# Measuring Vacuum



## dtbaker (Jan 5, 2008)

TheSGC said:


> So I have my new US8VGC XC installed and during my test drive my brakes were less than stellar after a single slam. So I took out my dad's old Vacuum gauge and hooked it up to where my vacuum switch is connected and let the pump loose. It topped off at 15" Hg after, way under it's 22 Hg rating.
> 
> Now here is my question, is my pump not working right or the distance the gauge it from the pump throwing off the reading?



your brakes may feel a little less powerful than stock in part because your car weighs probably 500-700# more.... but as far as vac goes; the switch can 'probably' be cranked up to pull closer to 20 in-hg. You should have gotten some directions on adjusting the max level and the lower kick-in?

vac pumps generally do de-rate about 1 in-hg per 1000' of elevation, but max elevation in MA is only about 3500', so that probably is not the issue

As long as the hoses are not overly soft and flexing and collapsing, the length wouldn't really matter. as long as the check valve is in the right place, and system is not leaking, the the only thing affecting that first braking event is the max vaccuum set by the switch and pump capability.

I had to mess w/ mine a bit to set it 'almost' as high as I could without having it run too long trying to pull down the last little bit. I am at 7500', so I only get about -13in-Hg in a reasonable runtime. I tried for 15 and it just took a LOT longer to finish that last little bit.


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## TheSGC (Nov 15, 2007)

When I tested the vacuum I removed my switch from the setup so the pump wouldn't shutdown, just keep going and that's when it topped off at 15 Hg. I will play with it this afternoon, but I found my first pump was fine, which brought the reading down to 10Hg, then the second braking not so great, but the vacuum pump wouldn't come on until after the third braking event, so I am going to try and get my switch adjusted to come up after each braking event. I know it's a little noisy, but I need to stop these sucker!


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## dtbaker (Jan 5, 2008)

TheSGC said:


> When I tested the vacuum I removed my switch from the setup so the pump wouldn't shutdown, just keep going and that's when it topped off at 15 Hg. I will play with it this afternoon, but I found my first pump was fine, which brought the reading down to 10Hg, then the second braking not so great, but the vacuum pump wouldn't come on until after the third braking event, so I am going to try and get my switch adjusted to come up after each braking event. I know it's a little noisy, but I need to stop these sucker!


there are two settings on the switch... one for the trigger on, and one for off. you set the 'off' to just below where it takes a while to finish. you could set your 'on' close to that to come one after one or two pumps... I think its fine at three.

part of the learning curve is as you are sitting at a stop, before you take off, give the brakes a couple taps till the vc comes on and you will be at full vac prior to next stop.


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## TheSGC (Nov 15, 2007)

dtbaker said:


> there are two settings on the switch... one for the trigger on, and one for off. you set the 'off' to just below where it takes a while to finish. you could set your 'on' close to that to come one after one or two pumps... I think its fine at three.
> 
> part of the learning curve is as you are sitting at a stop, before you take off, give the brakes a couple taps till the vc comes on and you will be at full vac prior to next stop.


Oh, there are two adjustment screws? I have the MPL vacuum switch, and I have only been able to find one large adjustment. I'll have to look again.

I usually ease into most of my stops so normally it's not a problem, but on occasion I demand locked wheels. I actually never noticed the brake problem until I decided to slam on them doing 25 MPH just to see what would happen. It was weird, I had the brakes to the floor, the EV slowed and the vacuum depleted, then the pump kicked back in and locked up the wheels.


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## dtbaker (Jan 5, 2008)

TheSGC said:


> Oh, there are two adjustment screws? I have the MPL vacuum switch, and I have only been able to find one large adjustment. I'll have to look again.


I dunno about MPL; I have a D&D switch that has two screws, one for setting max, other for range. Do you have documentation?



TheSGC said:


> I usually ease into most of my stops so normally it's not a problem, but on occasion I demand locked wheels.


I hope 'locked' is not the goal.  But do keep in mind that traditional non-ABS 'pumping' is probably not a great idea as it will deplete vaccuum assist. A panic stop is a panic stop, and the car will be slower to stop with extra weight for sure.


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## TheSGC (Nov 15, 2007)

I have tweaked my vacuum switch a little so it comes on after one pump of the brakes, but it still tops off at 15 Hg. My EV feels like a tank, which is awesome, but I may upgrade to one of those "quiet" vacuum pumps that have ratings of 22.5 Hg. I don't need a kit since I have everything else, just the pump, which in it's own right is pretty expensive.

I drove 20 miles yesterday to break in my new batteries and I started to teach myself to make sure the pump comes on after each braking event, even if I have to pump the brakes a little to get it going.


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## JRP3 (Mar 7, 2008)

15 inHg should be plenty, I run my system right around there, maybe a bit higher, and have plenty of vacuum. Are you sure there are no leaks in the system? Maybe you need more reservoir capacity.


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## TheSGC (Nov 15, 2007)

JRP3 said:


> 15 inHg should be plenty, I run my system right around there, maybe a bit higher, and have plenty of vacuum. Are you sure there are no leaks in the system? Maybe you need more reservoir capacity.


There is a slight leak, but it takes about 30 minutes to get low enough for the pump to kick back on from just sitting there. 

Here is a pic of my reservoir (it's the big white PVC tube): http://4.bp.blogspot.com/_ighXUBbIRfk/ShXAwW6nkuI/AAAAAAAAAPw/L7p2xugEfVE/s1600-h/S5001741.JPG

One thing I might try is connecting my vacuum gauge straight to the pump and see how much vacuum is at the source. I finally found docs on my pump, and it's rated for 23 Hg continuous. My leak may be large enough to keep the vacuum from getting past 15 Hg.


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## dtbaker (Jan 5, 2008)

TheSGC said:


> There is a slight leak, but it takes about 30 minutes to get low enough for the pump to kick back on from just sitting there.


I would track that down before buying a new pump! Make sure your check valve is BETWEEN your pump and tank as there is always leak-back thru pump exhaust.

I operate with 13in max, and set so I get 2 or three pumps before 10in when the pump kicks back on... vac stays constant for days without ever coming on ...


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## TheSGC (Nov 15, 2007)

dtbaker said:


> I would track that down before buying a new pump! Make sure your check valve is BETWEEN your pump and tank as there is always leak-back thru pump exhaust.
> 
> I operate with 13in max, and set so I get 2 or three pumps before 10in when the pump kicks back on... vac stays constant for days without ever coming on ...


HHmmm check valve... whoops, dont have one of those.


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## dtbaker (Jan 5, 2008)

TheSGC said:


> HHmmm check valve... whoops, dont have one of those.



bingo.....


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## TheSGC (Nov 15, 2007)

dtbaker said:


> bingo.....


I will visit the hardware store tomorrow, in my EV. I am also going to bypass my reservoir setup and hook my gauge right up to the pump and see if it will pull 23" Hg.


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## JRP3 (Mar 7, 2008)

Just so you're aware, I went through 2 cheap check valves that wouldn't seal my system before getting this one that worked: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=180490054758


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## TheSGC (Nov 15, 2007)

JRP3 said:


> Just so you're aware, I went through 2 cheap check valves that wouldn't seal my system before getting this one that worked: http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=180490054758


After checking out the local hardware stores, I am going with that one from eBay. It's inexpensive and should do the job just fine.


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