My new '67 Firebird got delivered today (pics to follow when weather clears) and had kind of a scary experience. I was driving it home from a buddies house, doing about 50 mph, when the stop light changed suddenly. I must have hit the brakes a little too hard, because the rear end did it's best to become the front end (e.g. nearly spun). The road was wet from the recent rains in the area, but I didn't expect this to happen. The car is a four-wheel power drum car. Should there be a proportioning valve in the system, or is that function handled by other means? Any suggestions on how to keep the rear wheels from locking up so quickly, other than not braking so hard?

I'm looking into buying a four-wheel disc setup, but that probably won't happen for several months. Is a proportioning valve required there as well? Sorry for the confusion, but surfing through Google provided more disagreeing opinions than any useful information. Thanks in advance for the help.