2002 Pathfinder - NO Brakes After Changing Pads and Rotors

[AlekAuto]

When I say "no brakes", I don't mean a spongey pedal. I don't mean you have to bottom out the pedal to get braking power. I mean it has NO brakes. 0. It's like the brake pedal is attached to thin air. Brake light is on. This leads me to believe possibly air got into the ABS module?
You can build up some, extremely minimal pressure with the engine off. Like, "I definitely wouldn't drive this" minimal pressure, and it loses that pressure if you keep it depressed until you're at the floor. And then once the engine is on, any semblance of resistance in the pedal instantly goes away. Master cylinder? Worked absolutely fine when it was parked.
Vehicle sat for a few months, I changed the pads and rotors in the front, and on the passenger side, I didn't crack the bleeder before compressing the caliper piston because I'm an impatient bastard who didn't wanna get up to grab the brake bleeding bottle.
I didn't perform a fluid flush after this, and I plan on doing so on Monday, but the situation in which not doing the flush when using the bleeder bottle on one single caliper and topping off the brake fluid could result in NO brake pedal whatsoever is strange and esoteric to me.
No leaks, fluid level never changes.
I should also note, I recently changed the valve covers out, as well. Maybe something I forgot to reconnect when doing that job? You have to take the intake manifold off. I can't imagine what could cause NO brake functionality, seeing as the only thing I could imagine forgetting to reconnect is the brake booster, and that'd result in a hard brake pedal. Everything I know can cause these issues causes a spongey pedal or extreme travel, NO brake pedal while having a full master cylinder reservoir is unheard of for me. Basically makes it impossible to diagnose when the symptom is "nothing happens for no apparent reason". Except I guess the behavior when the engine is off feels kind of like how a bad master cylinder feels?
Rear drums needing to be re-adjusted? Wheel cylinders leaking? Master cylinder's completely f***?
Basically my flowchart right now is:
Do the full fluid flush, starting from the proportioning valve and moving up to the front driver side
Redo the shoes/wheel cylinders if it's still a problem
Replace the master cylinder, and if I can see a leak from the back of the master cylinder, replace the brake booster as well.

After that, I'm out of ideas. I'd like to hear if anyone has any better ideas than me or ways I can actually properly troubleshoot it before I just start throwing the parts cannon at it, though. Could there bed something wrong with the pedal itself? That seems difficult to imagine, seeing as you can build some resistance when the engine is off.
​​​​​​​Thank you for any of your ideas.

[mdmellott]

Sounds like the master cylinder is shot. Often is the case, when brake systems are not maintained with periodic fluid flushes, a build up of gunk on the master cylinder piston, from the worn out and contaminated brake fluid, is about the only thing keeping up the brake pressure when you press the brake pedal. When you depressed the front brake cylinders to replace the disc brake pads, more worn out and contaminated brake fluid was pushed into the master cylinder, breaking up some of that built up gunk on the piston. Like pulling off a big scab, the master cylinder is now bleeding internally. The same worn out and contaminated brake fluid will wreck the wheel cylinders as well.

[AlekAuto]

Sounds like the master cylinder is shot. Often is the case, when brake systems are not maintained with periodic fluid flushes, a build up of gunk on the master cylinder piston, from the worn out and contaminated brake fluid, is about the only thing keeping up the brake pressure when you press the brake pedal. When you depressed the front brake cylinders to replace the disc brake pads, more worn out and contaminated brake fluid was pushed into the master cylinder, breaking up some of that built up gunk on the piston. Like pulling off a big scab, the master cylinder is now bleeding internally. The same worn out and contaminated brake fluid will wreck the wheel cylinders as well.

The response I knew was true but was trying to weasel my way out of lol
I should just cave and get a new master cylinder, redo the shoes+wheel cylinders while I'm at it because they've literally never been replaced. I just kinda wanted to be done fixing things, ygm?
Unfortunately, it looks like the rebuild kit was discontinued and nobody's stepped in on the aftermarket side. Now I gotta decide if I wanna gamble with a used master cylinder, or with an aftermarket one. Tough decisions.

[Mike W.]

Probably a little late, but you've got air in the system. How, where, why doesn't matter, you've got air in there and it needs a good bleeding. Pump pump pump. Bleed, bleed, bleed.

Now, there is a but... Once I had a Toyota Celica, didn't bother to crack the bleeder to change the brake pads and ended up with a bunch of junk in the M/C. Which I then had to replace. But it was driveable, I had pedal, it would just sink to the floor in time.

[VStar650CL]

Probably a little late, but you've got air in the system. How, where, why doesn't matter, you've got air in there and it needs a good bleeding. Pump pump pump. Bleed, bleed, bleed.

Now, there is a but... Once I had a Toyota Celica, didn't bother to crack the bleeder to change the brake pads and ended up with a bunch of junk in the M/C. Which I then had to replace. But it was driveable, I had pedal, it would just sink to the floor in time.

Quite right about the OP's issue, but I don't think I've ever cracked a bleeder doing brake pads unless the owner had "topped" the reservoir instead of recognizing that the pads were worn and replacing them. What causes master seals to crap out after a bleed is pushing the pedal all the way to the floor. Never do that, go halfway and stop. A ring of crud always builds up in the bore of the master at the point where the pedal normally stops. Pushing the pedal to the floor causes the seal cups to overrun that crud, which can tear or abrade them. Limiting the bleed push to halfway prevents that.