1992 Grand Caravan ABS Issues

I have a 1992 Grand Caravan with ABS. Both the ABS and Brake Light come on at startup, ABS light flashes then remains on, brake light remains on as well. Light will usually go out after running in the drive for a couple minutes. Both come back on after 10-15 minutes driving, depending on braking. I have had it to the dealer and they have replaced the hydralic assembly and pump. They say their diagnostics come up with a controller fault. I replaced with a used one. This did not fix the problem. So I guess I have two questions, first, is there any to check the controller indepent of the system. I just don't want to spend the money to buy a rebuilt one and have the same problem. Two, is it possible to simply change the system to standard power brakes? Could I simply go to the junkyard and take the system off another van and replace the system on mine. Thanks for the help.