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Proportioning valve changes on rear disc swap?


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15 replies to this topic

#1 freedster

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Posted 07 August 2005 - 11:33 AM

Talked to my mechanic and he was concerned about whether or not there were any proportioning valve changes needed when I swapped to rear discs on my 87 GL hatch. He said that I likely wouldn't get much better braking with the discs because the line pressure would be drum brake line pressure, and wouldn't be correctly suited to the application.

I've asked about the rest of the system, not the proportioning valve, so he might have a point, I guess. Looks to me like the existing proportioning valve is a non-adjustable unit, and a cursory check at my Chilton manual seems to indicate that the units are the same across both disc and drum rear brake models. Do I have to switch to a disc brake proporting valve, or can I just leave the rear drum one there unchanged? Would there be any advantage to going to an aftermarket unit?

Thanks.

- Freed

#2 beauregaardhooligan

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Posted 07 August 2005 - 12:02 PM

Do I have to switch to a disc brake proporting valve,
No
or can I just leave the rear drum one there unchanged?
Yes
Would there be any advantage to going to an aftermarket unit?
No

If you scroll down and look under "Similar threads" you'll find a thread about just that.
The search function here works well also.

#3 edrach

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Posted 07 August 2005 - 12:45 PM

I'm with Ed on the previous post. I've done over a half dozen conversions and was never concerned with the proportioning issue and have never noticed a difference to be concerned with. That being said, the primary reason I convert over is the ease and symplicity of changing pads compared to changing brake shoes; also, the fact that you never have to readjust the rear brakes or any of the problems associated with rear drum brakes. Other than occasionally replacing the rear pads, I've never had a problem with any rear disk conversion I've done.

#4 Syonyk

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Posted 07 August 2005 - 01:04 PM

How big of a braking difference do you normally see with the rear disks?

I'm planning to swap mine, because I tend to run heavy (there's currently 500-800 lbs of assorted rotary engine parts in the back of my Subaru), and with the lack of power, I prefer to get a running start on merges and slow down if needed. Plus, I just prefer disk brakes. :-)

Also, is there a good place to buy stainless steel brake lines for older Subarus, or do they have to be custom made?

-=Russ=-

#5 GLCraig

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Posted 07 August 2005 - 01:31 PM

You know I've looked at my FSM several time, trying to locate a proportioning valve and I just can't find one. Maybe subaru didn't use a proportioning valve. Also if they did, wouldn't they need two since the brake circuit split is Front Left/Right rear and Right Front/Left Rear?

#6 edrach

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Posted 07 August 2005 - 02:42 PM

Frankly I don't see much of a braking effectiveness difference other than no tendency to lock up the rears. As to the brake lines, I just used the original lines from the donor car and carefully bent the excess line out of the way without kinking it.

How big of a braking difference do you normally see with the rear disks?


Also, is there a good place to buy stainless steel brake lines for older Subarus, or do they have to be custom made?

-=Russ=-



#7 VaporTrail

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Posted 07 August 2005 - 03:27 PM

I've done a few of these as well, and the braking is much better in my opinion.

the soob has a dual diagonal system.

Jerry (bratsrus) did a lot of research on the rear disk swap, and the actuating pressure for the discs is the same as used by the drums, so no need for a proportioning valve...

#8 john in KY

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Posted 07 August 2005 - 03:51 PM

Doesn't this swap also require changing the MC? I seem to recall reading somewhere one system, can't recall which, maintains a few pounds of line pressure for some reason and the other system does not.

#9 freedster

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Posted 07 August 2005 - 03:59 PM

MC is the same part number. That much I do know.


- Freed

#10 Petersubaru

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Posted 07 August 2005 - 05:23 PM

I got an older Haynes manual with a picture of it...keep in mind this is an English car... proportioning valves where not put on every subaru, it really depends in what market they ended up in,..for ex my 86 wagen does not have them and here in Canada according to the Subaru dealer there were no listings for these valves until 1992 and they were located on the fire wall ( I wanted those valves after having the car spin around on me in the snow & ice ?one of the more important safety items to have)...in the mid 80's a lot of things fell into the gray area, like seat-belts...Europe/England had 3 point belts and here they had lap belts..same issue with colapsible steering columns and it took years for the gov't to allow the volvo safety item to take place here.....enough of my rambling....

You know I've looked at my FSM several time, trying to locate a proportioning valve and I just can't find one. Maybe subaru didn't use a proportioning valve. Also if they did, wouldn't they need two since the brake circuit split is Front Left/Right rear and Right Front/Left Rear?



#11 edrach

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Posted 07 August 2005 - 05:26 PM

I've never changed the master cylinder on any swap.

Doesn't this swap also require changing the MC? I seem to recall reading somewhere one system, can't recall which, maintains a few pounds of line pressure for some reason and the other system does not.



#12 Qman

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Posted 07 August 2005 - 05:58 PM

I have never owned an EA series car that I didn't do a rear disc swap to. Subaru drum brakes suck! The braking is improved. You do not have to adjust the rear brakes any more(I hate manually adjusted brakes).
I am the second person to do this particular mod on the USMB. Jerry(BratsRus) was the first. He did alot of research on the matter. I did as well. I am not that trusting(sorry Jerry). M/cyl and proportioning valves do not need to be changed.

#13 NV Zeno

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Posted 07 August 2005 - 06:04 PM

I concur with Edrach, Qman and Jerry. The only thing I changed besides the actual brake hardware is the last portion of brake line. I managed to find the appropriate length with the correct ends then carefully bent the line for a perfect fit (I'll show ya at WCSS 7)..just remind me :rolleyes: .

Just like downtown.

#14 edrach

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Posted 07 August 2005 - 06:17 PM

(I'll show ya at WCSS 7)..just remind me :rolleyes: .

Were you sleeping through WCSS7?

#15 NV Zeno

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Posted 07 August 2005 - 11:52 PM

Were you sleeping through WCSS7?


LOL! Yeah, I realized I goofed that one when I was out at dinner...let's make that WCSS 8.

Thanks Ed :D.

#16 TahoeFerrari

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Posted 08 August 2005 - 09:37 AM

How big of a braking difference do you normally see with the rear disks?

I've done the rw disc conversion to both of my ea82's and am going to do it to my one running '82 hatch one of these days.

You most likely will not see any big difference in your around-town or even freeway driving unless you really need to brake hard. Then you will notice 1) - as already mentioned - no rear wheel brake lockup and 2) significantly greater stopping power.

Apparantly Subaru had the rw calipers designed so that they use the same input hydraulic pressure as the drum brake cylinders - very nice (expecially for those of us doing conversions!).

I also originally did the swap due to ease of maintaining the rear pads vs brake shoes, but am very glad I did. I think the rear discs saved my butt on at least one occasion.

FYI: If you pull the discs from a JY car, remember to get 1) the flex line connected to the caliper, and 2) the short hard brake line just before the flex line. With them it's a bolt on conversion. Without, you'll have to do some fabricating. This is in the conversion post but is easily overlooked - especially the small hard line.




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