Mast back will result in more load on the rudders when going to weather. This will typically increase weather helm - but that depends on how far forward the rudders are raked under the boat. If you have heavy weather helm it is possible your rudders are not raked forward enough.

On some rudder designs this is directly adjustable (e.g. later model Hobies), on others it may require filling and redrilling the holes in the blades.

Sometimes as a rudder system wears it allows the blades to be pushed back by the drag of the water when sailing, and this can increase the weather helm.

Chris.