There's a good chance that the booster- if sized for a disc/drum set up- is not large enough. For four wheel disc brakes you need a 7"-9" dual diaphragm or equivalent booster capable of 1200 psi. Usual vacuum required is 18 in/Hg although you might get by w/less, but pedal effort will increase as vacuum decreases. Vacuum reservoirs do not usually help very much; you can look into a vacuum pump if you have inadequate vacuum. But if this engine is basically stock it should be making enough vacuum.
You may need an adjustable proportioning valve, but other valves aren't really needed other than as a warning for system pressure loss, which can be plumbed separately.
From a post
It is easy to tell the difference between a combination valve for disc/disc and one for disc/drum.
The rear outlet looks much the same, but on the disc/drum combination valve between the 2 outlets for the front lines, there is a small screw in fitting with a button that sticks out, usually these are covered with a rubber boot. That is part of your metering valve. The disc/disc combination valve does not have this.
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