Close



Results 1 to 18 of 18
  1. #1
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default Front brakes rubbing after sn95 calipers

    Trying to sort out an issue with my front brakes on the 79 pace car. I used 94-95 sn95 spindles, rotors, and calipers. New brake hoses (I believe they are sn95 as well, but I could be wrong...did the swap last year). The car stops great, until you drive it for a good 20 minutes or so, then the front brakes feel like they're grabbing. I pulled in today and got out and shot them with the temp gun. Both front rotors over 500 degrees and I could feel them grabbing while driving. The more I drove, the worse they got. Let the car sit for a couple hours and cool down and everything is fine again.

    While checking the brakes, I noticed the master cylinder (93 cobra mc and booster) was pretty warm. I figure this is from the calipers heating up the fluid. With the wheel off, it appears the outer pad is touching the rotor, while the inner pad had 1/8" or maybe 3/16" clearance. The pins are not seized at all. Everything was greased properly and pins are allowing for good movement. Pistons do not appear to be locking up.

    I'm not really sure if the pads sitting closer on the outside is the normal resting position for them, or if the caliper is literally mounted to one side due to a misalignment of some sort. Is it possible that doing this caliper swap will require me to use washers to shim the caliper to line up correctly? Am I missing something else?

    It also dawned on me that this might not be a caliper alignment issue, but that heat from the header is heating up the brake fluid and causing pressure through the front lines. They are basic unequal-length shorty headers and don't sit right up against the prop valve or hard lines, but I figured it's worth mentioning as another possibility.
    Thoughts?
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  2. #2

    Default

    You've eliminated all my first guesses, so I'd go with brake lines. At least those are cheap and easy to replace.

  3. #3
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default

    I know bad/old brake hoses can cause brakes to stick, but these are basically brand new. If you're referring to heat from the headers affecting the hard lines, that's a possibility. I'm leaning more toward an issue with the calipers/outer
    pads being closer to the rotor. Or....it may just be that the pads are a little too thick. I measured thickness on all 4 front pads and they're all the same. I don't recall the reading I got, but the measurement didn't vary from one to the other. I guess it could also be that the rotors themselves are too thick. The fix might be to have them turned and slap on a fresh set of pads (measuring thickness first, just to see what they are compared to the old set and making sure I don't put any pads on that are thicker).
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  4. #4

    Default

    If the pads were too thick I would expect both to rub, or have same clearance from rotor. But the outers are closer. First guess would be slides not letting go enough, but if that feels good... did you do evil in a previous life?

  5. #5
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default

    Quote Originally Posted by FirstOnRaceDay View Post
    If the pads were too thick I would expect both to rub, or have same clearance from rotor. But the outers are closer. First guess would be slides not letting go enough, but if that feels good... did you do evil in a previous life?
    Quite possible...on both counts lol!

    When you press the brake pedal, the pads squeeze the rotor and the pins allow them to locate themselves to where each pad has the same pressure. When you release the brake, it may just be normal for the inner pad to retract more than the outer pad. This is just a guess. Either way, it seems like a thinner rotor and/or thinner pads may fix the problem. Shimming the caliper bolts with a washer would probably do the same thing, but I think the rotors may need turned at this point anyway, due to the heat cycles.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  6. #6
    FEP Power Member STL79Coupe's Avatar
    Join Date
    Dec 2009
    Location
    St. louis MO
    Posts
    2,424

    Default

    Just wondering if you did anything to the brake pedal rod going into the master cylinder? I thought my calipers were seizing up, but my MM adjustable brake pedal rod wasn't adjusted correctly.
    Keith formerly STLPONDS
    '79 V8 coupe in the works!
    Build thread http://vb.foureyedpride.com/showthread.php?t=89153

  7. #7
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default

    I don't believe I did anything to it. I went to the parts store today and dropped off the rotors to be turned. They were actually a little on the thin side. Also, new pads were slightly thicker than mine. Presuming he measured correctly, that rules out fitment issues with pads and rotors.

    I tried moving the caliper to provide clearance to the outside just to see if there was even room for them to not lie against the outer rotor. The caliper is over as far as it's going to go. That pretty much leads me to believe that the caliper is just plain ol sitting too far to one side. The question is "why?". And I know a way to fix it. A washer on the mounting bolts placed between the caliper and spindle should offset it just enough to give the needed clearance. As long as I use big enough washers, I don't think there are negative effects of doing it. I'm so perplexed as to what is going on and I don't really like the idea of ignoring the underlying problem and rigging it to work.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  8. #8
    FEP Senior Member Patrick Olsen's Avatar
    Join Date
    Mar 2013
    Location
    Where the Navy sends me...
    Posts
    543

    Default

    Which specific SN95 spindles, calipers, and rotors are you using? '94-98 GT/V6, '99+ GT/V6, or Cobra/Bullitt/Mach1? Did everything come from the same car, or did you source new parts?

    "At rest", both pads should be basically right against either side of the rotor, so the mismatch you're seeing isn't normal. And your discovery that the caliper is slid out as far as it will go, and the outer pad is still being forced against the rotor, confirms that sumthin' ain't right!

    The only two possibilities I can come up with are that either (1) the hubs aren't seated all the way onto the spindle correctly, or (2) the rotors aren't seated fully onto the hubs correctly. Did you replace the hubs when you installed the SN95 setup? If so, are they OEM parts are something aftermarket? How were they torqued in place? (As I recall the torque spec is 250ft-lb or something really high like that.) If you pull a rotor off, is there any sign inside the "hat" that the rotor is making contact where it shouldn't? Can you measure the hub diameter and the inside of the hat, just to make sure the rotor will fit properly?

    That's all I can come up with. I was initially thinking it was something hydraulic, having had that issue once before, but based on what you've found it appears to be something mechanically wrong.
    '89 GT convertible - not a four-eye
    '82 Zephyr Z7 - future track car

  9. #9
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default

    The spindle, hub, and calipers all came from the same car....94 or 95 v6 car, iirc. Rotors are new aftermarket... one of the major brands, but don't recall which. I had the rotors turned today and reinstalled them with the old pads (which are still basically new, having maybe 50 miles on them. Didn't want to ruin a new set of pads before figuring this out. ) I verified that the rotor spun freely before putting the wheel back on. There was no contact between pad and rotor. Brakes worked perfectly on the test drive for about the first 15-20 minutes, and then they started grabbing again. I started thinking that maybe heat from the header was affecting the prop valve and master cylinder, but I tried another test. I let the car idle for 20 minutes and the brakes worked fine. It doesn't have issues until driving for a while. And I'm starting to think that it doesn't really matter how long it's driven, so much as how many times you brake. In other words, every time you brake, the pads/caliper do not retract like they should. If that's the case, then it has to be the calipers, mc, or prop valve. Calipers/pistons seem to move in and out fine and it's happening at both front wheels, so that makes me think it's not the calipers.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  10. #10

    Default

    Check for any preload between the brake booster pushrod and the m/c piston. There must be a tiny amount of clearance here, so that once the brake fluid heats up, the compensation port in he m/c isn't covered. If that happens the brakes will stay locked on.
    Jack Hidley
    Maximum Motorsports Tech Support

  11. #11
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default

    Quote Originally Posted by Jack Hidley View Post
    Check for any preload between the brake booster pushrod and the m/c piston. There must be a tiny amount of clearance here, so that once the brake fluid heats up, the compensation port in he m/c isn't covered. If that happens the brakes will stay locked on.
    That's very possible. How would I check clearance?
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  12. #12
    Jack Hidley
    Maximum Motorsports Tech Support

  13. #13
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default

    Quote Originally Posted by Jack Hidley View Post
    Thanks for the info. I was afraid this was going to be the way to do it. I literally bled these brakes probably 15 times, while in the process of restoring the car. I've been through 3 brand new master cylinders that were bad right out of the box. I've bled the standard "2-person" way, used the gravity method, and used a mighty-vac. I was hoping I wouldn't have to bleed them again for many years.
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  14. #14

    Default

    If you unbolt the proportioning from the inner fender, you can pull the m/c forward and inward enough to take these measurements without disconnecting any brake lines.
    Jack Hidley
    Maximum Motorsports Tech Support

  15. #15

    Default

    Maybe it's just the wrong rotor? On Rockauto is see 2 different ACDelco PNs for 94 V6; 18A654SD with a height of 2", and a 18A649SD with a height of 1.8". I'm thinking the 2" rotor will pull the disc away from the pad.

  16. #16

    Default

    18A654 is a rear rotor. 18A649 is a front rotor.
    Jack Hidley
    Maximum Motorsports Tech Support

  17. #17
    FEP Supporter Broncojunkie's Avatar
    Join Date
    Jan 2015
    Location
    Barboursville, WV
    Posts
    1,048

    Default

    Quote Originally Posted by Jack Hidley View Post
    Check for any preload between the brake booster pushrod and the m/c piston. There must be a tiny amount of clearance here, so that once the brake fluid heats up, the compensation port in he m/c isn't covered. If that happens the brakes will stay locked on.
    Thanks for the advice. You were spot on! I didn't really have enough room for an accurate measurement, but a couple turns on the adjustment rod and brakes appear to be working fine!
    79 Pace Car - 331, t5
    79 Cobra - working on 351w, t5
    86 coupe - rotbox roller
    86 coupe - working on 408w, c4
    88gt - 302 efi, t5

  18. #18

    Default

    LOL I thought I had it figured out! Thanks for setting the record straight.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •