Not a design issue...though in my opinion, the oem disc/drum combo braking system overall on the Scout II is a marginal system...but typical of most oem systems of that era. My caution is to not compare braking action/performance/feel of the sii system with later vehicles of a more modern design and mist especially...lower vehicle weight!
And the oem design is only compromised further with the addition of the typical significantly greater rolling diameter tire/wheel combos most folks like to run on these rigs.
That said...here's some possibles that might contribute to your described condition, and don't overlook having more than a single cause unless you can rule any out due to recent component replacement with verified good parts:
1) flex hose(s) are "ballooning" due to internal failure. Replacement of 30+ year old flex hoses is a maintenance item and is the most overlooked item regarding brake system maintenance. Another related issue regarding hoses is internal "delamination" of the hose material allowing the inner layer to act as a "check valve" when pressure is applied.
2) the master cylinder is by-passing to a degree, but not completely. A common issue I find with "remanned" master cylinders that are crap...I will not use a commercial reman mc, I use either new or a I rebuild 'em myself so that I control quality and then become responsible for proper operation.
3) not likely but something I have seen more than a few times on disc/drum combos of all oem types...a rear drum brake shoe that is actually cracked through the web (even partially) and allows the shoe to "flex" when pressure is applied by the wheel cylinder. I have a couple of shoes now in my pile of crap, those cannot be "rebuilt" by my friction materials source since that is a liability issue on their side...though they do routinely repair mangled shoe cores if the shoe "table" is damaged.
4) missing/in-op residual pressure valve in the rear drum brake circuit only. The sii service manual does a really nice job of describing how this system works for any disc/drum brake system. This is very common if the master cylinder has been replaced with a service part that did not include an rpv.
