Because most trucks have more rear brake than they need when not loaded(talking drum and wheel cyl size)and even with down stream regulation (ride height, rear abs dump etc) rear volume to apply brake shoes can be higher than front discs, while rear pressure can be lower.
As far as m cylinder pistons being connected... that happens through hydraulic pressure.
Another form of bias inside the master can be controlled through the transfer springs. The transfer springs can be installed to which chamber compresses first.