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  1. #1

    Default Rear Disc Brakes/Proportioning Valve Tuning

    So I have these new brakes on the rear. They are SN95 solid rotor style. All new/reman parts. I went out to try to dial them in today. I did lots of aggressive braking, stopping to open up the PV (it started fully closed), and trying again. Well, the valve is now at the end of it's adjustment. Wide open, I assume. The rear end still is not trying to come around on me like I remember it doing when i went to do the same procedure on my last set of brakes (whole different setup). So what's the deal? My rear brakes don't seem to be functioning. The parking brake DOES hold, so I'm confused.

    I was about to jack up the car and go to look at potentially adjusting the pistons outward so there's less space between them and the rotors, but i'm a bit wary of that because on my last set of brakes i had a bad sticking problem with my rear calipers. The first time i drove the car with this current set of brakes, i noticed what I thought was excessive heat at the left rear wheel (and very little at the right rear), so i adjusted the piston inward on that one.

    For a little background, I had SVO rear brakes with used calipers for years and then swapped to the current SN95 style. I realize not everyone does that sort of thing, so it's probably necessary information here.

    So, what do you guys think? Maybe I shouldn't have adjusted the left rear piston inward because it was just a balance problem? And now NEITHER is functioning as a result? If that's the case then why does the parking brake hold? Could it be a tire thing? I have MUCH better tires on the car this time around. Maybe they're not allowing the rear to grab unevenly or something?
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  2. #2

    Default

    Which master cylinder do you run? Have you tried hitting the brakes on a dirt/gravel to see if they work at all?
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  3. #3

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    The master and booster are '94 V6. The front brakes are single piston '94-98 V6-GT. So, in theory, a matched system except for the Fox pedal.

    I haven't been on a dirt road with them yet.

    Sent from my XT1650 using Tapatalk
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  4. #4

    Default

    I guess what I said about the tires doesn't really make sense, but it IS something that's different last time to this time. Maybe they weren't allowing me to slide as easily.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  5. #5

    Default

    The parking brake is mechanical and designed to be able to move the caliper pistons a large distance. The m/c is sized so that when you press the pedal about 1/3 of its travel, this is enough fluid flow to move the caliper pistons as far as the o-ring seal has moved them back. If the rear pistons are artificially pulled back from the rotors with an incorrect adjustment of them, the pads will probably never touch the rotors, but the parking brake may seem to work fine.

    If you cycle the parking brake enough times, the pistons should adjust themselves to the rotors.

    Another possibility is that there is air in the rear brake system, but this would make a mushy brake pedal.

    BTW, higher grip tires at both ends of the car will increase the chances of rear tire lockup as it allows increased weight transfer.
    Jack Hidley
    Maximum Motorsports Tech Support

  6. #6

    Default

    Thank you! I'll fiddle with those things.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  7. #7

    Default

    I have a similar setup. Jack Hidley told me to run a 95 Cobra MC not the standard V6 MC as the bore was too large. I have all the parts but just haven't finished the car to test it yet. Also I installed the whole rear brake setup off an 02 V6 car including the axles.


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  8. #8
    FEP Super Member erratic50's Avatar
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    Default

    I agree that cobra MC is a good approach. It's not quite as easy as some other options so do your homework and know what you are getting into and giving up, etc.

    It's preference for sure. Many like 15/16" bore master cyl. Personally I like the Mark VII master cyl - it bolts right up on my stock 86 mustang booster. One line fit, one required an off the shelf adapter that Autozone carried.

    for me, a 1" bore is a great combo with sn95 brakes on a fox with fox booster.

    FYI I've driven SN95 brakes on a fox with SVO master cyl and stock booster. It was not the answer. It sucked - way too sensitive. Too little pedal travel before lockup.

    Best news with Mark VII mastercyl is all metal which helps with fluid heat dicipation.

    Make sure you have all the air out of the lines. My rear SN95 brakes worked better than the drums did even when I had a stock 7/8" master cyl. I drove it that way for a while before I rebuilt the frontend and added the new master cyl with 99GT front brakes and gutted the proportioning valve and added a summit adjustable valve.

    Also check for leaks.
    Last edited by erratic50; 07-27-2017 at 01:39 AM.

  9. #9

    Default

    While I know the mc and booster I have aren't the best option, the booster was a BITCH to put in, and I've been fighting with my brakes for over a year now (long story). I'm going to wait to see how I like the feel with everything working properly before I think about swapping any more parts. The fact is, my mc choice is not what's causing my rear brakes not to function. I'm only going to look at swapping parts if I hate the feel a LOT.
    Last edited by ZephyrEFI; 07-27-2017 at 12:17 PM.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  10. #10

    Default

    Well, i did the back up-parking brake thing several times the other day, and my rear brakes do seem to be working now. The rotors were new, and you can clearly see they are operating now.



    They seem to be getting too hot, but i don't know how else to test the balance front to back besides braking aggressively. Anyway, I took it down a dirt road today (which pained me to do), and locked up the brakes several times. It seems that even with the proportioning valve ALL the way open, they are good and even. Anyone ever heard of that being the case? I guess I'll keep tweaking them as i go on with other things if I need to. The pedal does seem to be more responsive now too. I'm liking the feel so far.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  11. #11
    FEP Super Member erratic50's Avatar
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    What are you running for front brakes and suspension as that influences front to rear bias.

    I have 99GT dual piston calipers in front and stock 95GT in rear. I had to dial out some rear brake to keep them from locking but I need to take out some rear spring with the 4 cyl rears to get it to sit level with my dropped frontend. It has about 1 1/2" of rake if I take the weights out of the back. Less than 3/4" the way I have weights so I can drive it for now. with a stock rear suspension there is always a ton of brake dive. I hope to eventually go to a torque arm to solve that, then dialing in rear brake should be an operation in setting it at about full kill out back.

  12. #12

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    The front brakes are '94-98 GT. Springs are H&R Sport, with Tokico Blue dampeners. Control arms are all new stock replacements with rubber bushings except the rear lowers. They are Steedas with urethane. The car sits level after I added Global West 1/2" spring spacers. The rear drooped without them. Mustang to Zephyr difference, I assume. Car also has thru floor SFCs.

    I guess most of the suspension is different, as well as the SFCs, since the last time i had to set the balance at the prop valve too.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  13. #13
    FEP Super Member erratic50's Avatar
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    With the slightly reduced braking force you should have out of the 98 setup up front you have, there should be less brake dive. Seems like it could be reasonable.

    Might be interesting to wait for some Ice (before anyone salts it) and see. I have to dial out about 5 clicks of rear brake on slick stuff.... this said honestly my car is a holy terror on crappy roads now. I blame the type of tires plus they are wide tires and rear track width is about 2" wider than front overall. I don't think my negative camber I run up front helps in those instances at all either.

    I got stuck driving home from the airport in around 2" of snow last year when I landed late at night. The road crews hadn't been out yet. Was 100x worse than it ever was back when the car was stock and had studded rear tires back when it was my daily. Took 3 hours to drive it 30 miles in that crap but visibility was a major factor too.

  14. #14

    Default

    I wouldn't be surprised if brake dive and weight shifting was MUCH more of an issue the last time. You can see, the car was practically lifted before. Whereas it's lowered now.



    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

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