Combination valve problems
Would the wrong combination valve cause poor braking. Yes Here's why.
The disc brake pad is normally in contact with the disc while the drum brake shoes are normally pulled away from the the drum. Because of this the disc brakes are in position to engage before the drum brakes, when the brakes are applied. The metering valve compensates for this by making the drum brakes engage just before the disc brakes. The metering valve does not allow any pressure to the disc brakes until a threshold pressure has been reached.
I suspect with your set up you are getting most of your braking action to the rear brakes and very little to the front.
Try this. Put the car up on jack stands. Have someone apply the brakes with the brake pedal all the way down. Go around to each wheel and try turning the tires. You should not be able to turn any of the wheels by hand. Check to see if the front wheels are easier to turn than the back wheels.
Is your master cylinder on the firewall above the level of the calipers, or has it been relocated to the floor level?
You say you saw some rust in the old master cylinder, you may have some rust in the calipers as well. Did you get clean fluid when you re-bled the system?