[teragram (OP)]

A friend, who does his own car maintenance, posed the following question:-
If dual circuit hydraulic brakes are supposed to allow one brake circuit to remain working ( i.e. the pedal to provide pressure in that circuit) in the event of loss of pressure in the other circuit, why does opening any bleed nipple in the dual system allow the pedal to travel to the floor ( implying that the circuit still closed has also lost pressure).?
I assume he is correct, and this is what happens.

[CliffordK]

Good question.  I think I've noticed the same thing myself.

This Honda website gives a good description of the dual chamber master cylinder. 

The two chambers are aligned in series, which in ordinary circumstances would give equal pressure to both chambers, and equal braking.  If they were arranged in parallel, then any reduction in pressure on one side would remain uncompensated and you would get unequal braking.

I think the problem occurs that if 100% of the pressure is allowed to drain from one chamber, then that chamber compresses enough to allow 100% pedal travel, while the other chamber remains uncompressed. 

It should be easy enough to modify the master cylinder design so that one would only get full pedal travel with both chambers compressed.  It would mean that with a leak (or a bad air bubble), half the system would reach zero pressure quicker, and one would only brake on half the system.

And, drivers might have the tendency to ignore problems causing excess pedal travel on half the system (although an unequal pressure sensor should be activated).

But, I agree that having half the braking system is much better than having none.

[teragram (OP)]

Thanks CliffordK for your reply and link.
The linked site is interesting, and gives a good explanation of how the dual brake master cylinder works.
I think however that something is missing. It points out clearly that the “solid spacer” (yellow) will bottom out on the cylinder housing if fluid is lost in #1 circuit. The #2 part of the piston assembly will then (under further pedal movement) still provide pressure for the unaffected circuit.
I think that what is not stated is that the #2 rod (blue, within the yellow spring, and which obviously? can slide into the yellow solid spacer), must also bottom out inside the spacer. This I think will then allow the #1 circuit to operate, again with further than normal pedal movement.
As far as I can tell then, opening a bleed screw should not result in the pedal being able to move to the floor, although its distance of movement will be very great and perhaps give the impression that it has done so.
I hope this makes sense.