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Rear disc conversion: proportioning?


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17 replies to this topic

#1 Snowman

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Posted 11 September 2004 - 06:29 PM

Okay, so I was talking to my automotive professor about doing the rear disc conversion on Roxanne now that I've got bigger tires and stuff. I explained that it's been done countless times before with pretty good results. The only thing that we're worried about is the proportionin, metering, etc. valves. Before going fourth with this conversion, I need to get ABSOLUTE proof (i.e.: identical part numbers or extensive first-hand testing) that the hydraulic system doesn't need any modification, or if it does, what modification, in order to function as if the rear discs came with the car from the factory.

Any concrete information or pointing me in the right direction to research would be great!

#2 Will Smith

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Posted 11 September 2004 - 07:49 PM

this has been the subject of very heated discussion just recently. there are adjustable proportioning valves available for aftermarket apps. just as you describe. Your prof has good reason to wonder and with comments like "the rear wheels lock up first" and "kinda squirley" makes me wonder also.

#3 NoahDL88

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Posted 11 September 2004 - 09:08 PM

It probably wouldnt' hurt to get a different proportioning valve, I've heard both ways from very respectable people. in my experience though, my rear breaks don't seem to clean very well, which would seem to indicate that the back ones arn't getting enough pressure. talk with ken and eric, the've both got the rear disks.

#4 carfreak85

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Posted 11 September 2004 - 09:25 PM

I'd also like to hear some input on this.

#5 GLCraig

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Posted 11 September 2004 - 09:48 PM

you know i'm not convienced that ea81s and ea82s even have a proportining valve. I went looking through my FSM and the section on brakes dose not mention there being a proportining valve. Also if they did have one wouldn't they really need two, since these models use crossed brake channels? I.E. left front and right rear are on one channel and right front and left rear are on the other. You would think that an FSM would mention that they had one.

#6 Turbone

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Posted 11 September 2004 - 10:11 PM

I dont think there is any difference between the EA81's and EA82's on the brakes. Theres alot of EA82 models that have drum brakes and/or disc brakes, with no change in the system as a whole. The MT's still have the HH also, but I'm not sure when the last year was. I think the ones that are having troubles or no increase in braking after the rear disc conversion should do a complete flush and bleed of their system. This should rule out any common problems with the brake setup.

#7 VaporTrail

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Posted 11 September 2004 - 10:28 PM

Jerry (Bratsrus1) did a bunch of testing on the actuating pressure of the rear drums/dics, and they were identical. he did this testing before doing the conversion on his Brat.

no proportioning valve on a ea81 for sure.

#8 bratsrus1

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Posted 11 September 2004 - 10:40 PM

Hey Snowman This is Jerry, i was the first one to do this project. If you want to do the math get a 86 and a 84 factory subaru service manuals. Bolth books have the psi that it takes to work the brakes. Look at the size of the brakes and the psi wow it is the same, thats why it works. Thanks Jerry

#9 GLCraig

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Posted 11 September 2004 - 11:24 PM

hey jerry, dose the EA81 master cylinder push enough fluid volume to maintain the required pressure when the rear disk pads are near the end of their service life?

#10 edrach

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Posted 12 September 2004 - 02:03 AM

Craig, I would guess the answer is yes. I've done the conversion on 5 cars now without problems. When I had rear drums the brake pedal would grab lower as the rear pads wore down (using more brake fluid to do so). With the rear disks this doesn't happen since the rears are now self adjusting to compensate for wear (needing less brake fluid than the drum brakes).



hey jerry, dose the EA81 master cylinder push enough fluid volume to maintain the required pressure when the rear disk pads are near the end of their service life?



#11 SubaruRichie

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Posted 12 September 2004 - 02:11 AM

Can this conversion be done on a gen 1 brat too?

#12 Qman

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Posted 12 September 2004 - 03:59 AM

And I was the second. I have done rear disc conversions to every Sube I have owned. There are not any issues with brake pressure. I have never had an instance when the rears locked up before the fronts. Squirrley is also something that has also never been experienced.

I understand your prof's concerns. However, he probably isn't too familiar with the Subes yet. Keep educating him and he'll get it sooner or later.

#13 ezapar

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Posted 12 September 2004 - 10:05 AM

The rear discs we use come from a tubo loyale body style, same type of car as your Roxxanne, Snowman.

I'll tell you this. Not "concrete" by any means, but my testimony.
Your braking will improve after you add the rear discs to your car.

You don't need a proffessor to help you do the swap, it's a take off/bolt on project. period.

#14 edrach

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Posted 12 September 2004 - 10:20 AM

You don't need a proffessor to help you do the swap, it's a take off/bolt on project. period.


The only issue I've had with the conversion is removing the backing plate on some cars can be a problem. Have plenty of rust penetrant and patience ready if you find one of those.

#15 ezapar

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Posted 12 September 2004 - 10:26 AM

The only issue I've had with the conversion is removing the backing plate on some cars can be a problem. Have plenty of rust penetrant and patience ready if you find one of those.


If you take the time to scrape off the road dirt, it comes off a lot easier. But I've wrestled with one or two of those backing plates myself. :)

#16 Snowman

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Posted 13 September 2004 - 01:07 PM

Alright, thanks guys!

Wow, that makes me really happy that our cars don't have proportioning/metering valves or any of that crap that can go wrong. And with the pressures being the same as you say, that really goes to show that Subaru did believe in the "Lego engineering" philosophy (note: not the same as and much better than GM parts-bin engineering).

I'll start looking for the parts immediately, hee hee!:slobber:

#17 MorganM

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Posted 13 September 2004 - 01:37 PM

EA82's do have a porportioning valve. It's in the rear of the vehicle, tucked up by the rear main crossmember on a bracket bolted to the unibody. That is all the 'concrete' evendince I can provide on it :)

#18 Partsman

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Posted 13 September 2004 - 02:23 PM

On a previous posting of this question, I called my local dealer to ask about the proportioning valve. At least for the 1988 model year, there is only one proportioning valve no matter whether you have disc or drum rear. Any other questions, check this thread:

http://www.ultimates...ortioning valve




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