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.