The combination valve is a proportioning valve and shuttle valve in one
On the Mustang, the proportioning valve is in the front part and is usually removed with aftermarket brake setups to distribute equal pressure to both ports and is adjusted via an aftermarket adjuster.
The shuttle valve shuts off pressure to either rear or front brakes in the case of a massive leak (hit debris which slices a brake line) Basically with near equal pressures, the valve stays centered, but if there is an imbalance, the shuttle valve slides over and severely restricts flow to the side with lease resistance (where the leak is)
It doesn't work with small leaks. Only really works if a line completely opens.
The stock 3-port fox MC blows ass. One line runs direct to the front left brake while the other 2 lines go to the prop valve and are split between the rear brakes and front right brake.
With a 2-port MC, you can set the brakes up the way it "should" be. Both lines run down to the gutted combo line and both front brakes T into the combo valve with a single line going to the drums with a proportioning valve installed at the brass union on the firewall.