I am no expert on anything let alone brakes.. and always happy to be corrected if I am wrong.
Were I suffering poor brakes I might be inclined to crawl underneath and use brake hose clamps to close all the flexible lines and then check pedal travel. The idea of locking them all off is to test all of the kit involved in creating and initially distributing pressure to the system, such as servo ABS etc. Neither the servo or the abs system will operate without the engine running, though brakes should still operate with a dead engine and no ignition, just that it takes more pedal pressure to gain a "reasonable" braking.
Pedal travel with all flexible hoses locked off/clamped should be as good as it ever could be.... With possible issues and the cap removed from the fluid reservoir I would be looking for either signs of bubbles of fluid flow within the reservoir indicating possible master cylinder seal failure allowing fluid or air to escape back past the seal into the reservoir..
By releasing the clamps and comparing pedal travel on each individual brake drum/disc set-up it should be possible to detect if there might be an issue affecting one brake more than the others.
Logic dictates that if brake shoes are not correctly adjusted it can require more pedal travel/fluid displacement to bring the shoes into contact with the drums to them create the braking effect
With discs there is no adjustment to go wrong, however on poorly maintained brakes I have seen seized pistons in a calliper that stop one side of the caliper pushing it`s piston out and the pad being unable to move. in this situation it is possible that the piston(s) on the opposite side of the calliper do operate normally when the pedal is pressed forcing the pad to contact the disc which in turn forces the disc to flex to the extent that it is forced over to make contact with the seized pad on the other side of the calliper.. The effect of this feels not a lot different in relation to pedal pressure and the amount of spring normally associated with applying the brakes, however the brakes are less efficient when out on the road.
Mixing fluid types or thewrong type of fluid can cause all sorts of issues for seals, ABS etc....
