mark kibort

Here is a post that basically describes how the brake vacuum circuit in relation to the aux vacuum accumulator, work. And how they work on separate circuits with no assist from eather one. so, it basically shows that the vacumm accumilator is not needed.

I posted a question with some assumptions and tests i did, to see what some of the experts thought!

what do you think?

MK


JKF says:

"Both are segregated from each other with on way valves. The one way
valve works by opening at a spring loaded threshhold when enough vacuum
pulls on it from the intake side to flow air contained within either the
booster or the reservoir (accumulator) into the intake. But the thing
they can't do is feed each other so there cannot be a cumulative effect.
So unless someone took the check valves out and teed the accumulator and
booster together and put a one way valve in the tee towards the source
(intake) then you couldn't get a net better effect in the booster by
adding in another volume. You wouldn't get better braking anyway because
braking is proportional to the largest vacuum value achieved as opposed
to amount of stored low pressure in the accumulators. Stored volume is
proportional to how many times you can push the pedal before you run out
of stored vacuum. "

jfk
79

> -----Original Message-----
> From: mark k [mailto:mkibort@compuserve.com]
> Sent: Wednesday, June 29, 2005 5:12 PM
> To: 928
> Subject: [928] Brake Vacuum Booster question (once and for all)
>
>
> Folks, we are having a debate on the other list about the
> brake vacuum booster. as far as i can tell and test, the
> vacuum booster draws vacuum from the intake at all times
> other than full throttle. then, the vacuum is stored in the
> brake booster via a one way check valve. yet another 1 way
> check valve (little blue and white valve comes off the brake
> booster and leads to the aux. vacuum booster tank, in the
> driver side fender, used for cruise control and flappy on the
> S4s and later.
>
> the question is, whether the aux. vacuum reservoir can help
> brake booster vacuum in racing or otherwise conditions. I
> say absolutely NO, but there
> are those in disagreement. The reason i disagree, is that
> the aux vacuum
> tank is fed through a one way valve. meaning, it can pull air
> out of the aux tank to make vacuum equal to that of the brake
> booster, but if the brake booster was bled down, by repeated
> brake pumps, the aux vacuum tank couldnt provide vacuum or
> help it, because air CANNOT go from brake booster to vacuum
> tank due to the one way valve.
>
>Some have said their brakes were bettter with the
> vacuum tank connected. Im thinking its imagined or when the tank is
> removed from the circuit, maybe they had a leak on that hose leading to it, when not connected or plugged correctly, and it bled down the brake booster and gave a bad pedal early.
>
> ive had some leaks in my vacuum lines and recently found
> flappy was not happy. anyway, after discovering that the
> vacuum can be provided for flappy operation by the brake
> booster (and it barely uses any vacuum, or air volume to
> operate) , ive disconnected the vacuum reservoir in the
> fender, and am now driving the flappy directly off the blue check valve and brakes feel the same, and WHY WOULDNT they.
> Since the vacuum
> reservoir cant make or help the brake booster due to that
> one way valve, why are folks thinking its needed??
>
> any vacuum experts out there that can comment?
>
> Several characteristics of a well functioning brake booster
> is that you usually get 1-2 more full power assisted brake
> pumps without the engine
> running. (after it is turned off) under idle, vacumm creates brake assist pretty quickly, and
> lighting fast if under decel (ie 17" Hg vacuum vs near 27"
> Hg, or nearly twice the vacuum under decel)
>
> mk
>

IcemanG17

Mark
First of all I am not an expert...but after doing some minor repairs to my vaccuum system I thought of this:

If the blue-black check valve is failed and pulls vaccum both ways, then in theory the cruise control vaccuum tank "could" be used to boost the brake booster tank "if" it ran low? "if" it got through the blue-black check valve AND the check valve at the brake booster? It might make a difference...however if both check valves (or just one) are working then it shouldn't make a difference?
Brian

mark kibort

Nahh, i understand what you are saying, but if the check valve on the brake booster was bad, there would be a huge vacuum draw to constantly be replacing vacuum every time you went near WOT. , as the one way check valve on the booster is there for that reason. ive you ever look at a vacuum gauge under idle and part throttle, the vacuum is all over the map. the one way check valve gets the vacuum to a specific level, and it pretty much stays there unless the vacuum is used up by repetitive brake pushes under WOT. (probably good for 2-3 full brake presses without the engine running at part or idle or decel.

so, if what you are saying is that both check valves have to fail, then yes, then the reservoir would be in parallel, but they also would be depleated together. but the odds of that happening is pretty nil!

MK

BrianG

Either way, JFK is correct. The volume of vacuum available wouldn't make any difference to braking effort if they were connected. The fact that they are mutually isolated by check valves puts the lie to the assertion that there is a change in performance when the Aux system is removed.