to increase the size of the return line and fittings.
