brake pedal drops half way, then gets firm...confusion - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-24-2010, 05:03 PM   #21
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The Lotus crew reports similar issues and adjusting the pedal works. Any thoughts on pedal adjustment?
I'm not going to argue because I don't have all the details, but I have difficulty thinking that an OEM would not use the full stroke of the master. And as long as it releases far enough that the port to the reservoir is open, I'm not sure what else you could do. Easy enough to test I suppose.

Edit: How much did the piston surface area change anyway?
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Old 09-24-2010, 06:24 PM   #22
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I suspect it's knockback. I will try rebuilding the front calipers.
I bet we have a winner!

I say NO to adjustment. As you said, you can make pressure at the top of the pedal, so that wouldn't really be a good thing to have the brakes dragging heavily.
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Old 09-24-2010, 10:08 PM   #23
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I tried snugging the rod up with no success, I have the same symptom until passing the return-weep hole. So I put in a 3rd junk-yard master cylinder, tomorrow I'll build a reverse pressure bleeder, weigh my success, then order a Wilwood master and mount the tanks in my wallett since I have room after unloading it again on brake parts.
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Old 09-25-2010, 08:49 PM   #24
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I give up. I've swapped in a 3rd master cylinder (that sat on the shelf for a while) and the brake pedal still drops lower than the gas pedal, unlike my street car. When I unplug the booster the pedal drops a bit when you press it, but not much.

I'm at the point where I can order $30 rebuild kit, order a $130 stock replacement, or pay $230 for the 949/V8R Wilwood. I'm happy to hear advice on where to do and I wouldn't care about the money but my office owes me $3080, I dropped about $120 chasing the brakes, $200 on pads, dropped $550 to run at Hallett last weekend including hotel, dropped $200 to run NASA next weekend, and moving in 3-weeks. I'm broke.
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Old 09-25-2010, 09:21 PM   #25
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I should note that I have new wheel bearings in now that I'm repacking them every 3 track days with Amsoil grease. This seems to work.
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Old 09-26-2010, 03:49 AM   #26
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suck! i hate dropping coin on track days and then something stupid lets you down.

my bet is on rear brakes. whenever ive encountered this scenario its almost always been rear brakes. the only times where it wasnt, constant pedal pressure would result in the pedal hitting the floor. to find the smoking gun i would proly take the rear calipers off and c-clamp the pistons good and tight in their bores and see if it changes the pedal position. drive the damn thing around the block if you have to.
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Old 09-26-2010, 05:05 AM   #27
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I should note that I have new wheel bearings in now that I'm repacking them every 3 track days with Amsoil grease. This seems to work.
I was just about to say my car did similar **** when the wheel bearings were fucked up front. But it looks as though you have that covered.
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Old 09-26-2010, 06:41 AM   #28
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Try putting one of the "bad" master cylinders or boosters in your daily, just to see if they're actually good or bad. Cause if you havent noticed it's looking like the booster and MC aren't your problem.
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Old 09-26-2010, 07:47 AM   #29
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What is your bleed procedure and equipment? Do you open the brake balance **** fully when bleeding?
Parking brake adjusted? Try with parking brake full tight.
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Old 09-26-2010, 09:45 AM   #30
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What is your bleed procedure and equipment? Do you open the brake balance **** fully when bleeding?
Parking brake adjusted? Try with parking brake full tight.
Open valve, e-brake pulled.

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Try putting one of the "bad" master cylinders or boosters in your daily, just to see if they're actually good or bad. Cause if you havent noticed it's looking like the booster and MC aren't your problem.
I don't fully trust used master cylinders that sat dry for a while.
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Old 09-26-2010, 06:39 PM   #31
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Do your four-piston front calipers have round o-ring pressure seals or square-cut pressure seals?

Do you get the same result driving in a straight line, or only when going around turns between braking?

Are the rears adjusted properly? I've found that backing off 1/4 turn instead of 1/3 turn helps at least parking brake travel without adding any drag.

Are your rear sliders in reasonably good shape and well lubed? (Uh-oh, I just asked Hustler that!)

How much remaining thickness do you have in front and rear pads? Are your brake pads worn with taper longitudinally, radially or indicating either caliper flex or rotor coning, or are they nice and flat, like 0.5mm or less difference in wear all across? Pad taper is the simplest explanation for the symptoms you describe.
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Old 09-26-2010, 07:16 PM   #32
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I think they're o-rings because Wilwood sucks.

Same results on the highway, cruising home.

I adjust the rears by spinning the rotor and adding pressure to the adjuster to get it as close as possible.

They look great and lubed with expensive Amsoil grease.

Front pads have 0-miles, rears have 3-sessions on them. I get a slight longitudinal taper but flip the pads around to mitigate this. Rotors are flat according to the micrometer.

Thanks for the advice.
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Old 09-28-2010, 01:00 AM   #33
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It might be fixed, but in general shame on me for being a failure and dealing with terrible brakes for a year and spending about $1000 to fix the problem when it was basic bad bleed. I'd like to think the bad bleed was made extra challenging from something like the rear prop valve restricting flow in front of the brake lines pointing up, above the master cylinder.

Before work today I cracked the bleeders and let everything drain out. I wrapped every brake fitting in Teflon tape and it made me very happy since I was tired of battling with the crappy Wilwood calipers.
Yesterday I had a vision, where the racing gods spoke to me, and I remember watching leMans a few years ago when the BMW team complained about baking the brakes when trash clogged a brake duct. I watched them manually bleed the line, then bring the car back and hook up a hose to it and magic happened.

So today I built this little rig:

I hooked it up to the left rear and watched smooth fluid come from the right rear and up to the reservoir, perfect fluid all the way with zero bubbles. I then hooked up the pressure bleeder to each Wilwood caliper and got a good bleed on each rotor then filled the master, drove it, and it feels great...better than ever before. I'll call it "fixed" if it works-out at the track next week.

I highly recommend reverse bleeding to anyone with dry brake lines, it was very easy and worked right the first time.
I did a reverse pressure bleed starting at the left rear caliper.
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Old 09-28-2010, 03:53 AM   #34
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I am still not exactly sure, even after you called me for phone sex a few hours ago. I was under the impression you lifted the front of the car higher than the rear AFTER draining all the fluid on your entire system, master and all. Pressurized the drivers rear w/ the pressure bleeder. cracked open the passengers rear untill no air was within that part of the system.

Then, poped open the willwood calipers, while still having a pressurized left rear caliper. Once you got that far, you continued untill the master cylinder filled, from the back up? I guess my confusion is the picture, your pressurizing the front left, and also bleeding it at the same time.
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Old 09-28-2010, 09:02 AM   #35
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Bleed all lines from the bottom-up, since the right rear is on the same "circuit" you need the pressure feed from the left rear. I may do my clutch tonight.
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Old 09-28-2010, 11:33 AM   #36
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Check your rears i had the same issue on my miata and it was because my rears were frozen. I bought two new calipers from pep boys.

I also had the same issue on a EG Civic in which the wilwood brakes were installed in the front it had ITR master and rear brakes along with a 40/40 prop valve. It would do the same thing like your car it braked fine and locked up just fine. Just the pedal feel was very weird.
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Old 09-28-2010, 11:58 AM   #37
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Check your rears i had the same issue on my miata and it was because my rears were frozen. I bought two new calipers from pep boys.

I also had the same issue on a EG Civic in which the wilwood brakes were installed in the front it had ITR master and rear brakes along with a 40/40 prop valve. It would do the same thing like your car it braked fine and locked up just fine. Just the pedal feel was very weird.
Dem rearz is rebuilt, son. I drove the car to work today and the pedal is pretty epic. There is a slight drop because when you reverse bleed from the caliper, a small amount of air sticks in the top of each piston cylinder due to fluid filling from the bottom up. I will crack that line today and push out just a touch of air and it should take up the very minimal slack. Its crazy, I bedded the new pads this morning and the pedal stayed rock hard and high. not bad for a first attempt at reverse bleeding.
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Old 10-01-2010, 01:52 AM   #38
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Day 6 is still a failure, brakes are worse than ever. When you pump the pedal you can hear some weird popping/clicking noise through the rear line. I don't know what to do at this point other than take apart everything, reassemble, try again. I'm totalland its starting to affect my personal life, lol.y lost
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Old 10-01-2010, 01:35 PM   #39
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And now, for my next act, I will purchase a Sport brake master cylinder and booster from local swap yard and install at 3am the, morning before a track party.
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Old 10-01-2010, 06:47 PM   #40
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Keep fighting the good fight!!
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