Vacuum brake connection

Technical discussion on converting internal combustion to electric
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Williamo
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Vacuum brake connection

Post by Williamo » Sun, 02 Jun 2013, 22:27

I am converting a 1974 BMW. Currently there are two vacuum brake reservoirs. This is a common way to maintain braking capacity should a line rupture.
When I install an electrical vacuum pump, having removed the existing reservoirs, do I need to replicate the volumes of the existing ones, or is there a better arrangement?
Last edited by Williamo on Sun, 02 Jun 2013, 12:28, edited 1 time in total.

Renard
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Vacuum brake connection

Post by Renard » Mon, 03 Jun 2013, 00:10

The bigger the reservoir, the less often the pump will run, but it will run longer each time. So that's not really relevant. And in any case, it will depend on the efficacy of the pump. I'd say the reservoir needs to be as big as necessary for one severe braking stop.
What other situation needs to be considered? Well if you're going down a very long hill and frequently applying the brakes, and therefore apparently needing a large reservoir, I'd say you should be in a lower gear.
The pump needs to be sized adequately to restore low pressure within a reasonable time. All the parameters mentioned here will depend on your individual situation.
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4Springs
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Vacuum brake connection

Post by 4Springs » Mon, 03 Jun 2013, 03:22

Is there a reason you want to remove the exsisting reservoirs - why not use them? It is a pain to make a new one.

I am surprised at how effective my electric vacuum pump is. Even constantly pumping the brakes doesn't make the pump run full time.
I'm no expert, but this was the way I went about it:
I made a reservoir and hooked everything up. The switch seemed to have very little hysteresis - the pump would run for a couple of seconds, then jitter on and off for a few more seconds, then settle down. I didn't like this, so I made another reservoir and added it into the line. This didn't really improve the situation much, the pump still had the jitters. So I made myself a circuit to extend the run time of the pump. The circuit extends the run time to eight seconds. So if the pressure switch tells it to run for one second, it will run for eight. This works very well, since the extra seven seconds means that the vacuum is well and truly enough to ensure it doesn't jitter back on again soon after.

Which is probably a long way of saying "suck it and see"! Different combinations of vacuum pumps, vehicles etc. will mean that you will just need to try it out.

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Vacuum brake connection

Post by Williamo » Mon, 03 Jun 2013, 04:55

Thanks . The existing reservoirs are quite bulky, so I was considering the poly pipe option in another location to maximise battery space.

Williamo
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Vacuum brake connection

Post by Williamo » Mon, 03 Jun 2013, 19:53

Thanks for that. my problem is really how to hook up the pump to two new reservoirs and preserve the 'fail-safe' aspect of the orifinal set-up

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4Springs
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Vacuum brake connection

Post by 4Springs » Mon, 03 Jun 2013, 22:16

Williamo wrote: Currently there are two vacuum brake reservoirs. This is a common way to maintain braking capacity should a line rupture.
How exactly does this work? Is it really for if a line ruptures, or is it more likely to be for if your engine conks out?

I could imagine that a good size reservoir would be a good idea if your ICE stalled while driving. In that case your source of vacuum has ceased and you need to pull over quickly.
It is a different situation in an electric vehicle though. If your motor stopped working, and you needed to quickly pull over, you would still have vacuum for your brakes.

Have you tried the car's brakes without vacuum? The brakes on my car work quite adequately without vacuum, so I don't feel that I need any "emergency" capacity. Combined with the above logic, I feel that a smallish reservoir (and only one of them) should be fine.

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woody
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Vacuum brake connection

Post by woody » Mon, 03 Jun 2013, 23:39

Test how many smashes of the pedal you get assistance with after turning off the engine, and change the size of your custom reservoirs accordingly.
Planned EV: '63 Cortina using AC and LiFePO4 Battery Pack

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