Yes. Drum side doesn't need the large fluid reservoir. When disc brake pistons move out of the caliper due to pad wear, the caliper has to fill with fluid from the reservoir. Duo-Servo drum brakes have the pistons return to the same place every time, with shoe wear compensated at the adjuster assembly.
Leading/Trailing shoe brakes are kinda in-between. The pistons move outward like caliper pistons, due to shoe wear, but the pistons are way smaller so it takes less fluid volume to compensate.
Three causes:
1. Park brake pedal switch
2. Certain ABS faults
3. "Safety switch" within the combination valve. NOT the "proportioning valve" which is also within the combination valve assembly.
It doesn't.
Bottom-feeder falsely-advertised Amazon junk.
Any one or more of the three things listed above...but...given the known leakage, I'm expecting it's the pressure differential tripping the Safety switch.
There's many grades of "copper" tubing. I've never seen copper pipe, although I've seen brass pipe, and brass is an alloy of copper plus zinc. For the record, bronze is an alloy of copper plus tin. NiCopp is a brand name for an alloy of copper and nickel, which--I guess--is strong and durable enough for brake and fuel systems.
There's many ways to plumb "dual circuit" brakes. These trucks, and most older RWD cars were split front/rear. Rear brakes inherently have less braking effectiveness than front brakes, so there's nowhere near a 50/50 split. Maybe 60/40.
FWD cars have such a massive difference between front brake power and rear brake power that they rarely split the two circuits front/rear. If the front circuit failed, the isn't enough braking power at the rear for safety. GM used to use "dual-diagonal" circuits, LF/RR and RF/LR, to always have one front wheel with braking power, and the diagonal rear wheel to sort-of mitigate the directional pull of one effective front brake and one ineffective front brake. It was not a great system, but it was cheap.
Volvo and others used twin-piston calipers, where one piston of each caliper was on one circuit, and the other piston of each caliper was on the other circuit. That is the better way to plumb the system--all the brakes "worked" even if the caliper had only one of the two pistons pressurized. it maintained directional stability far better than "Dual-diagonal" systems. Tougher to bleed, though, since each caliper would need two bleeder screws.
My expectation is that the valve is self-centering, although I don't know that for sure.
I have heard--but not seen or experienced--that the British Isles routinely use "copper" tubing for all sorts of things that Americans can't get by with.
I don't know the root cause. MAYBE you folks have access to really thick-walled copper tubing. MAYBE you folks got "copper" tubing that is really the equivalent of NiCopp tubing--a copper alloy rather than genuine copper. Maybe it's something I just can't imagine.
Anyone in America that tried to use copper tubing on brakes would need to be completely reckless, totally un-caring, ignorant as all get-out, or actively suicidal.
Copper tubing as sold in America won't withstand brake-system pressure, and it work-hardens and cracks. Our Federal DOT will not allow copper tubing for brakes or fuel systems.
As a result, all I can say is that around here, copper tubing on brakes is bat-shiit crazy. NiCopp is acceptable, double-wall seamless steel tubing is the norm; and stainless steel is a niche product that gives considerable problem in flaring and sealing of the flares.
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