I used the search, couldn't find anything similar so...

I was driving at an autocross and while braking at around 80% the pressure seemed to drop off and my foot hit the floor, the ABS turned on and my stopping power dropped to about 10%.

You can currently still brake, but the sensation is weird. When you put you foot to a position and hold it (for example 40% of the way) the brakes bite as normal then drop off to almost nothing and I can feel the resistance in the pedal fall away. If I want to maintain even braking pressure, then I must press at the same tension and follow the pedal till it hits the floor. If I'm still not stopped, I can pull it out and repeat. No leaks. Nothing to do with speed, driving style, heat, car on or off.

I have new slotted rotors and hawk hp+ pads all under 100mi. Stainless lines with maybe 1000mi. I bled the system, didn't work. Took it to my neighborhood shop cause I needed the NY state inspection and had them bleed it, no luck. My mechanic thought it was the master cylinder as you could sort of pump the brakes back up to reload the pressure. He replaced the cylinder with an OEM one ordered from NISSAN. No change. Whats left? Can my calipers be bad somehow?

I could really use help on this...

 

What method did you use to bleed them? The old push-the-pedal trick or a power bleeder?

If you swapped to SS lines, then chances are pretty good you've still got air up in the system. Read this thread... https://my350z.com/forum/brakes-and-...m-brembos.html then find somebody who can powerbleed them.

 

brake booster failure? or leak?
when my s13 when the brake booster failed @ track.. i couldn't stop unless i was standing on the pedal. i heard a huge pop noise and the brake pedal shot back at my foot- and then when i applied force the car wouldnt stop unless i literally jammed it to the floor. but i guess you're not 'fighting' the brake pedal?

what worries me for your case is that you "must press at the same tension and follow the pedal till it hits the floor. If I'm still not stopped, I can pull it out and repeat."

that sounds like air in the lines.. squishy squishy firm pedal feel? i would definitely recheck the bleed procedure. Passenger Rear, Driver Rear, Passenger Front, Driver Front.

if you have brembos, remember you have two bleed screws per caliper. you might want to gently knock the side of the caliper with a rubber mallet to loosen up the air bubbles.

 

yea I would suspect some bubbles too.

how about the condition of the brake fluid? they're probably old. It's best to get fresh fluid while you're at it.

 

Thanks for the input guys. It may be more serious than air in the lines, here's why.
My mechanic came up with a way to test the whole thing. He made some "dead-end" fittings, essentially the end of a brake line with the tube cut and pinched off. Then exhaustively plugged off different sections of the system to see what was leaking. The new master and booster were fine, super firm, when separated from the system. But when blocking off segments of the ABS he found that the side that splits front-left and rear-right from one to two lines drops pressure somehow inside the ABS unit.
Anyone ever heard of that?
Thought I'd ask before I try and source an ABS system...

 

I should mention that in the process of blocking and un-blocking lines the entire system was flushed four times. I doubt a bubble in the lines or my calipers would survive that.
Oh, it's non-brembo btw.