Inconsistent Brake Pedal feel/stopping

Discussion in '2005+ Mustang GT 4.6L Tech' started by CammedS197, Oct 4, 2019.

  1. CammedS197

    CammedS197 forum member

    Hey Guys tried searching and didn't seem to find anything.

    2006 Mustang GT

    So not too long ago I changed my front calipers and rotors out after my car started to get a terrible shudder like some really warped rotors. But now the odd part was sometimes the car would brake as smooth as silk and stop hard and quick just how it should then most of the time it shuddered very badly. When the pedal was firm and responsive with instant response was when it stopped smooth. When it was I guess you could say squishy it would shudder. This is why I thought the calipers were bad. Had over 100k on them.

    Now after swapping to the new calipers and rotors actually went to the 13/14 non brembo brakes. It is 95%-99% better and it will pull to the left more then shudder but when braking it will either be even left to right or pulls left with a twitch to the right. Its not very aggressive almost like trammeling but you can tell its not. Now sometimes the pedal will feel soft and not very responsive until a little bit of travel then the braking power is almost linear with the light vibe/shudder literally barely there and it is more of a pull to the left. Then sometimes its super responsive with great feel and firm and stops super quick and hard and silky smooth no pulling left or right.

    I have no idea why. Do the rear brakes play in this? I was thinking the brakes may need bled again. I only bled that one time and used only a motive power bleeder never used the pedal actuation method. Is it possible that the brakes needing a bleeding again could do this?

    Oh also I forgot, after I installed and drove for two days I noticed a small leak from the driver caliper where the line goes in. Only happened when pedal was pressed. Took wheel off and tightened it and never leaked since. Stupid me just realized that I never bled after. Guessing some air may have gotten in and could cause it?

    I'm attaching a rough picture representation of the pedal feel/the stopping power to the pedal travel. Brake lines are less than 2 years old and are boss 302 lines.

    Thanks guys,

    Braking power to travel graph.JPG
  2. Dino Dino Bambino

    Dino Dino Bambino I have a red car

    Yes it's most likely that you still have air in the brake lines and that would explain the inconsistent, sometimes spongy brake pedal feel.
  3. CammedS197

    CammedS197 forum member

    Thanks Dino. Will hopefully get to do it this weekend.

    I do feel like the motive power bleeder doesn't get it all like the old fashioned pumping the brakes. So I'll do this with the pumping.
  4. 01yellerCobra

    01yellerCobra forum member

    If you have a dirt lot by you by go there and slam on the brakes a few times. Sometimes air gets stuck in the ABS pump. Getting the tires locked up activates the pump. Then go home and bleed them the old school way. I did that and they work great.
  5. Juice

    Juice forum member

    I like to let gravity do most of the bleeding. I attach a clear hose to the bleader, open the master cylinder cap and watch for the bubbles to stop. Then do one or two "pump it up and hold" bleeds.
  6. OX1

    OX1 forum member

    When I swapped to brembos, I swear I didn't let fluid leak enough to have air in ABS module. But I tried manual bleeding, pressure bleeding, gravity bleeding, vac bleeding (on caliper end, just fluid, and then at MC end). I bought a bi-dir scan tool that actuated ABS and tried all those methods again.

    I actuated emerg brake before bleeding (which I found one article that claimed it helps with rear bleeding, even though I never opened up that end). GT/BRAKES/BRAKE BLEEDING/IMG950189.jpg GT/BRAKES/BRAKE BLEEDING/IMG950190.jpg

    I even tried using old vac pump to remove air from brake fluid.

    Before GT/BRAKES/BRAKE BLEEDING/20180811_114133.mp4
    After shutting off vac pump and only releasing vac pressure. GT/BRAKES/BRAKE BLEEDING/20180804_092422.mp4

    Different fluid levels in both videos, but I tried it upwards of 30 times, so befores and afters were same results every time, but different levels in each of the videos above, as they were different "cycles" per-say. It was the same result, before and after, no matter the fluid level though.

    When the level was really high, it seems like it wanted to suck fluid into vac pump. If you try this, make sure you wear a really good chemical mask. Vac pump was discharging atomized brake fluid, can't imagine anything much worse to breath than that, UGH!!!. I put a rag over outlet of pump to keep it from seeping all over. Helps that I have 3200 sq ft garage with 14' high walls, so lot of air to disperse in.

    At first I thought it was just "vacing" out air in upper part of bottle, but that same air was there the second time after venting out vacuum (CAREFULLY), so who knows if it helps, but it sure looks liked it worked. I did seem that fluid that had been shipped or had sat for along time had more air in it. If you vented vac really quickly, a good bit of the air got back in fluid. Also, if I didn't pour fluid into pressure bleeder, like you would pour a beer not to get head, it also introduced some air. I heard manufacturers use this vac method on the production line.

    I also tried vac idea on the brake system and a complete draining of brake fluid to see if it would really suck out air. This was the start level GT/BRAKES/BRAKE BLEEDING/20181003_165448.jpg

    Then sucking GT/BRAKES/BRAKE BLEEDING/20181003_165456.mp4

    Then after about 1/2 hour. GT/BRAKES/BRAKE BLEEDING/20181003_171421.jpg

    I even did scan tool ABS procedure during vacing on system, but that gave weird results as ABS is creating pressure and you have a vac on system at the time. So anyway, it worked, but still had some bubbles coming out of calipers doing manual bleeding after that, but it is a fast way to get most of air out after fully draining.

    At one point, I even tried PP 2015+ MC, which is not super easy, but not impossible either,
    which has larger pistons, but it didn't seem to help at all and wasn't compatible with level switch. All the pics, if anyone interested......... GT/BRAKES/BREMBOS FRONT/2015+ PP MASTER CYLINDER/

    I had also read that bottoming MC with very low brake pressure damages it, but that is not true, as you can see pics of inside MC's, where pistons are smooth all the way to rear ends. The seals are internal to bore, so no real way to damage anything. I even put pistons back in/out several times and then pulled O-rings, no damage.

    I finally brought it to a mustang shop that has Fords IDS and they were able to do it. Overall, I feel my 14's brakes were spongy from day one, and they are still longer travel than I like, but at least now they work great once you get enough pedal travel.

    So bottom line, worse case, might be worth a shot to have a shop do it that has the Ford IDS.
    I don't know what it does over my scan tool, and I think I went WAY over the top doing every form of bleeding known to man (so I don't think it was incompetence or even my method, as I tired bleeding a zillion different ways, upwards of 100 times and 100's of dollars on new brake fluid, but I was on a quest LOL!!)
    Last edited: Oct 5, 2019
  7. 08MustangDude

    08MustangDude Resident Fuktard

    You need two people to properly bleed brakes.

    You on the bleeder, and someone in the car.

    Motor not running, no key in, nothing:
    Have them pump the brake pedal THREE times and hold.
    Open the bleeder slowly, then close when the pedal floors, making sure they do NOT let up on the pedal.
    When closed, they release the pedal, pump three times and hold, you open the bleeder again.
    You repeat till there is no air.

    Check the fluid level after each bleed. I have a bottle of fluid that sits over top the
    reservoir and lets fluid in when it gets low, it's pretty neat...

    You start with the caliper farthest way from the master, rear right, then work your way
    to the driver side: Rear right, rear left, front right, front left, in that order.
    Dino Dino Bambino likes this.
  8. 08MustangDude

    08MustangDude Resident Fuktard

    31 GT/BRAKES/BRAKE BLEEDING/20181003_165456.mp4
    That vacuum method like you did, does not work for brakes. That's
    how you replace coolant in a motor, with a special canister that releases
    coolant in when you release the vacuum, but that will not work for brakes.

    You can't find any videos or instructions that show brake bleeding with
    vacuum at the master cylinder/reservoir like that. You are pulling the
    fluid out of the calipers, because the caliper pistons will not suck in with that
    vacuum. All you did was put tons of air into the brake lines, hence the
    bad pedal.

    I have never, EVER seen anyone apply vacuum at the brake reservoir
    like that, for any reason.

    Vacuum bleeding is using a vacuum source @ individual bleeder screw(s) so that atmospheric
    pressure on top of the master cylinder reservoir forces brake fluid through to that lower pressure

    Pressure bleeding is when a pressure higher than atmospheric is applied to the fluid in the master
    cylinder reservoir & bleeder screws are in turn opened & closed until all air has been removed. It is also
    where one applies pressure to the bleed screw/s rather than the master cylinder & is rarely used.

    Gravity bleeding is accomplished very easily when the master cylinder is firewall mounted. This
    takes longer, and is also not as reliable as any other method.

    The problem is, NONE of those methods move the master cylinder piston inside the master
    cylinder bore. Pumping, like I mentioned, moves the piston at full stroke inside the bore, and
    is the best way to bleed brakes. No instructions I have ever seen, show an application of vacuum
    at the reservoir.
  9. 07 Boss

    07 Boss Senior Member


    Hahahaha, I skimmed over this vacuum bleeding and just assumed it was being done from the caliper bleed screw which works OK, not great, but OK. I had to go back and see that it was being done from the reservoir. That just seems like it wouldn't work without even trying it. I've been bleeding brakes for 30 years the same way and it has always worked just fine. Don't know why we gotta re-invent the wheel.
  10. CammedS197

    CammedS197 forum member

    Thanks guys. I ended up being way too busy this weekend and wife and I were spontaneous and went to UF/Auburn game. Going to get the wife out with me to pump and hold the brakes! Will also try and get the ABS activated lol. Thanks for that tip!
  11. OX1

    OX1 forum member

    It filled an entire, dead empty brake system, including the calipers, except for a tiny bit of
    air removed by pressure bleeding at the end. And for 99% of the air, I did not have to open the brake system and/or run brake fluid through system, wasting it (unless you re-run brake fluid through your system, I don't typically).

    Degassing hydraulic fluids is nothing new, my "chamber" just happened to be the brake system.

    Pumping MC in the car does not get you full stroke. You can only do that on a bench
    (at least with MC's on S197's that I pulled apart, inspected and Mic'd, and then measured
    pedal travel and pedal ratio).

    The real issue was air in the ABS. I've looked all over (inclduing factory shop manuals) and could not find an internal hydraulic schematic for the S197 ABS hydraulic "block" and solenoids. My scan tools allows individual opening/closing of each solenoid, or has an ABS bleed procedure. I don't know exactly what the procedure does with each valve or ABS pump, but as I understand it, ABS actuation does (at some point during it's cycle) send fluid back towards the MC to attempt to compensate for the pedal dropping when it relieves pressure at caliper.
    So even after a lot of bleeding, you cold be sending air back up to the beginning of the circuit.

    I didn't know if it would or not either, but that never stopped me before, LOL. I won't ever use it for final bleed, but it works great for an empty fill, without losing/wasting ANY brake fluid in the process.
  12. datmbn

    datmbn forum member

    The need is the mother of inventions !
    A while ago when I helped a person to change a brake tube, a bleeding nipple
    couldn't get loose, and then I started to think about whether there was any other way to get the air out?
    I then found that the volume of liquid present in a fully pumped brake piston is quite large, probably larger than the volume in
    the brake pipe!
    OK little count the inner diameter of a "normal brake pipe is 2.8 mm cross area then becomes 1.4x1.4x3.14 = 6.1
    The diameter of the brake pistons on the eg Mustang is 60 mm front and 38 mm rear with a stroke length of 50 and 40 mm, respectively.
    The volume of liquid then becomes 30x30x3.14x50 = 141300 and 19x19x3.14x40 = 45341, respectively, if we then divide these volumes with the cross-section of the pipe
    we get the pipe length corresponding to the volume of the piston 141300 / 6.1 = 23163 mm 45341 / 6.1 = 7432 mm
    We can thus conclude that the volume of the piston more than enough is enough to fill the tube (even if we count on ABS and main cylinder)
    The only thing you need to do to get the brake aired is to pump out the piston as far as possible and push it back.
    let's say 2 times to be sure.
    If you are unsure whether the rear volume is really sufficient, then work diagonally (two-circuit brakes)
    and also use the piston forwards (pump out both pistons press back rear and then forward)
    How did it work in practice? After the change of tube, the brake was perfectly aired and the car went through the inspection.
    This method I will use the next time I change brake fluid
    regards Mats
  13. Norm Peterson

    Norm Peterson corner barstool sitter

    If you look carefully, you'll see that the master cylinder inclines upward going forward from the firewall end. I've read (forget where) that it's possible for a small air bubble to get trapped at the forward end of the M/C where it can't be bled out or sucked out. Jacking up the rear of the car to level the M/C was proposed as a solution. Can't hurt to try this.

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