To test the condition you describe I would install a fitting we built instead of buying the real tool. It is a fitting about an inch long that screws to the booster and the line screws to it. It also has a section of steel brake line welded to the tube in the center making a test port for the vacuum gauge on the booster side of the plastic line's check valve.

It is most likely that you are at very low vacuum when the condition exists. There are two possible causes. Too little source or too much leak. When shut off a gauge placed at the point I describe should hold vacuum for significant time. Fully charged with vacuum three applications of the brake pedal can deminish the vacuum (if not replenished) to the state causing hard brakes.

You need to check for leaks, evaluate the use rate of several brake applications with motor off, and then watch with no vacuum how quick it builds after starting. The only other thing to evaluate is whether the same finding take place when the event occurs.

The system is designed with enough vacuum capacity that all the small source tees from the line could be left open and the size of the hole would still be small enough that the pump will still quickly supply the more important braking vacuum.

The brown line is the shut-off vacuum and if there were vacuum in it when running you wouldn't be running. You would be shutting-off. There are actually two brown lines one has a tracer. The source line off the booster line will have vacuum if the pump is working. That line goes to the steering/ignition lock/switch and when shut down the switch allows the vacuum to flow through the other brown line to the shut-off diaphram on the back of the injection pump.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician