Truth is, a combination valve applies rear brakes first (I guess that would be a 1:0 or 100%). This is the 'metering valve' function that ensures stability to the car. With more foot pressure, the front brakes get the majority pressure through the range all the way up to lock-up.
I'm not sure if it is 70/30 but understand, the proportion changes with pedal pressure. THAT is the major difference between a combination valve and a proportioning valve. This also explains why you wouldn't manually adjust it.
In a hard stop, when the front dives (and the rear lifts), a massive amount of weight shifts to the front wheels which lightens the load on the rear wheels. This can be exagerated using the example of a pickup truck with an empty bed. The back wheels will simply skid when improperly proportioned.
The car manufacturers got it right. If we mimic their system by using OEM parts from a similarly proportioned car, our brakes will work the same. Notice I didn't say, 'similar weight' car. That doesn't matter (engine size doesn't either). Avoid using pickup truck valves for your Thunderbird as those vehicles are proportionaly NOT the same.
Disk brake SPINDLES from a Granada/Mustang will work on your Thunderbird. The bottom ball joint hole needs to be opened slightly, but many conversions from Thunderbird drums to Mustang disks have been done. That is exactly what Eric did for his 'bird. (Spindles have mounting provisions for the brakes.)
Fact is, those 'Mustang/Granada/Versailles' spindles are compatable with LOTS of Ford suspensions. I'm using the Granada setup on my '55 Ford Customline. It is every bit as hearty as the original spindles, and it works great! The bearings swap, which means the diameters are the same.
There are many ways to convert to disk brakes for your T-bird. All of them are a vast improvement over the original drum brakes. - Dave