Gents,

New to the forum, lots of fantastic info in here!

I replaced the master brake cylinder (OEM part) thinking this would solve my brake pedal leak down. While changing, I replaced the rubber lines, back calipers, rotors, full set of pads on all 4. I’m now facing this problem again. No leaks in system.

I’ve found if I get the car (2012 M37x) up to 60mph, hit the brakes to activate the ABS....problem goes away for a couple months.

Possible leaking valve in ABS?

Thank you

P.s. I’ve owned this car since new, used to have everything done by the dealer, but my local dealer had an ownership change and service stinks now (ie getting charged for work that wasn’t done).

When passive, the ABS just lets brake pressure through. A bad valve can cause high effort or uneven braking, but there's nothing inside the ABS block that can cause leak-down of the pedal. Leak-down comes pretty exclusively from the Master Cylinder or Booster. Booster diaphragm problems generally cause erratic pedal effort because of changing engine vacuum, and since you don't mention any of that, most likely you got a bad MC off the shelf. It wouldn't be the first time I saw a torn seal (or in one instance machining debris!) in a parts-store MC. I did see it once in an OEM as well, which I'm pretty sure got dry on the shelf.

Thank you Vstar.....it’s a real odd ball issue. Many years ago I had a master cylinder completely let go in a ‘78 Camaro....blew through an intersection, crapped my pants. $35 for a new one, bled the brakes, good to go. This one....looked all except ABS and diaphram....not thinking it could be the diaphram.

Like I said, booster issues aren't usually consistent. They also can't produce a floored pedal, just a low or high one. The MC is really the only component that can bypass the pedal all the way to the floor.

Did you bleed it in the exact order that the service manual says or the traditional way from the furthest to the MC? The traditional way doesn't work on nissans. There's also a vaccum check valve that goes to the mc. Maybe that's busted.

I had a brake pedal leak down when the car was turned off and it was because i didn't bleed it properly and thought the mc was broken. now it works fine again after bleeding properly.