I figure the accumulator but not sure.
Even a total failure of the hydroboost system would not cause your "Brakes to just disappear without warning". They (the brakes) are designed to continue to operate without any assist - you just have to push them harder.
The hydraulic circuits of the braking system are completely segregated and independent of the hydraulic assist circuit. The only thing that connects the master cylinder and the hydroboost unit is the hydro's output rod - and all it does is multiply the force applied by your foot on the pedal/pedal rod. As long as the pedal rod pushes on the input rod and the input rod pushes on the output rod and the output rod pushes on the master's piston, the brakes will develop hydraulic pressure - provided the master is working properly and the hydraulic lines (wheel cylinders/calipers) are tight.
I would take look at the MC.
Just some general info from GM on the hydroboost system:
In order to properly service power brake systems, a thorough understanding of the various power assist systems is necessary. These systems provide a mechanical assist, acting to increase the force applied to the master cylinder piston by the operator while decreasing the effort necessary to obtain acceptable stopping performance. Two basic types of power assist mechanisms are used: vacuum assist diaphragm assemblies which use engine vacuum and in some cases vacuum from an external vacuum pump,
and hydraulic assist assemblies which use hydraulic pressure developed by an external pump (usually the power steering pump).
Hydraulic assist units incorporate a basic open center spool valve, hydraulic cylinder and an accumulator assembly incorporated into a common housing. When the brake pedal is depressed the hydraulic piston moves forward closing off fluid relief passages, the spool valve applies fluid pressure to the rear of the piston, and pressure applied to the piston is transmitted to the brake master cylinder through a pushrod. When the brake pedal is released, fluid pressure behind the piston is exhausted through the open center of the spool valve to a return line and the pump reservoir, and the spool valve diverts fluid pressure away from the hydraulic piston. The accumulator stores sufficient fluid under pressure to provide 2 or 3 power assisted stops in the event that pressure from the pump is interrupted.
The design of the hydraulic cylinder allows the brakes to be applied without power assist, however, greater pedal effort is required to apply the brakes manually.
The Hydro-Boost system, Figs. 1 and 2, provides an additional cylinder in the brake system.
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This cylinder contains no brake fluid. The Hydro-Boost cylinder is hydraulically operated, by pressurized fluid from the power steering pump, providing power assist to operate a dual master cylinder brake system.
The booster is composed of two sections; the linkage section and the power section.
The booster is designed so that if a total absence of power assist occurs, the brakes can be applied manually, but somewhat greater pressure on the brake pedal is required.
