Sinking/spongy brake pedal --- bad ABS unit?

[attofarad]

On a January trip, I noticed that the 2001 QX4 brakes seemed not so great, pedal a bit soft, travel a bit excessive. Getting a bit worse. I decide to lube/adjust the rear drums, since I read somewhere that unadjusted rears could give more brake pedal travel. I was due to do that anyway. I flushed the brake fluid while I was at it (had been 3 years). After that, brakes were slightly worse and quickly got really bad. Re-bled brakes, getting no air and no improvement. Unhooked lines at master cylinder, plugged the ports, and got a good firm pedal, with expected action engine on and off -- so both master cylinder and booster seem okay.

So, it seems that I am down to a bad ABS module. I don't even see how to get access to the bracket below the module, which holds it in. Super tight space to get to heat shield bolts, then bracket underneath that.

Any other ideas what else could be wrong, or is time to say goodbye to the QX4?

[VStar650CL]

If your rear brakes have a load-sensing valve like many of the older Pathies, that needs to bled first and then the rear cylinders. If you bleed the cylinders without bleeding the load valve, you'll have a permanent bubble in the back side of the system.

[attofarad]

If your rear brakes have a load-sensing valve like many of the older Pathies, that needs to bled first and then the rear cylinders. If you bleed the cylinders without bleeding the load valve, you'll have a permanent bubble in the back side of the system.

Thanks, I did bleed the LSV first, each time I bled the system.

The service manual actually says to:
"Turn ignition switch OFF and disconnect ABS actuator
and electric unit connectors or battery ground cable."
while bleeding. I don't think it will make any difference,
but I'm going to do it again anyway, with ground cable removed.

[VStar650CL]

I dunno then. Bad ABS units don't usually cause a spongy pedal. Pulling or false triggering are more typical. Not that it's impossible, an ABS with blown seals can "bypass" just like a master cylinder and basically cause the metering function to not work. I'd get some plugs the right size for the ABS and block it off just like you did with the master. If the pedal gets hard then it isn't your culprit and the problem is downstream.

[VStar650CL]

Wait a minute, I think I understand this. Block off your master one outlet at a time. It occurs to me that if you have a bypassing seal on only one end of the piston, blocking both outlets will cause the good seal to stop the pedal from going to the floor. So blocking both doesn't necessarily prove anything.

[attofarad]

It seems to me that, if a seal in the master cylinder is leaking under pressure (both ports plugged, brake pushed), the fluid which bypasses the bad seal would return to the reservoir. So no hydraulic pressure would build up, just spring pressure followed by the mechanical stop. Except the seal closest to the booster, which would leak into it. Or maybe I don't understand how it works.

I may pull out the master and disassemble it to check for problems. I do have a new one on hand, but can return it if I don't install it.

[VStar650CL]

I'm assuming your QX4 has a dual-stage master, my understanding is that most Infinitis use them. We don't see them on a lot of Nissans so my expertise is limited, but I'm pretty sure the high-pressure stage can leak back to the high-volume stage at some point before they transition. If both rear seals are good then blocking both outlets won't let the pedal move far enough for the transition to occur, so the problem won't show up. I'm thinking blocking the outlets one at a time should give you movement only when the side with the leaky seal is blocked.

[attofarad]

Thanks for mentioning the 2-stage MCs -- I had never heard of them.

The QX4/Pathfinder has a traditional tandem master cylinder. The piston/spring
setup looks very much like this one,
https://youtu.be/WYB4vOv7Iag?t=353

except there is a slot in the secondary piston
for a screw, which limits travel in both directions,
which can be seen here (QX4 model):
https://www.stockwiseauto.com/images/tm ... 729209.jpg

[VStar650CL]

If it's a conventional tandem then forget what I said. Back to head-scratching.