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Old 11-28-2004, 04:55 PM   #1
Legacy777
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Default Ever Wanted to see the inside of your brake booster?

I've been meaning to do this for a while, and finally got around to it. The booster wasn't too bad to take apart. It had bent in metal tabs on the outer shell that kept everything together. I just flatened them, and started taking apart the booster.

Here's the pics. I have labeled things, so hopefully it'll help in knowing what is what.

http://www.main.experiencetherave.co.../brakebooster/

Here are scans that show the booster's components and its operation.

http://www.main.experiencetherave.co...kebooster1.jpg
http://www.main.experiencetherave.co...kebooster2.jpg
http://www.main.experiencetherave.co...kebooster3.jpg
http://www.main.experiencetherave.co...kebooster4.jpg


Now here's a couple things I learned from this whole process. The claim about a "dual stage" booster is complete and utter rubbish. Both diaphragms are fixed to the same plastic body and move as one unit. The poor pedal feedback and/or "dead pedal feel" at the top of the stroke is caused by having too much vacuum assist, and/or the design of the booster. The single diaphragm booster eliminates the "dead pedal feel" and provides much better pedal feel.

There is slop in the booster setup, i.e. the operating rod does travel about an 1/8" before actually coming in contact with the reaction disc, which by the way is made out of rubber. I believe this is done though because the poppet valve seals off the passages between the diaphragm & atmospheric chambers when the operating rod is depressed slightly.

If you take a look at the scans and the pictures I took it will hopefully make more sense. I don't claim to 100% understand exactly how the valve body is setup, but I think I have a good grasp at how everything is supposed to work.

Josh
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Old 11-28-2004, 06:54 PM   #2
duncangrant
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I did a similar exercise on a previous car with a single stage servo in an attempt to eliminate the mushy feel. Here's what I found -

The mushy feel is caused by too much assist for the initial pedal motion.

The reaction disc is made out of rubber not to form a seal but to act like a spring to allow the plunger valve to move relative to the valve body depending on the pedal pressure. Its this relative movement that controls the amount of assist. To understand think of it in stages:

* you press the pedal and that pushes the plunger valve into the soft rubber reaction disc.

* that allows the valve to open in relation to the applied pressure

* the pressure differential accross the diaphram pushes the valve body forward against the reaction disc so giving a boost to the braking pressure and reducing the relative shift between the plunger and body, hence reducing the valve opening and assistance until there is equilibrium. This is the negative feedback that all servo systems have.

If you look at the plunger valve in the valve body in this pic http://www.main.experiencetherave.co...r/DCP_3692.JPG you will see that the plunger valve is recessed relative to the valve body. That allows the valve to open for very little pedal effort so giving the mushy feel.

To rectify this I shimmed the plunger valve by 20-30 thou simply by cutting a piece of feeler guage roughly to a circle a bit smaller than the plunger diameter and then supergluing it to the centre of the reaction disc (you will see an imprint of the plunger at the centre of the disc).

It worked very well indeed! I could at last feel when the pads were making first contact on the discs and gave more pedal weight but still provided full servo assist when I wanted it.


BTW I wouldn't right-off the dual stage booster as rubbish. The design of these servos is very elegant and subtle. It took me a while to understand the single stage one. I think its likely that there is something going on that you don't yet understand - but keep at it.
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Old 11-28-2004, 09:41 PM   #3
Legacy777
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Just curious, what car did you do this on?

Yeah I'm aware the rubber reaction disc does not form any type of seal and is made out of rubber to act as a "cushion"

From what I can see from the FSM & from taking it apart, the plunger valve is either open or close, it does not vary with pedal pressure. It takes a very small amount of pressure to close in comparison to moving the diaphragms against the return spring. This initial motion closes off the passageways in the valve body between the atmospheric & diaphragm chambers.

To be quite honest the plunger valve (or what the FSM calls the plunger valve) just looks like a metal piece that's attached to the ball on the operating rod. The only sealing portion against the valve body is the poppet valve as seen in this pic
http://www.main.experiencetherave.co...r/DCP_3708.JPG

The plunger movement forward is limited by the key, which makes contact on the valve body, and moves it forward with the assist due to the pressure differential.

The shim idea is a pretty good one. That sort of popped in my mind as I was taking it apart.

I've looked over everything and I don't see how the valve body can be partially open/close and vary assist. It just doesn't work that way. It's possibe the booster you took apart was different, which is why I asked what vehicle it was from.

I'm not overly concerned with modifying the booster. I've got the single diaphragm booster on my car now and like it. I did this mainly to understand.
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Old 11-28-2004, 09:53 PM   #4
scoobdude
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so...any ideas on how to get a better pedal feel without taking the booster apart. Maybe limiting vacum or more pressure on the rod as to elimant the "play" in the rubber cushion.
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Old 11-29-2004, 06:41 AM   #5
duncangrant
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The ones I modified were Valeo servos fitted to Peugeots. The operating principles are similar.

Trust me the valve is not simply on/off, if it were it would be unusable. It will go from off to on over a very small distance though. One of my first experiments with this was to replace the reaction disc with a ring which allowed free movement of the plunger. That gave an on/off response - in servo terms the -ve feedback was disabled - and provided an extremely unpleasant driving experience!

On the second one I altered I was able to withdraw the push-rod from the master cylinder end of the servo without dismantling the servo. The push-rods were held in with a simple push-in jaggy spring washer type clip. By wrecking the clip with a long shart pinted screw-driver it was easily withdrawn.

Unfortunately looking at how the push-rod is retained in the Subaru servo http://www.main.experiencetherave.co...r/DCP_3686.JPG I don't see how it could be removed without dismantling the unit.

Yes you could reduce assist by limiting vacuum. I havn't done it though so I can't tell you how. That wont solve the initial mushy response though.

What I have done in my current Subaru is to change the pivot points on the brake pedal to reduce the leverage, also I've shortened the pedal slightly (big feet). That has reduced the pedal travel and increased the pedal effort. Also using a medium friction brake pad (EBC Red Ceramic) and metal sheathed brake lines. All together this gives me good feel even with the dual-stage servo and gives a pedal solid enough to allow easy heel & toe (which I find near impossible with the stock soft pedal).
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Old 11-29-2004, 09:26 AM   #6
Legacy777
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I looked at things a little more closely last night, specifically the passages and how the valving works in correlation. I've better grasp now. I'll have to snap some more pics and post what I've found. Hopefully I'll have time to do so tonight.

scoobdude, as duncangrant mentioned, there really isn't anything you can do to alter the booster unless you take it apart. The best solution if you don't want to modify (cut/hack/etc) anything is to get the single diaphragm booster.
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Old 11-29-2004, 03:13 PM   #7
jlevy
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Default Nice exploratory surgery!!

I'm a little mad I didn't keep my extra booster and do the same thing. From the pics it looks like the diaphrams are mostly made of flexible material as opposed to a speaker-type setup where the middle is rigid and the outer part flexible. I've been curious about that.

We need to get together again and compare brake setups now that you have the single diaphram booster and I've got the smaller dual booster.

-JL
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Old 11-29-2004, 03:57 PM   #8
Legacy777
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Yeah, let's get together after I get the B4 rears on my car. I can bring the disassembled booster as well.
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Old 11-29-2004, 08:57 PM   #9
Legacy777
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Update will have to wait until the weekend more then likely. I found out this morning I need to be at a construction job showing in SC on wednesday, so gotta pack tonight.
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Old 11-29-2004, 10:08 PM   #10
scoobdude
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Thanks for answering my questions guys. Keep me updated and let me know when you guys put together a write up.

Joe
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Old 12-03-2004, 03:36 PM   #11
jlevy
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Default One more question...

Is booster action affected by adjusting the length of the rods. In other words, shorten the bolt on the part of the rod that goes into the m/c and lengthen the other end that attaches to the pedal. The length would be the same overall, but would the booster react differently to changes of this type?

-jlevy
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Old 12-03-2004, 03:58 PM   #12
duncangrant
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J, that might take out a small amount of dead motion but really its the relative positions, and areas, of the valve plunger and valve hub that press against the reaction disc that control the response.

The recession of the plunger controls the initial amount of assist; greater recess = more initial assist and mushier feel, while the relative areas of the hub to plunger, combined with the level of vacuum, control the degree of assist for further pedal pressure; the smaller the plunger is, compared with the hub, the greater the assist.

Josh, yhm @surrealmirage

Last edited by duncangrant; 12-04-2004 at 11:19 AM.
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Old 12-05-2004, 06:56 PM   #13
Legacy777
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Got an update on things

I've looked over the booster a little more closely, and have identified the passageways and for the most part understand how it works. With vacuum hooked up and the pedal not depressed, there is vacuum in both sides of the diaphragm plate. The poppet valve seals the valve body and doesn't let vacuum escape. When you first depress the brake pedal, the poppet valve retainer/washer, comes in contact with the stops. When this happens the poppet valve seals off the vacuum valve passage. When this occurs passages A & B become isolated from each other. Also, when the poppet valve retainer/washer hits the stops, it allows the vacuum to escape through the center portion of the shaft. You can see what I mean in this pic http://www.main.experiencetherave.co...2/DCP_3761.JPG

The only thing I haven't fully come to grips with is how the plunger valve seals itself between the vacuum and atmospheric side. There's no rubber seal, so in this pic, http://www.main.experiencetherave.co...2/DCP_3757.JPG vacuum can pass through the center hole to the atmospheric side. I had thought that the plunger valve gets sealed on the reaction disc since it's rubber, however I don't think that's the case because the plunger valve extends further forward then the surrounding valve body.

Here's some more pics
http://www.main.experiencetherave.co.../brakebooster2

I wish there was a way to test out air flow through some of the passages. It might help in understanding things more.

As for modifying the booster to reduce assist. I don't really see a way. As Duncan mentioned, reducing the spacing between the plunger valve and reaction disc might eliminate some, but for most people that's not really an option. Best solution....get the single diaphragm booster.

BTW Ducan sent me this scan which looks more like the booster I took apart. It's got more detail.
http://www.main.experiencetherave.co...ke_booster.jpg

Pdf version
http://www.main.experiencetherave.co...ke_booster.pdf

Last edited by Legacy777; 12-06-2004 at 01:14 PM. Reason: added pdf booster diagram
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Old 12-06-2004, 08:37 AM   #14
duncangrant
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I guess I was wrong when I said that the reaction disc didn't act as seal. When the brakes are applied the hub will be pressed against the disc, so providing a seal. When at rest it won't be but then a seal isnt needed since the poppet valve does the sealing. Although the plunger can protrude beyond the level of the hub (with brakes on) that doesnt mean it breaks the seal - it will just push into the rubber.

You've got to admire the design of these things - why have a bit of rubber do one job when it can do two?

Josh the pdf I sent for the US-STi has the drawing in a vector format so it can be zoomed-in to gain resolution. The jpeg snapshot you have taken has limited resolution. Maybe you could stick the pdf on your site along with the jpegs?

It looks like your dismantled booster is slightly different from both the old and the new drawings. I expect like every other Subaru part there is a process of continual evolution going on.

It looks like you were right about the '2-stage' being incorrect. The manuals describe it as tandem and by that and from the drawings and your findings I take that to mean 2 diaphrams and chambers acting as one but not a low / high assist 2-stage booster. I wonder where this 2-stage myth came from?

Good work Josh!

If I've time over Christmas I'll try shimming the plunger on my tandem booster. Although its not bad a bit more feel would be nice. After dismantling I think I can modify it so that the push-rod will come out through the master cylinder hole, simplifying any further tweaking.
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Old 12-06-2004, 09:39 AM   #15
Legacy777
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When I was first looking at it, I thought the reaction disc sealed the front portion of the valve body. The only reason I doubt myself on it, is that the metal collar around the reaction disc makes contact with the valve body first. When this contact is made the reaction disc still has a little slop between the valve body. However that must be taken up once the valve body is depressed against the spring.

I was just thinking that when the booster is together, there is no slop in the pushrod, so it has to be taken up some where, and the reaction disc most likely is acting as a sealing surface.

Yeah I noticed a slightly different shape on the booster that came out of my 1990 legacy vs this one that came out of the turbo legacy. My 1990 legacy booster had a little protruding round bulge on the one side of the metal casing. So yeah, I'm sure they're continuing to tweak them.

I think the 2-stage myth came about partly from Mike Shields at spd tuning
http://spdusa.com/soft.htm He knows his stuff, but I don't know if he's ever looked at the drawings, or taken a booster apart. To make a "2 stage" booster, it would require a lot more intricate passageways and complexity to the booster, and wouldn't really provide too much of a benefit IMO.

Thanks Duncan, I've been wanting to take one of these apart for a while and see how it works. I feel like mythbusters...... "2-Stage brake booster myth" BUSTED

Yeah I can throw the pdf on my site as well. I'll post the link when I get it up there.

Let us know what you come up with when you take apart your booster.

Josh
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