Bob's Studebaker Resource Website

Studebaker Hydrovacs

This is a Type A Drum Brake Hydrovac

Here's how it works

As pressure is applied to the brakes, fluid is passed into the Hydrovac (yellow). It by-passes the hydraulic brake piston (# 20, thanks to the check ball), and pressure goes through the lines to the wheel cylinders. At the same time it also passes through a pathway in the unit and applies pressure to (# 10) the control valve hydraulic piston. As more pressure is applied to the brakes, the control valve piston pushes up on the metal rod on the rubber diaphragm (# 7). The metal portion of the diaphram has an air pathway through the center which allows air flow circulation throughout the unit (through the tube (# 9) to the rear of the power cylinder piston). The atmospheric / vacuum valve (# 8 and # 13) is a two part valve located above the diaphragm. It blocks air coming into the Hydrovac due to the spring (# 14) under the intake tube. Brake pressure changes cause the vacuum valve to seat on the rubber diaphragm (shown in pink), sealing the air passage through it and effectively blocking any air coming into the power cylinder (# 1) between the ports (# 6) located in the end plate of the unit and the connecting tube (# 9).

As pressure builds and the diaphragm is pushed even further forward, the atmospheric valve portion opens (shown in purple) allowing air to enter the power cylinder (air path shown in green) and letting the vacuum from the engine pull the power cylinder piston (# 2) forward which pushes the hydraulic brake piston (# 20) into the hydraulic cylinder (# 22). The check ball has pressure from the brake lines pushing back against it now (orange) and seats inside the piston allowing no brake fluid to pass by it back into the master cylinder. Therefore, the piston pushes more brake fluid forward in the lines and braking power increases.

The whole system uses brake pressure to overpower the springs and engage / disengage the control body valves. The reason this system works the way it does is that the vacuum created by the engine is able to travel throughout the Hydrovac unit (shown in red). Therefore with vacuum in front of and behind the power piston, there is equal pressure on both sides. So until the valves close off that circulation and opens the air intake, the power cylinder piston won't move. Closing one valve stops that circulation throughout the unit. Opening the intake allows the cylinder to pull air from behind so the vacuum can pull the power cylinder piston forward.