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  #1 (permalink)  
Old 10-02-2010, 03:39 AM
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whats up with this booster

dont know If anybody can help i have just upgraded my braking system, i have got 1970 gto front disc & calipers rear gm discs & calipers new brake lines & a 7" chrome dual booster & chrome master cylinder, & a vacuum pump with resovior(due to the cam) i have bleed this sytem till my leg felt like it was about to fall off the brake pedal is fine while on tick over but as the rpm goes up the pedal slowly sinks to the floor which means when im driving down the road i have to knock it into n before braking i have tried a diffrent master just to make sure the new one wasnt duff but still the same i am now using a single painted 8" booster that works fine but i would like to use the chrome one.
ANY IDEA'S SO I CAN TRY AGAIN.

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Old 10-02-2010, 07:49 AM
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You now have it working with the 8in booster, does not work with the 7 in booster .... Just a guess but I think I would check the booster, reading your post common sense tells me this . Obviously you know how to install a booster and make the pedal too booster adjustment . Also if the pump is the source of the vacuum, engine rpm has nothing to with it.

I also run an electric vac pump, I did an experiment once using a reservoir. I was trying to cut the cycle time of the pump, I found no difference with the systems operation with or without the reservoir, so I concluded that they are a waste of time, money and clutter the engine bay. I must qualify the statement and that a power assist brake is the only accessory plumbed.

Last edited by pepi; 10-02-2010 at 08:11 AM.
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Old 10-02-2010, 09:15 AM
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hi
sorry the pedal did this when we had it connect to the engine it also did the same thing as the vacuum went up from the pump.

yep the booster or power assist is the only thing plumbed into the brakes the resovior is an old fire extinguisher under the floor 4" in diameter 20" long
fitted this to try & keep some vacuum but as you say hasent worked i have tried plumbing booster in to carb & still had same effect the pedals fine on tickover can sit with my foot on the brake for ages but as soon as the rpms start to climb it starts to sink to the floor very slowly it is as if the second diaphragm is collapsing over the first one. so basiclly when you push the pedal feels normal go's to the end of the travel in cylinder which doe's apply the brakes. but it then starts to sink very slowly to the floor as the rpms start to rise there is no hissing or air leak noises coming from it.

Last edited by mr zephyr; 10-02-2010 at 09:20 AM. Reason: adding
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Old 10-02-2010, 05:36 PM
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I do not understand what the problem, is .... does the pedal go ail the way to the floor and you loose the brakes? Your post just sounds like you experience the peddle going down a little further after you first apply the brakes. Does it keep going down until you have no pedal is that what you are talking about? I am not sure you have a problem or just splitting hairs .

To test a booster, keep the vac pump off and pump the pedal until the vac is completely exhausted, then put your foot on the pedal hold pressure and turn on the pump the pedal should go down some that tells you the booster is working. Does your vac pump cut off when the system has vacuum, or is it a continuous run type?

The question is do you have brakes that you can drive with, stops the car and holds it stopped ? When the brakes are applied the booster will vacate the vacuum and the pump then must make up for that maybe that is what you are feeling. This "tip" phrase is off the planet not a clue what you are saying.

I am out of guesses and if you do indeed have a working brake system; stops the car and holds it stopped. get out a drive its working.
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Old 10-03-2010, 05:25 AM
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hi pepi
im sorry im not very good at explaining this so 1 more try. forget the engine i have connected the vacuum pump to the brake light switch so it comes on when the brakes are applied. i have also fitted an on off toggle switch in line to switch of the pump when the vacuum reaches 25. i tried exhausting all the vacuum so that the brake pedal was at the very top & hard to push i then turned on the vacuum pump & the pedal drops slightly as it should but as the vacuum bullds up the pedal sinks all the way to the floor releasing the brakes it would be nice to drive this but it only slows me down allowing me to stop eventually. sorry not splitting hairs just trying to sort out a problem. thanks for your help
chris
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Old 10-03-2010, 05:57 AM
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Both boosters doing this ? If so it now sounds like you have other problems, hard to believe the same problem occurs on two different parts .
I question the way you have the pump wired, you really need that vacuum there before you apply the brake, as if the engine were the supply.
I would correct that pronto, you want vacuum ready before you need it, it very well could solve the problem. Thinking more I would do that first.

I have run an electric vac pump for 10 years, the only change I made is I went to a pump that auto stopped when the vac was reached, because I did not like the pump constantly running when not needed..

Change the wiring of the pump; should be on when the ignition is on. I will keep watching and try to answer your questions until I run out of ideas .. I am smart enough to know what I do not know; so I won't be pulling ideas out of thin air.


I am enclosing a link that you should be able to get some answers from and directions to follow to troubleshoot this; but again rewire that pump first before you go any further ,

http://www.mpbrakes.com/technical-su...leshooting.cfm
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Old 10-03-2010, 04:08 PM
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hi have ordered a new relay & vacuum air switch from summit racing today these will take a least 2 weeks to reach here at the moment the 8" single diaphragm booster work ok but i just have to push hard on the pedal original size was 11" the reason for changing too a 7" dual diaphragm booster was to get near to the original size.& it is a chrome one & looks better on the stainless fire wall. as soon as the new parts arrive i will get back .thanks for the link i will have a good read.
thanks chris
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Old 10-03-2010, 08:45 PM
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I think I have your problem solved you say; " i have connected the vacuum pump to the brake light switch so it comes on when the brakes are applied."

Then you say; " just have to push hard on the pedal "

Here is what is happening, the hard pedal indicates no power assist; no power assist indicates no vacuum.

The vacuum pump does not have enough time to vacate the booster wired the way you have it, it needs to be wired so that it will run when the ignition switch is turned on.

The booster has vacuum when the brakes are off, when you hit the pedal the vacuum bleeds off to assist your application of the brakes. Without the vacuum you do not have power brakes, without the vacuum you are feeling the pressure it would take to manually apply the brakes.

Just because you do not have a working booster does not mean you do not have brakes, you have brakes they are just hard to apply
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Old 10-04-2010, 05:48 AM
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I agree totally w/the rewiring of the pump- you need a vacuum in the booster before you touch the pedal, there's no way the vacuum pump will be able to evacuate the booster fast enough the way it now is.

I would also do what Pepi mentioned- wire in a cut-out switch so the pump isn't constantly running.
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Old 10-04-2010, 08:17 AM
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The cut out switch is actually a pressure cut off switch that is plumped into the line,between the booster and pump. Takes all the guess work out of it.

http://www.mpbrakes.com/products/pro...product_id=130

notice the brass fitting to the right on the head of the pump.

And oh yes that pedal going down towards the floor ..... happens because the vac pump is evacuating the system, just like described when trouble shooting the boosters operation.

You will be able to use the chrome booster just fine, correct the pump wiring and you will be golden.
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Old 10-04-2010, 10:22 AM
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as soon as the parts arrive i will be out in the glorious english weather (rain)& giving it a try many thanks for all your help.
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