1979 GMC Dura Van 35 (motor home) w/hydraulic brake booster

First thing you should do is check the power steering fluid level. It is also is what provides the hydraulics to the brake booster.

Hydro Boost Brake Troubleshooting Chart:
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High Pedal and Steering Effort (Idle)
1.Loosen/broken power steering pump belt
2.Low power steering fluid level
3.Leaking hoses or fittings
4.Low idle speed
5.Hose restriction
6.Defective power steering pump

High Pedal Effort (Idle)
1.Binding pedal/linkage
2.Fluid contamination
3.Defective Hydro-Boost unit

Poor Pedal Return
1.Binding pedal linkage
2.Restricted booster return line
3.Internal return system restriction

Pedal Chatter/Pulsation
1.Power steering pump drive belt slipping
2.Low power steering fluid level
3.Defective power steering pump
4.Defective Hydro-Boost unit

Brakes Oversensitive
1.Binding linkage
2.Defective Hydro-Boost unit

Noise
1.Low power steering fluid level
2.Air in the power steering fluid
3.Loose power steering pump drive belt
4.Hose restrictions