Inconsistent brake pedal feel. Doesn't look be your everyday problem
 

I did a z32 brake upgrade on my s13.

Here are the parts that were swapped during the upgrade:

New 1 1/16 brake master cylinder
z32 brake conversion lines
Front calipers remanufactured aluminum calipers from Advance Auto
Rear calipers remanufactured aluminum calipers some guy off ebay.
Forodo DS2500 brake pads front and rear
Powerslot, slotted rotors front and rear
z32 ebrake conversion
z32 2+2 ebrake cables
ichiba 5 lug conversion hubs front and rear
arp extended wheel studs front and rear.
Motul RBF 600 brake fluid
Recently swapped remanufactured brake booster from advance auto.
Z32 NA rim used. Kind of beat up.
Falken Azenis front and rear. 215fronts 225 rears.

Details on the swap:

I had problems from day one. Mainly spongy brakes. It turned out the problem was my front passenger caliper was actually a front driver side caliper. Because the caliper was sitting upside down i couln't bleed the brakes properly. I swapped in the correct caliper. Rebled and I got rid of the sponginess. I have had the brakes pressure bled to be certain there is no air in the lines.

The problem:

90% of the time the brakes feel fine, nice solid brake pedal feel. Then sometimes I have to press further to get the brakes to work properly. It feels like there is air in the lines (but there isn't). If you were to step on the brakes immediately after the initial spongy reponse, you'd get a nice solid brake pedal feel. Sometimes the brakes respond perfectly, sometimes the brake pedals goes 50% lower before you actually get brakes. It hard to determin when the brakes will work properly and when they won't.

What I have done to remedy the issue:

Bled till the cows came home. I swapped in a new brake booster thinking perhaps it was screwed up. I pressure bled the system once afterwards and the problem is still there. I did the brake booster swap this week. The brakes feel solid 90% of the time but if you're driving hard around turns the problem is very apparent. I have replaced the entire brake system minus the hardlines. I'm really not sure where to go from here.

What to do next?:

I'm not sure what else to do to diagnose the problem. I'm hoping you guys can help me on what to do next.

Here are a couple things I am thinking of doing.

Remove the front calipers and pads and see if I see anything strange.
Contact Forodo if perhaps my pads are the issue, and see what they say (doubt it's there).
Could the suspension have something to do with this problem?

Any help is greatly appreciated.

I'm posting this here instead of basic because I've tried most of your basic things to solve the problem.


Please help.

During driving your engine may not be producing enough vacuum to operate the brakes properly. Or your check valve in the booster line to intake may be bleeding pressure.

With the engine at idle if you press on the brake peddle & HOLD it what does it do? If you pump the brakes at idle does the engine almost stall out.
At idle you need ATLEAST 16" of vacuum. Bigger brakes require more vacuum from the engine to operate. I have done quite a few big brake upgrades just to tell the owner that the engine cannot handle the extra requirements. But if the engine is in known good condition an addition of a vacuum pump or canister may be in order.
A canister stores the needed vacuum to operate the bigger brakes.

With the engine at idle, and I presson the brake peddle, it does nothing weird. I have a stiff pedal.

If I pump the brakes at idel, the engine behaves like normal. Comes not where near stalling.

I will pickup a check valve tomorrow and see what it does.

Update:

I finally got a chance to work on the car this past weekend. I believe I've figured out the issue.

http://www.bumnah.com/brakes/28frontdisplay.jpg


I'm running Ichiba 5 lug conversion hubs, with ARP lug studs. These are two individual pieces, that work together, not engineered for one another.

If you look at the base of the studs, you'll notice the taper to where the threads start is sitting past the base of the hub. It appears my rotors are sitting on this taper instead of the hub. The only way I noticed was by putting the rotor on backwards. When I did that, I noticed the rotor wasn't sitting flush against the hub. I took the rotors and hubs to a machine shop this morning. Showed them the problem, the guy said he'd chamfer the holes on the rotor for me. I'm going to reassemble everything tonight, and hopefully solve my problem.

Thanks for the suggestions. I'll update the post for future searchers.

So. It looks like the problem is finally fixed.

Issue 01:

The studs as I described in the post above. The taper at the base of the wheels studs was too longer. The rotor wound up sitting on those instead of the hub. The rotor was not rotating properly.

Resolution:
I had the holes in the rotor enlarged so the rotor would sit around the studs, and flat against the hub. It helped a lot, but the problem was still there.

Issue 02.
After the studs issue, it was better but not fixed. I guessed because of how they sat on the studs for so long I may of warped the rotors. So I borrowed a tool from a friend that could let me measure how straight the rotors were.

Driver side rotor was .004 of an inch off.
Passenger side rotor was .010 of and inch off.

The .004 wasn't too bad I was sure machining the rotor would fix that problem. The .010 was something I really didn't expect. I was measuring the rotor at it's edge. I decided to see how much better the reading would be if I went towards the center more. Too my surprised not much of an improvement. Closer to the center the passenger side rotor was .008 off.

I decided to keep checking on how it was that bad on the passenger side. I took the rotor off and measured the hub. The hub was .004 and it was not rotating smoothly at all. The good hub was measuring .001 if not less. I hypothesized with a bad hub and the rotor was warped even more. I made sure the issue wasn't anything else, so I swapped rotors around on the front and measured. Sure enough when the passenger side rotor was on the driver side hub (the good hub), the readings improved.

Resolution:
I replaced the hub. I also had the front rotors machined. I did not take any reading after the machining though.


So the rotors not sitting properly was causing the rotor to push the pistons back in the caliper. That was causing the horrible pedal feel. The bad hub and warped rotors explains why it was shaking so badly when I braked.

So machining the holes in the rotor to clear the arp studs was part of the solution, the other was replacing the warped hub. Machining the rotors also helped fixed the problem.

I pressure bled the system once more, and I pumped the brakes while I pressure bled also. It took 20 mins to pressure bleed. The pressure bleeder was worth every penny.

I drove it for a good hour and the pedal feel did not fluctuate. I did the bedding in procedure again since the rotors were machined. The brakes are working really well.

I hope to get some time behind the car now so I can get a better feel for the brakes. It's great to have properly working brakes again. It make the car that much more enjoyable to drive.

good stuff.. someone will benefit from this if they follow you steps..