Gee, I've been doing this all wrong for the 40+ years I've been in and around the auto repair industry. It's no wonder that all the cars I've worked on and race cars I've built don't work properly. If you think the VW TDI recall is massive, just wait until I generate my own recall.

On the cars you reference, don't confuse both MC lines entering a frame mounted combination valve with an actual diagonal system. This combo valve contains a piston and switch to warn of a brake pressure imbalance and a proportioning valve that reduces rear braking pressure in proportion to front pressure, this isn't a regulator where outlet pressure is constant.

On the rear drive cars you reference, do they have one brake line running to the rear or two? Assuming a solid rear axle, is there one rubber brake hose running from the body or two? ( Short whip brake hoses from the brake caliper to the axle don't count on rear disc cars. )