by Ken Nelson…..
Regarding the brake button advantages, the hydraulic system (steering, brakes, suspension) is a high-pressure system – at 2500 psi (IIRC). Braking force is modulated by the foot pressure applied to the button, rather than how far the pedal travels. In a panic stop situation, bringing your foot down to the button and “mashing it” produces full power braking immediately.
Braking up front is via large, well cooled. inboard discs. Another thing is, when you mash on the button, the rear of the car will stay level, if not drop slightly because when the DS was introduced, it was built-in load sensing/brake proportioning, regardless of front/rear load balance.
The other thing about DS braking is that the champignon/mushroom pedal allows for quick foot movement from the accelerator pedal to the button. Unlike a conventional brake pedal where you lift your foot to the other pedal and then mash on it, on this you drop your foot off the gas pedal directly onto the button. Activating the brakes in a DS happens a lot faster than a conventional brake.
Underneath the mushroom knob is a split VALVE system, not a master cylinder type system – one tiny 1/4 in. diameter piston for front discs, another identical piston behind the front one, serving the rear brakes. Total travel from zero to full stop is – about 3/8 inch! The valves simply admit the stored high pressure in the system to the brakes. This sounds crazy, but it’s a true feedback proportioning system so your foot does get used to the gradation in force. Sheer genius!
To make it more brilliant, the mushroom knob pushes not directly against the two pistons, but on the CENTER of a teeter-totter plate which straddles both pistons. AND a tiny hydraulic piston forward of this twin hydraulic valve block pushes the TT plate forward/aft based on the pressure coming to the auxiliary piston from the rear suspension, whose height is affected by the trunk load. So, when a heavy load is in
And further to the brilliance of the design, as the rear suspension arms are very long, to give lots of soft wheel travel, when the rear weight is high and/or front brakes are pushed hard enough, the rear drums engage and rotational torque from them YANKS the rear end DOWN – it squats automatically, and counters any tendency to nosedive. In fact, if a DS doesn’t show the squatting rear under hard braking, a knowledgeable owner will know the rear brakes on a DOT 3 early car will have stuck rear cylinders, and you know you need to free them up with an overhaul.
It’s the tiny travel which throws people – literally – thru the windshield if you stomp on it! So – be GENTLE – this is NOT a vacuum-boosted system. There is 2500 psi of pressure, enough to operate a forklift, is under that mushroom knob!
I could go on and on. . . These are brilliantly, thoroughly engineered automobiles.