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You described exactly what is happening even with 18" at idle or coast.
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Pedal feel is subjective, but 18" vacuum at idle is generally not low enough to be a problem. 8-10" at idle is where significant problems usually show up. Are you aware that several GM vehicles from the '80s used calipers with similar outside dimensions but different diameter pistons? If available, larger pistons would help reduce pedal effort.
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theres a little info around on piping options (like connecting pump "exhaust" to manifold vacuum)
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Vacuum pump exhaust probably should not be directed to manifold vacuum. It can alter manifold pressure and in some cases will alter airflow readings to a computer in fuel injected vehicles. I would direct it to the air intake tubes in front of the air filter or the air cleaner.
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Those who use the pump seem to like the results but complain mostly about the noise.
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I had an '82 Cavalier with a factory installed vacuum pump. For me the noise was simply part of using the vehicle. We have owned fleet vehicles which have Hydroboost and use electric assist pumps. Again, noise is simply part of proper operation of the system.
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Being a somewhat anal retentive engineer I installed a shut off switch
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Interesting.
As a discussion point, there are several design considerations for powering the pump. Most OE installed applications that I'm aware of allow the assist pump to operate if the brake is applied when the key is off. A brake operated switch could be wired to battery power so the pump is powered whenever the brakes are applied. This could cause the pump to operate unnecessarily at idle however it would also ensure brake assist is available in the case of an engine stall. Wiring the brake switch through the vacuum switch limits operation to those times when vacuum is needed, however it could prevent the booster from maintaining the minimum level of vacuum and would not provide assist if the engine stalled. I believe the GM and Delco based systems connect the pressure switch to battery power. The consequence of the circuit malfunctioning and draining the battery is far better than the consequence of an accident caused by insufficient braking.