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Old 04-04-2012, 06:25 AM   #55 (permalink)
mechman600
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Location: Langley, BC
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Vacuum Pump & Brake Testing


I drove to work and back, and no, I didn't crash and die. But it did not work well. When applying the brakes there was virtually no vacuum assist until you held the pedal down for at least 5 seconds. It was a weird feeling pushing HARD on the pedal coming up to a stop, and as the vacuum assist started working the car started slowing down increasingly abruptly with the same pedal pressure. If I let off the pedal to reapply, it always started off with no vacuum assist all over again.

This is all because of the very way vacuum assisted brakes work. When the engine is making vacuum and the brake pedal is released, there is vacuum on the engine side of the diaphragm, and the port is open from the engine side into the pedal side to allow vacuum to go there as well. (I speak as if vacuum is a thing, when it is really an absence. But you know what I mean, right?). When you press on the brake pedal, vacuum remains on the engine side, but the port to the pedal side is closed and another port is opened on the pedal side to atmosphere to allow some atmosphere to get into the pedal side to push the diaphragm forward, assisting braking. When the pedal is released, the atmosphere port is closed and once again the port opens to allow vacuum on the engine side to get into the pedal side. When you switch the vacuum pump using the brake pedal switch, at this very moment the overall vacuum level drops because the pump is shut off and there is no vacuum supply to fill both sides of the diaphragm again.

So I thought maybe a latched circuit will work - the brake pedal will turn the pump on but with some magic circuitry, it will remain on for 10 seconds after I release the pedal. Not having such circuitry skills (or parts), I wired in a momentary switch and taped it to the gear shift. I drove home from work simulating a latched circuit setup. What I found is that it is much better this way. The brakes work as they should under moderate/normal braking. But as soon as I pumped the brake once and reapplied, there was nothing. Also, there isn't enough vacuum with this pump (only 13.5" HG) for a proper hard brake application.

Conclusion: a latched circuit controlled by the brake light switch will work, but my cheap junky vacuum pump does not have the volume, nor the amount of volume required for this to be safe.

I wonder if one of those 12V cigarette lighter powered air compressors could be used as a vacuum pump on the suck side.....
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