Front Brakes won't release

I have an 82 XV750 and I have installed the front suspension from an 86 XV700. I can't get either of the the brake calipers to release on it. I'm going from a one, single piston caliper system to a two, dual piston system. I'm using the master cylinder from the original one caliper system, since the master for the two caliper system is trashed. I checked the calipers before installing them and they both checked out ok. All pistons are free and the seals are in good shape. I have no leaks and the brake lever feels solid when applied. I'm wondering if the master is just too small for this new system? It worked fine with the original system, but is almost half the size of the dual caliper master. Thoughts??? Thanks