brake pedal drops half way, then gets firm...confusion
 

My brake pedal drops about half way to the floor, then gets firm and its done this ever since I swapped in the turbo. I can pump the pedal up I have Wilwood front calipers, 1.8 rears (from a junkyard), wilwood prop valve, and nothing else special.

I've changed master cylinders, bench blead the master, changed front calipers, inspected rear calipers. I tried bleeding again last night with no success. I'm thinking about reverse gravity bleeding later today.

What are the chances of 2 bad masters?
Should I get a master rebuild kit?
If the calipers aren't leaking, what else should I look for?
Could it be the booster?

This is making me CRAZY!!! I've had this problem every single time and at my wit's end. Dear Lucifer, please help me!!!

So it will build pressure back up when you pump the pedal a few times?

Correct. I typically drive down the straights and pump the pedal with my left foot while at WOT to build pressure to it will engage at the top of the travel, so I don't think its the booster.

You can pump the pedal to bleed and the pedal is rock hard and sexy, then start the car and it drops half way to the floor. I suppose have I have old/bad master cylinders.

My '99 started doing this after I swapped a Sport Brake setup onto it, if you mean you push the pedal half way to the floor before you get resistance. I haven't figured out why (though it might have a lot to do with just vacuum assistance from the engine running), but braking is very strong, no loss of fluid, etc.

Mine did this after sport brakes, and currently with the Wilwoods. Maybe 949 is about to get more money; paging Emiliano Cervantistana and his revolution.

I had a small leak at the head of the stainless line going into the caliper on the TSE kit after some offroading at buttonwillow. Didn't find it till my next track day when 9 different spots decided to leak at the same time and the brake pedal did the same thing as you're describing. The fitting on the end of the line must have made contact with something because it was tweaked and no longer air/water tight. It doesn't sound like the problem you're having, but maybe it's worth checking the stainless lines.

you have ABS?

No ABS, no leaks anywhere, I've visually inspected every part and the only questionable leak lies at the "feet" of the master cylinder. I'm ready to put a stop to this because I'm tired of the chemical burns.

pm bbundy about the master replacement. I remember reading a thread where he was experimenting with larger stock MC's that corrected the issue's of running larger brakes. I think they were from a 929 or 626 ask Bob.

It probably takes x amount more volume to move the caliper pistons to the point of firm engagement if you are using larger bore calipers than stock. You might need to likewise increase the bore diameter of your master cylinder to compensate.

I was thinking similar to this. If there is any increase in diameter of the fluid passage from master to piston, then you would see this. The current master cylinder piston only pushes a given volume of fluid regardless of the diameter of the rest of the system.

In other words......If we say the volume of fluid that is moved by a piston in a system with one caliper is 100cc.

The old system required 50cc to get firm pedal feel. The piston has to travel x distance to push this volume.

Now you changed systems and the new system requires 75cc to get firm pedal feel. The piston has to travel a greater distance to move 75cc than it did to move 50cc. Resulting in longer pedal travel before a firm feel.

I'm interested in this but by the time I find the master, and booster, and make new lines...I've run out of time and give-a-shit.

There is no gradual engagement, it drops half way with very little effort. I don't think Savington has this problem and we have identical hardware. Also, don't tons of people run the Goodwin or FM BBK with the stock master?

No one locally has a rebuild and the replacement master is $100...so I'm considering the Wilwood master if 949 has them in stock.

This, with maybe some pad knockback from worn wheel bearings or loose caliper sliders??

Mine has done the same thing also after bleeding numerous times. Stock calipers, no leaks, stainless lines, no ABS. I was going to change the master cylinder too.

In for details...

I have this same problem on my 97 M3 after changing the front BBK back to the factory brakes. Bleeding til my eyes bled didn't do squat. It's driving me nuts too.

I suspect it's knockback. I will try rebuilding the front calipers.

The larger pistons up front mean you will have more travel than ever before. In other words: in driving conditions you will never have the same firm pedal.

I've got the Wilwood master on the shelf, needs to go in still.

Mine did this exact thing after I swapped to 1.8 brakes, although I thought it was just me doing a shitty job of bleeding it :crx:

edit: maybe i'll try using a 2001+ master cylinder? any difference between in the part for miatas with sport brakes/regular 1.8s?

You'll need the boost and the lines don't line up properly.

The Lotus crew reports similar issues and adjusting the pedal works. Any thoughts on pedal adjustment? The ability to build pressure at the top of the travel tells me not too...but this is a cost-free option.

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?

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.

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.

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.

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.

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.

I was just about to say my car did similar shit when the wheel bearings were fucked up front. But it looks as though you have that covered.

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.

What is your bleed procedure and equipment? Do you open the brake balance knob fully when bleeding?
Parking brake adjusted? Try with parking brake full tight.

Open valve, e-brake pulled.

I don't fully trust used master cylinders that sat dry for a while.

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.

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.

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:
http://i56.tinypic.com/x0nthf.jpg
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.

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.

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.

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.

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

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.

Keep fighting the good fight!!

Right now I'm trying to decide if I want to relax and wing it tomorrow with bad brakes, or work for about 4 hours and get the bigger master in

Just wanted to put these 2 together...

Try to at least get parts from a non-ABS car. The ABS M/Cs have only two ports (like a 929 1-in. m/c), so you'll need an M10x1 tee somewhere to connect the second front line. Even at that, I think the second front port on the non-ABS car is on the bottom, rather than the side, so the LF line will take some special work to connect.

You have more options if you already have an adjustable prop valve installed, as the union for the front circuit that no longer has to pass through the prop valve is an opportunity for a tee. See Bob's setup over on m.net for a pic.

Wirelessly posted

It took 90-minutes to move the lines and bolt it in on my back...now, the reverse bleed. If you have the FM prop valve kit, get the Lines and fittings and stock prop valve to bend them up instead of making lines.

sure hope that works. i'm like the worlds laziest bleeder and haven't had any problems except with brake fade... and the time i lost the brakes completely after brian swapped the 1.8 brake hardware in. some problem with a T and it leaked all the fluid out and the pedal went to the floor...

i still say a c-clamp on the rears would tell you what to chase. or cap the fucking things off completely where the hard line ends.

It feels good up and down the street, I'll be on the track in 6-hours.

Go hustler!

I know it was already mentioned in this thread, and disregarded, but I did adjust the rod on the MC, only sightly. I wanted there to be as little travel as possible. It worked well but took 2 or 3 trys to get it adjusted to the point that the brakes would not drag. Now my pedal moves about .5 to .75" total, I feel it gives me very quick and precise brake modulation, I don't have to "push" the pedal down at all, just apply pressure. O yes my set up is the 4 wheel Good-win BBK, w/ stock MC, and Flyin miata prop valve kit, Amsoil brake fluid, and Cobalt friction XR2, XR4 set up.

tell me why you didnt bleed it from the top two bleeders on that brake caliper instead of the bottom?

do air bubbles not rise in your garage?

Wirelessly posted

You have to bleed all 4 on each caliper, crap always comes out.

I've had fine luck with just bleeding the top two... I just thought maybe you hadn't.

it still comes out if you do the wheels after....

So, how was the track?

I fucking hate that track, first of all. The pedal felt good but I definitely have pad knock back...and I'm not sure why so I'm tightening all the hubs and going to see what that does at the track. The pedal felt good until the cool down lap...then it dropped to the floor. I just bled the brakes and the fluid is clear with very few bubbles if any so I'm stumped. Caliper temps show now higher than 400* so I doubt I'm boiling fluid.

If fluid boils does it immediately darken or can it stay clear?

heat soak into the master cylinder or nearby lines? if i'm thinking about this right, engine bay temps should peak at the very beginning of cooldown lap and then drop quickly.

you can move the mc out of the engine bay and add some redundancy at same time.
http://www.outlawraceparts.com/image...view/til72.jpg

u need a pan and a stove ;) probalby not the one you got for wedding present...