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  1. #1

    Default Master Cylinder Tech Talk

    I'm going to end up going 4-wheel disc on my Fairmont. This is a by-product of the rear end I snagged, a Mark VII rear. Rear discs wasn't a must, but I wanted to upgrade to better gears, an 8.8 and 5-lugs...this had all of that and also rear discs.

    So my question is, I know I will need a new proportioning valve for sure to be able to fine tune it. But what I want to know is, how does one know when you HAVE to upgrad a master cylinder? In my head I'd assume Master Cylinders are sized to push certain amount of fluid based on what components they have to operate. But I have no clue just because that makes sense if I'm actually right.

    Because I'm a budget build, I'd love to get by with not having to upgrade another component. But I also acknowledge that I might not actually be able to functionally operate 4 discs with something that was never designed to. What's a good source I can read up on to better wrap my head around Master Cylinder tech?

  2. #2
    FEP Power Member mcb82gt's Avatar
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    https://www.maximummotorsports.com/1979-86-C519.aspx

    poke around some on this site, there is some good tech in there somewhere. LOL
    Mike

    Now stang-less.

    88 Cougar 5.0

  3. #3

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    Haha, thanks. It's a place to start at least. That's my biggest issue right now. I can't tell how to support whether or not my combination will or won't work.

  4. #4
    FEP Power Member mcb82gt's Avatar
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    I suspect you will need to swap the MC. Even the stock one is tired and may be leaking internally. I have a turbocoupe rear in mine with a turbocoupe proportioning valve. I have TC brakes on the front also. So it made sense to throw in that proportioning valve. My brakes feel soft pedal and I'm not happy with it. I suspect my MC is shot, its the orginal one. I wasn't getting good fluid flow to the rears when bleeding them. So dunno whether its my PV or the MC?
    Mike

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    88 Cougar 5.0

  5. #5
    FEP Power Member mcb82gt's Avatar
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    https://www.maximummotorsports.com/M...tang-P667.aspx

    Says this is the MC for your swap with the bigger Lincoln brakes.
    Mike

    Now stang-less.

    88 Cougar 5.0

  6. #6
    FEP Power Member 4-barrel Mike's Avatar
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    Gut your current prop valve and plumb a manual valve into the rear brake line.

    Attachment 128109

    Mike

  7. #7

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    Quote Originally Posted by 4-barrel Mike View Post
    Gut your current prop valve and plumb a manual valve into the rear brake line.

    Attachment 128109

    Mike
    I'm planning on buying a new manual prop valve regardless, but what I need a solid answer on is whether or not I HAVE to swap my M/C out. I would suspect yes, but I want to understand best the why.

  8. #8

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    Quote Originally Posted by mcb82gt View Post
    https://www.maximummotorsports.com/M...tang-P667.aspx

    Says this is the MC for your swap with the bigger Lincoln brakes.
    So this is a good place to start...thank you.

    I plan on keeping the front calipers stock, and the rears will be the lincolns. So at the moment this appears to be the most accurate information I can have at the moment. May be another answer out there, but for now this is the best information.

  9. #9
    FEP Power Member Broncojunkie's Avatar
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    Quote Originally Posted by Str8sixfan View Post
    So this is a good place to start...thank you.

    I plan on keeping the front calipers stock, and the rears will be the lincolns. So at the moment this appears to be the most accurate information I can have at the moment. May be another answer out there, but for now this is the best information.
    I completely understand your question and I'm not an expert on this subject, by any means, but here's my understanding. You need a larger mc when swapping in larger calipers. They just have more fluid displacement with the larger piston (or dual pistons). Drum brakes require more fluid because the shoes have to move further to meet the drum surface. When switching to rear disc, a smaller percentage of the fluid from the master cylinder is required. You have to replace the proportioning valve with an adjustable one in order to reduce the amount of fluid pressure going to the rears. By this reasoning, you shouldn't have to upgrade the mc. Just gut the OEM valve and install the adjustable one into the rear line.

    Now... here's my disclaimer: I have absolutely zero experience with rear disc conversion and I haven't performed the swap because everything I read presumes a front caliper upgrade thrown in the mix. I do have sn95 front calipers on my pace car and it stops very well. Upgraded mc and booster (93 cobra). Adding rear discs, although fine for ease of pad replacement, doesn't really interest me because of the work involved with modifying the e-brake.

  10. #10
    FEP Power Member mcb82gt's Avatar
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    Its my understanding that disc brakes require more fluid than drums, hence the need for a different MC to push more fluid to the rear disc?

    Im no pro on brakes though, LOL

    So you are planning to keep the stock spindles and brakes on the front of the Fairmont, and just putting on a 5 lug rotor? Maybe you could get by with just gutting the PV and adjusting the rear bias with a aftermarket valve.

    Try it and if it doesn't work then get the MC afterward.
    Mike

    Now stang-less.

    88 Cougar 5.0

  11. #11

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    Quote Originally Posted by Broncojunkie View Post
    I completely understand your question and I'm not an expert on this subject, by any means, but here's my understanding. You need a larger mc when swapping in larger calipers. They just have more fluid displacement with the larger piston (or dual pistons). Drum brakes require more fluid because the shoes have to move further to meet the drum surface. When switching to rear disc, a smaller percentage of the fluid from the master cylinder is required. You have to replace the proportioning valve with an adjustable one in order to reduce the amount of fluid pressure going to the rears. By this reasoning, you shouldn't have to upgrade the mc. Just gut the OEM valve and install the adjustable one into the rear line.

    Now... here's my disclaimer: I have absolutely zero experience with rear disc conversion and I haven't performed the swap because everything I read presumes a front caliper upgrade thrown in the mix. I do have sn95 front calipers on my pace car and it stops very well. Upgraded mc and booster (93 cobra). Adding rear discs, although fine for ease of pad replacement, doesn't really interest me because of the work involved with modifying the e-brake.
    Haha, LOVE IT! Yes, this was the answer I was looking for. This all makes sense to me too as you describe it. Hopefully someone else out there can come in and help support what you were writing and back it up. I totally heed your disclaimer, but this is helping. I'm not sold I'm safe to keep my current MC or not, but I feel like it's helping me think about this more critically, which is what I was after. THANKS!

  12. #12

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    Quote Originally Posted by mcb82gt View Post
    Its my understanding that disc brakes require more fluid than drums, hence the need for a different MC to push more fluid to the rear disc?

    Im no pro on brakes though, LOL

    So you are planning to keep the stock spindles and brakes on the front of the Fairmont, and just putting on a 5 lug rotor? Maybe you could get by with just gutting the PV and adjusting the rear bias with a aftermarket valve.

    Try it and if it doesn't work then get the MC afterward.
    This is kind of my thought. Go for one questionable test drive at low speeds in the residential area, and if I need to do the MC upgrade, can always do that later.

  13. #13
    FEP Power Member 4-barrel Mike's Avatar
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    OK. FWIW, my McLaren (4-wheel 5-lug discs) has SVO master, booster, and prop valve , while the Fairmont
    (also 4-wheel 5-lug discs)
    has '93 Cobra master, booster, gutted prop valve with manual as pictured above.

    Mike

  14. #14
    FEP Power Member Broncojunkie's Avatar
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    I believe, if you were to install a larger mc, it wouldn't necessarily make it better or worse, as far as stopping power. All it would do is require less pedal travel than a smaller mc. This based entirely on theory and not experience lol!

  15. #15
    Moderator wraithracing's Avatar
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    You don't state what year Fairmont, so I can't guarantee the information is correct for your year model. It would appear that you most likely have a 7/8" bore M/C with 60mm front calipers.

    The Mark VII rear calipers are 54mm bore just like the SVO. The Mark VII and the SVO both use the same 73mm front calipers which are larger than the standard Mustang/Capri and most likely your Fairmont 60mm front calipers. The Mark VII and the SVO both use the 1 1/8" bore M/C to feed the larger front and rear calipers.

    The issue you have is the huge rear calipers and the smaller 60mm front calipers are not going to be matched very well to begin with. Personally if running the 54mm rear calipers, IMHO I would install the larger 73mm front calipers as well as the SVO/Mark VII 1 1/8" M/C that way the system is a bit more balanced and once you gut your Proportioning valve, install the adjustable proportioning valve in the rear line and adjust it properly. You should have a fairly decent set of brakes. Essentially the SVO brake setup on a Fairmont.

    If you choose to stay with the 60mm front calipers, more than likely the 7/8" bore will be too small and have a soft pedal feeling with poor braking performance. A 1" bore might work better, although the 1 1/8" might be too large considering the 60mm front calipers. Good Luck!
    ​Trey

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  16. #16
    FEP Super Member erratic50's Avatar
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    The 1" bore master cyl that bolts up to the foxbody lines is $15 last I looked. M1858 will get you there with 1 line adapter needed on most foxbody Mustangs. It might go right on a Fairmont -- not something I've seen documented anywhere.

    From experience you can get away with an SN95 rear-end with a stock Mustang master cylinder and stock proportioning valve. 7/8" master AFAIK. I say this because I've done it before It stopped better than the CRAPPY rear drum brakes for what its worth. Pedal feel was just as terrible as always. Improved after i bled the brakes in all 4 corners but was never a better feel than the average minivan.

    Where things greatly improved is with the 1" bore master cyl. that was exactly what the doctor ordered. I run all braided lines and high performance pads and drilled/slotted rotors in front and stock SN95 rotors and rubber lines in back. Brakes improved a ton with the adjustable valve adding more rear brake bias.

    IMO a stock foxbody could use a lot more rear brake. Biggest problem preventing that is the brake dive from the factory is terrible. Not much other than a torque arm will fully address that albeit lowering the rear of the car substantially does help.

  17. #17
    FEP Super Member Ken P's Avatar
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    If you're looking to save some bucks and don't care if the MC is cast iron there are some 70s model F-150 that have a larger piston. Also be sure to watch which side the ports come out.

    I have one in my 90LX with SVO front calipers and prop valve. Rear end was 94 GT 5 lug disc. Raced it at Road Atlanta and Talldega Grand Prix tracks and never had any fade problems.

    Also consider getting yourself a Mity Vac: https://www.eastwood.com/silverline-...yABEgIlRPD_BwE

    Even after doing a Pump / Hold / Bleed there is sometimes air left in the system causing a mushy pedal. By "Pushing" fluid backwards in through the bleeder screws you will get air bubbles being forced out.

    The Mity Vac is also good for things like vacuum testing brake boosters, or flushing steering racks, and filling trans & rear ends etc. etc.
    86 LX Coupe 4E
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  18. #18

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    Dude...Thanks everyone, this is really good info. I'll try and catch up with you all, so...

    wraithracing - I have a 78 Fairmont, 302 car. I know it's going to be unconventional having that big of a rear brake compared to the front, it really wasn't planned this way just more like that's how the cards fell
    erratic - as always, thanks for your input! If I'm reading that right, $15 I'd be dumb not to? 1" bore does seem like the sweet spot
    Ken P - thanks for the suggestion on the F-150 MC, though don't know if I'll run into the strut tower?

    All in all it sounds like 1" MC might be the sweet spot. Another friend was recommending a 93 cobra MC (which according to mcb82gt's link above 93 cobra = 1" bore).

  19. #19
    FEP Power Member mcb82gt's Avatar
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    If you can get a 1" bore that bolts pretty much right in, for $15....... Id be all over that!
    Mike

    Now stang-less.

    88 Cougar 5.0

  20. #20
    FEP Super Member erratic50's Avatar
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    So putting this together....... you're looking at LSC style rear brakes and standard foxbody 60MM front brakes. Off the top of my head if memory serves me correctly..... the LSC brakes are the same as the SVO brakes. The SVO rear disc use an enormous rear caliper piston that has to be paired with an equally enormous front caliper piston and master cyl. There are no smaller rears calipers available that bolt up to the SVO style brakes AFAIK.

    73MM front caliper paired with those rear brakes and a 1 1/8" SVO master cyl is what the SVO used.

    So that said -- the next part of this -- is your front brakes. The early fox spindles do not have any 73MM calipers available that I know of. Only the 84-86 SVO spindles and purely by chance the 87-93 V8 spindles have one 73MM caliper that will fit.

    I would abandon those particular rear brakes and select something compatible based upon my choice in lug count.

    SN95 style rear brakes are compatible with the M1858 and your 60MM front brakes and are 5 lug. I assume you may want 5 lug since you have the Ranger front rotors for your spindles AFAIK

    FYI - 87-88 Turbo Coupe / 93 Cobra rear brakes are compatible with the M1858 and your 60MM front brakes and are the 4 lug choice I was referencing.

  21. #21

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    What Trey and erratic said.

    Either install 73mm front calipers, matching spindles and struts, 1.125" m/c or dump the rear brakes and axles and use rear brakes from a 1994-98 Mustang GT/V6 (38mm).

    If you try to use the 54mm rear brakes with 60mm front brakes, the car will always have too much rear brake bias along with really poor stopping distances. You can't fool physics. It always wins.
    Jack Hidley
    Maximum Motorsports Tech Support

  22. #22
    FEP Super Member Ken P's Avatar
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    Quote Originally Posted by Str8sixfan View Post
    Ken P - thanks for the suggestion on the F-150 MC, though don't know if I'll run into the strut tower?
    Ports come out on engine / right side of MC. But make sure you ask first. I think mine is from a 1976 but it's been a very long time ago.
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  23. #23
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    1980 F-150 4x4 has ports on engine / right side of MC also, if you have to go that route.

  24. #24

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    Quote Originally Posted by Jack Hidley View Post
    What Trey and erratic said.

    Either install 73mm front calipers, matching spindles and struts, 1.125" m/c or dump the rear brakes and axles and use rear brakes from a 1994-98 Mustang GT/V6 (38mm).

    If you try to use the 54mm rear brakes with 60mm front brakes, the car will always have too much rear brake bias along with really poor stopping distances. You can't fool physics. It always wins.
    So the follow up (potentially stupid) question would be...are the 94-98 V6 car rears, do they have the same bracket mounts for the calipers? Because I was probably going to just buy new house brand calipers anyway.

  25. #25

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    Quote Originally Posted by Jack Hidley View Post
    You can't fool physics. It always wins.
    This got me curious, so I went looking and found this explanation: https://www.apcautotech.com/getmedia...e_8-2018_1.pdf
    1985 GT owned since new

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