1. The warning light just means there is an imbalance between the front and rear circuit. That could be a actual problem or it could mean the brakes need to be bleed in order to center the warning switch (which is what Froggy was explaining).
2. I have never heard of a Dodge product of this era having any form of safety lock unless you talking manual emergency brake.. The front rear valve is combination warning light switch proportion valve. The warning light section only indicates there is a front to rear imbalance. The proportion valve section only reduces the pressure to the rear brakes because disk brakes require more pressure. This prevents the rear drum brakes from locking up before the front disk breaks.
3. RM400 - Look under the rig to see if you have two big boosters just forward of the rear axle like this:
if not, then you should have a hydroboost unit just inboard of the front drivers side wheel like this
I have to know what system you have to continue further.