The dual circuit system provides two independent hydraulic brake systems. If for any reason a wheel cylinder or brake line should fail in one system, the other system remains functional.
The main cylinder front reservoir and outlet is connected to the front wheel brakes. The rear reservoir and outlet is connected to the rear brakes. The different thread sizes are purposely provided ot ensure proper connecting of brake pipes to the cylinder outlets. The front outlet is 1/2 -20; rear 9/16-18.
The main cylinder is compased of two independent hydraulic systems (fig 16). the primary piston operates in the same manner as the piston in the current production main cylinder. The piston contains a secondary seal, a short lip primary seal and a thin steel cup protector, a secondary spring retainer, and a secondary return spring. A stop screw which retains the secondary piston while the primary piston is being assembled also prevents the secondary piston from damaging cups, in the event that bleeding presure is applied to the front chamber before the rear chamber.
The reservoirs are completely independent and are sealed at the top with a diaphragm member (fig 17). This diaphragm is not vented and behaves in the same manner as the diaphragms used in current production. The diaphragm is always assembled with the convolution pointing up, a feature which is different that present production diaphragms which can be assembled from either side. The diaphragm is protected by a metal cover which is vented to atmosphere. Diaphragm and cover are held in place by a bail wire. The bail wire limits the pressure build up in the reservoir to 4-20 PSI.
In normal operation the primary piston and caged spring assembly move the secondary piston to close off the by-pass at the front chamber. At the same time the by-pass hole is also closed at the rear chamber. Under these conditions hydraulic pressure is then built up in both front and rear chambers and the brakes operate normally.
When a failure occurs in the front braking system, the operation is the same as just described except that the secondary piston will move through its full stroke and bottom out on the end of the main cylinder body. After this occures hydraulic pressure can then be developed in the rear chamber htus actuating rear brakes. This failure is also accompanied with a noticeable increase in pedal travel and pedal effort.
When a failure occurs in the rear brake system, the primary piston and caged spring assembly move forward and pressure starts to build up in the front chamber. Since the primary spring is the only source of load no increase in pressure will occur until the extension screw on the primary piston makes contact with the secondary piston. From this point on all pressure build-up in the fornt chamber is by direct mechanical loading via the primary piston, extension screw and secondary piston. Again, this type of failure is accompanied by an increase in pedal travel and pedal effort.
source: Chevrolet Service News Volume 38 - September-October 1966, Number 9.
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