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 Post subject: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Thu Jul 28, 2016 10:16 pm 
Fifth Gear
Joined: Mon Nov 06, 2006 3:41 am
Posts: 8352
Location: Tenino, WA
Our cars have a dual diaphragm brake booster which creates a noticeable "two stage" brake pedal feel that is undesirable to some. You can distinctly feel two levels of firmness in the brake pedal as it travels through it's range of motion. The first stage is spongy and soft which makes bleeding your brakes a pain because it always feels like you've got air in the system still. The second stage is where you can feel the pedal stiffen up quite a bit. Changing out your brake booster for a single diaphragm booster will fix this problem by make the whole pedal travel one linear movement. It enhances the pedal feel a lot and I really like my brake pedal feel now.

The booster is hard to find and I was lucky enough to find a very old for sale thread on NASIOC and contacted the seller and he still had the booster. You might have a hard time finding one but if you do, it should be pretty cheap. I couldn't find any concrete evidence for exactly which cars came with them, unfortunately. It seems they came on older GC imprezas but only a few of them. Some told me only cars with rear drums got them but others said they had rear drums and didn't get the single diaphragm booster. So, just scowl the web and the junkyards til you find one. They're easy to spot because they much thinner front to back.

Here is the Write-Up that I went off of but I made one very time saving change when I swapped mine out that I was surprised this guy didn't think of.

http://www.northursalia.com/modificatio ... oster.html





You can see the difference in the boosters in these pictures. The first picture is the single diaphragm booster. The dual diaphragm legacy booster looks like it has two compartments. The diameter steps down about halfway through the depth of it which probably houses the other diaphragm.

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Start off by removing your master cylinder. This will get very messy. Suck out all the fluid with a turkey baster or similar tool. Then put some towels under it to catch brake fluid and remove all the hard lines from the master cylinder. You'll be left with something like this.

Image




Then, go up under the dash and remove the four nuts holding the booster to the firewall and the cotter pin holding the brake pedal to the booster rod.

Image




This is where I took one extra step that saved me a lot of hassle and time. Remove the cotter pin holding your clutch pedal to your clutch master cylinder rod.

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Climb back out of the car and undo the two nuts holding your clutch master cylinder to the firewall and pull it out of your way. Now the booster can come right out really easily and the job becomes so much more enjoyable. I started to pull on my booster and saw that it was hitting the clutch master and thought,"I wonder why that guy never removed the clutch master? Seems like the only thing that its hitting." I was right.

Image
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Not to sound clitche, but the rest is the opposite of removal. Just put the single diaphragm booster in the firewall and the clutch master back in the firewall, install the nuts and cotter pins and put your brake master back on and put new fluid in and bleed everything. Then go out and enjoy your awesome pedal feel!

Image




I had to contact Grimmspeed for a longer rod for my brake master cylinder brace.

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Last edited by kimokalihi on Thu Jul 28, 2016 10:35 pm, edited 1 time in total.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Thu Jul 28, 2016 10:34 pm 
Fourth Gear
Joined: Mon Aug 31, 2009 9:09 pm
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Location: Rochester, WA
Good write up, but I really dislike the pedal feel from the single diaphragm booster. I much prefer the dual diaphragm. There's a reason only the cheap cars got the single diaphragm.

To me, the single diaphragm booster hurts brake feel. It's definitely a personal preference type of change.

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Last edited by Alphius on Thu Jul 28, 2016 10:37 pm, edited 1 time in total.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Thu Jul 28, 2016 10:36 pm 
Fifth Gear
Joined: Mon Nov 06, 2006 3:41 am
Posts: 8352
Location: Tenino, WA
Interesting. I hated mine. I've only heard like opinons on it. Good to know. I suppose, if you you don't like your legacy pedal feel, try this mod out.

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"Your testes are close to your bottom but you still play with them all the time." Jeremy Clarkson


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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Fri Jul 29, 2016 3:28 am 
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Yeah Kimo, I have this in my SS and I hate the pedal feel now. WRX fronts and H6 rears with aggressive pads. I really wish I had driven one before I did the swap.

I don't have abs but the abs pump is still installed (and properly bled!). Might be a factor.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Fri Jul 29, 2016 1:57 pm 
Fifth Gear
Joined: Fri Jun 20, 2008 8:17 am
Posts: 6287
Location: Lakewood, Wa 98439
Great information Kimo.

I think I'm on the fence with the single diaphragm upgrade but I might have to try some better parts/pads before I decide to remove my single and go back to the double. It seems like my car stopped much better in the past compared to how it did recently.

What did Grimmspeed charge you for the extended rod?

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Fri Jul 29, 2016 4:41 pm 
Fifth Gear
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Location: Tenino, WA
Something ridiculous like $12 plus shipping. Mike, I've got a dual diaphragm booster I'll trade you.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Tue Aug 02, 2016 6:20 pm 
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Kimo,

Nice write-up again!

I still don't care for the term "single" or "dual" stage feel. There is no staging or difference in how the booster operates. The difference in feel is due to the amount of brake assist, and the diaphragms are one connected piece. Here is a thread I posted on NASIOC many years ago with pictures and labeling of the dual diaphragm booster.

http://forums.nasioc.com/forums/showthread.php?t=672929


Gabe & Mike,

What don't you like about the single diaphragm booster? I agree the single diaphragm booster has a much more linear and short pedal feel which can make the brakes a bit more touchy. I think my 97 Impreza dual diaphragm booster/brakes feels better than my dual diaphragm booster/brake setup that was on my Legacy. But it has been a while since I had the dual diaphragm booster on my Legacy so I may have forgotten what it felt like.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Tue Aug 02, 2016 6:28 pm 
Fifth Gear
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Location: Lakewood, Wa 98439
I personally feel like the brakes don't work as good now with the smaller booster vs how they worked with thicker booster.

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92 SS Prefaced, GD dash swapped, 22T/205 Hybrid 20 psi - BEAST!
93 SS Bone Stock Gone!
94 TW Bone Stock Gone!
91 SS 4EAT Sold!
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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Tue Aug 02, 2016 6:33 pm 
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What doesn't work as well as before? Does the car not stop in as short a distance, does the pedal feel too stiff, what don't you like?

I'm not trying to be an ass, but your statement above is opinion and really doesn't give any information to support your claim.

I've learned this a long time ago after many conversations on NASIOC; brake feel is a completely subjective thing and can be difficult to measure. About the only thing you can do is measure pedal input force and the resultant stopping distance. But that doesn't really tell you anything about how the driver interacts with the brakes and provides confidence in the car's ability to stop.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Tue Aug 02, 2016 7:17 pm 
Fifth Gear
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In my case, it doesn't seem like there is as much clamping force on the pedals, but I could also have an issue somewhere else. I'm having to push too hard to really get some stopping power.
I had planned to swap the boosters out before I had my engine issue.

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91 L-TW Wagon with a full Swap -RIP
92 SS Prefaced, GD dash swapped, 22T/205 Hybrid 20 psi - BEAST!
93 SS Bone Stock Gone!
94 TW Bone Stock Gone!
91 SS 4EAT Sold!
98 LGT 4EAT
98 LGT Wagon 4EAT


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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Tue Aug 02, 2016 8:16 pm 
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You'll have to drive my setup Josh. I was hoping for that linear pedal feel you are describing. The reality is more pedal play, and brakes that take probably 50-100% more effort to stop than the original booster. Whether this is "normal" for the new booster or not, it is not confidence inspiring. I really hate the overboosted feel of modern cars, and having driven many 90s Subarus, I like the dual diaphragm stock boosters. Next time the engine comes out I'll swap the booster back to the original.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 12:41 am 
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CJ & Mike,

It really sounds like something isn't right with your brake setup. My brake setup is the WRX 2-pot fronts & LGT vented rears with SS braided lines and the braking effort is not that drastically different than my Impreza in terms of how much effort I have to use to stop the car.

It's a shame you're all the way up there and can't feel how mine is setup. If we have time to meet up next time I'm up there I can drive your cars and see how it compares.

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1997 Impreza OBS

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 12:44 am 
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kimokalihi wrote:
This is where I took one extra step that saved me a lot of hassle and time. Remove the cotter pin holding your clutch pedal to your clutch master cylinder rod.

Image



Kimo,

I forgot to mention this earlier, but if you are running the stock brake lines it may be more difficult to remove the clutch master cylinder. I believe on the 91 MY's it was a hard line from the clutch MC. I'm sure you can move things a little bit like you can on the brake lines but it may not be as movable like in your setup with the aftermarket clutch line.

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 12:48 am 
Fifth Gear
Joined: Fri Jun 20, 2008 8:17 am
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Location: Lakewood, Wa 98439
Well, you COULD bring your Legacy up and let us drive it.... :-D

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92 SS Prefaced, GD dash swapped, 22T/205 Hybrid 20 psi - BEAST!
93 SS Bone Stock Gone!
94 TW Bone Stock Gone!
91 SS 4EAT Sold!
98 LGT 4EAT
98 LGT Wagon 4EAT


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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 2:32 am 
Fourth Gear
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Location: Rochester, WA
Josh, I totally agree with your terminology point and the fact that brake "feel" is very subjective.

I find I have difficulty quantifying the reason I like the dual diaphragm booster better, but I'll try.
The dual diaphragm booster seems to result in a more communicative pedal. It is easy to modulate. The single diaphragm always seems to require too much pedal effort (and somewhat more travel), and as a result I lose a lot of fine control over braking so I always end up having to adjust my braking force after initial application. It doesn't communicate what the brakes are doing in a way that enables me to brake consistently. For me, it's more difficult to modulate a firm press than it is to modulate a light press on a very responsive pedal.

I've also never driven the same car with each booster back to back, so it may indeed have something to do with the brakes on each individual car. I've driven 3 or 4 single diaphragm cars now, and I don't like it any time I do.

I think it has something to do with growing up driving old GM products. My '84 Z28 is similar: low pedal pressure and travel, large brakes, very communicative and easy to modulate.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 4:45 am 
Fifth Gear
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cj91legss wrote:
In my case, it doesn't seem like there is as much clamping force on the pedals, but I could also have an issue somewhere else. I'm having to push too hard to really get some stopping power.
I had planned to swap the boosters out before I had my engine issue.


What pads are you using? Maybe the pads don't have a good bite? We're they bedded in properly when new? Are the rotors glazed at all?

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"Your testes are close to your bottom but you still play with them all the time." Jeremy Clarkson


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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 4:51 am 
Fifth Gear
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Location: Tenino, WA
Legacy777 wrote:

Kimo,

I forgot to mention this earlier, but if you are running the stock brake lines it may be more difficult to remove the clutch master cylinder. I believe on the 91 MY's it was a hard line from the clutch MC. I'm sure you can move things a little bit like you can on the brake lines but it may not be as movable like in your setup with the aftermarket clutch line.



I forget my clutch master setup is different than stock. No clutch damper in the way and the aftermarket braided clutch line.

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91 SS EJ20G Engine/Tranny/Diff Swap Build Thread Here
"Your testes are close to your bottom but you still play with them all the time." Jeremy Clarkson


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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 5:40 pm 
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cj91legss wrote:
Well, you COULD bring your Legacy up and let us drive it.... :-D


I hope that's a possibility some day, but unfortunately isn't something that's on the immediate radar.

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 5:50 pm 
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Alphius wrote:
Josh, I totally agree with your terminology point and the fact that brake "feel" is very subjective.

I find I have difficulty quantifying the reason I like the dual diaphragm booster better, but I'll try.
The dual diaphragm booster seems to result in a more communicative pedal. It is easy to modulate. The single diaphragm always seems to require too much pedal effort (and somewhat more travel), and as a result I lose a lot of fine control over braking so I always end up having to adjust my braking force after initial application. It doesn't communicate what the brakes are doing in a way that enables me to brake consistently. For me, it's more difficult to modulate a firm press than it is to modulate a light press on a very responsive pedal.

I've also never driven the same car with each booster back to back, so it may indeed have something to do with the brakes on each individual car. I've driven 3 or 4 single diaphragm cars now, and I don't like it any time I do.

I think it has something to do with growing up driving old GM products. My '84 Z28 is similar: low pedal pressure and travel, large brakes, very communicative and easy to modulate.



Gabe, those are interesting observations/description of the pedal feel between the two boosters.

Under normal braking what booster feels like there is more pedal travel?

Is the pedal effort between the two boosters linear throughout the pedal stroke or does it feel like it changes?

On the cars you've driven with the single diaphragm booster, what were the brake setups like? Were they all disc brakes? Did they have upgraded brake pads, SS braided lines, were the calipers or MC changed out from stock, and did they have ABS?

Now I'm really curious if there's something different with my brake setup or if it's purely a personal preference....

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Wed Aug 03, 2016 7:22 pm 
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I have a similar brake setup to you Josh. 04 Wrx fronts with akebono street pads, h6 rears with parts store pads. Stock SS brake master cylinder, but the standard rubber brake lines.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Thu Aug 04, 2016 6:13 am 
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My setup is 05 lgt calipers and stoptech slotted rotors with stoptech street pads all around. Stainless braided lines. No ABS with non-ABS brake master cylinder. The single diaphragm booster (of course) and a grimmspeed master cylinder brace.

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Fri Aug 05, 2016 12:06 am 
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I don't know if the rubber brake lines, pads, or even ABS play that much into things. We'd just have to drive and see I guess.

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Fri Aug 05, 2016 2:21 am 
Fifth Gear
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Location: Tenino, WA
I think the stainless brake lines thing is a myth/placebo/marketing gimmick. I upgraded mine and felt no difference. ABS probably makes no difference in pedal travel or effort until it actually sees wheel stoppage and engages. Pads made a huge difference in performance for me but pedal travel, no difference. Effort might be different because they have a more/less aggressive bite than other pads. Cj, we will have to drive each other's cars and see how the brakes feel

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91 SS EJ20G Engine/Tranny/Diff Swap Build Thread Here
"Your testes are close to your bottom but you still play with them all the time." Jeremy Clarkson


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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Fri Aug 05, 2016 5:37 pm 
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I could tell a difference between the stock lines and ss-braided lines, albeit a small one.

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Single Diaphragm Brake Booster Upgrade Write-Up
PostPosted: Fri Aug 05, 2016 5:47 pm 
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I felt a change when I went stainless all around....but I did have a hole in my Rt rear rubber, so it's kinda null and void point.

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