Mushy brakes

[msavides]

I'm a bit late to this thread but wow! I had a similar problem as to what you are describing, but after swapping my MC all my issues seem to be resolved...

 

Considering I just went down to my local O' Reily's and had them order a new one, I'm glad the one I got was the correct one and not for the drums as you said. My rear brakes definitely don't work near as well as my fronts, I'm assuming the ABS system is at fault for that... now if only I could find that damn computer lol.

 

I believe it is under the bottom of the center dash cluster under the radio at the very bottom behind the shifters. I have a 1991 SE and mine is not under the seat either.

 

I believe they moved the abs computer under the seat for newer models with air bags and put the air bag computer in the place of the ABS computer on these later models.

 

I had a problem with my ABS light being on and it ended up being The ABS pump. Once I replaced it the light went off. Try unplugging the abs pump and see if it changes anything

 

 

Edited May 14, 2018 by msavides

[Slartibartfast]

Unplugging the ABS brick (not really a pump, it's just two solenoids and an accumulator in there) would open-circuit both solenoids, which would keep the light on. Wouldn't surprise me if they moved the module around between years. (I don't have an FSM between '89 and '94 to confirm.) If they originally had it in front of the shifters, but relo'd it to clear something, it was probably the ducting for the optional rear passenger footwell vents. I don't think the US-market WD21 ever came with airbags.

I wish I had an update on mine, but I don't.

[msavides]

Unplugging the ABS brick (not really a pump, it's just two solenoids and an accumulator in there) would open-circuit both solenoids, which would keep the light on. Wouldn't surprise me if they moved the module around between years. (I don't have an FSM between '89 and '94 to confirm.) If they originally had it in front of the shifters, but relo'd it to clear something, it was probably the ducting for the optional rear passenger footwell vents. I don't think the US-market WD21 ever came with airbags.

 

I wish I had an update on mine, but I don't.

 

Miy abs light would come on after about a min after starting, when I unplugged the Modulater, pump, whatever they call it (seen it called a few different names) it would come on immediately. I just took a stab in the dark and replaced it. It took a few tries to get one from a junkyard, but I finally got one and it fixed the problem.

 

yea your right air bags I don't think were until 97ish. I seem to remember some box just above the floor vents . but I can't remember exactly. I know there is not anything under the passenger seat though.

Edited May 15, 2018 by msavides

[adamzan]

The abs light on my 95 would randomly come on and off, without any real rhyme or reason. Then after replacing a leaking brake line I bled all the brakes. The light never came back on.

[Slartibartfast]

Mechanic says he's ruled out everything but the master cylinder, so he's replacing that again.

[RedPath88]

Can your luck with MC's really be that bad?

[Slartibartfast]

Seems suspect to me too, but it's either the master or the booster, and it's a bizarre failure from either component. I am increasingly suspicious of the booster.

[adamzan]

I'm not sure how the brake booster could make only the rear brakes not work.

[Slartibartfast]

Mechanic replaced the brand-new MC with another one. Changed nothing. There's a shock.

It's not the master. It's not any of the hydraulic system from the master down because the pedal acted the exact same with all three ports plugged off. That leaves the booster and the brake pedal itself, and I think I'd notice if the pedal pivot was tearing off the firewall or something.

I don't know why the fronts are getting pressure while the rears aren't. My best guess there is that the prop valve is sealing off the rear brakes because there isn't enough fluid moving to work all four. My guess for why there isn't enough fluid moving is that one of the pushrods in the booster has come out of its cup (or whatever's supposed to retain it) and is being forced sideways instead of extending out and pushing the master pistons like it's supposed to.

This is of course a wild-ass guess. I have no idea how the prop valve works and I've never cut a brake booster open to see if what I suspect is even possible. At this point, though, I'm just about out of things it could be. I'm also remembering an odd noise I heard at some point during the bleeding process that sounded sorta like a fork flopping around in a can. I was not able to replicate the noise, so I didn't think much of it at the time, but now I suspect it might've been the sound of a pushrod striking out on its own.

[Slartibartfast]

Got my truck back. Mechanic's best guess at this point is a bad soft line to the rear axle. I can see his reasoning, but if that line was the problem, the pedal should've firmed up when I deadheaded the system at the ABS block, or when I had the master plugged off.

Tonight I'm going to make a block-off plate to bolt to the booster where the MC is now. If the rod has nowhere to go, and the pedal still goes squish, I'm calling the wreckers tomorrow to see what they want for a booster.

[Slartibartfast]

I unbolted the master, slipped a piece of angle in there to block the pushrod (with a washer welded in the middle so the rod couldn't slip sideways), and bolted that into the master's place. The pedal's got a little free play (opening the vac valve in the booster, I'm guessing) and then it's hard as a rock. No squish at all.

That rules out the booster rods flopping around. Maybe the rod is adjusted wrong and it isn't hitting the master, but I think I checked that before, and I can't imagine how it would've adjusted itself. (I guess something could've broken out from behind the rod, but I didn't hear anything rattling around inside when I shook the booster.) If I assume the master's good (and how the hell could I have gotten four duds in a row), this calls my previous tests into question. Maybe the bleed plugs aren't meant to handle full braking force and leaked enough to perfectly imitated a fault elsewhere.

Next step is to re-check the soft lines. Maybe the mechanic's onto something.

[Slartibartfast]

Soft lines are fine. No ballooning.

My dad's theory is that the prop valve built into the master is screwed up and it's bypassing fluid that should be going to the rear brakes back into the res. Apparently the wrong bleed order can kick in some kind of safety feature where the valve shuts off fluid to the line it thinks is breached. This could explain why the first MC I tried to put on locked up and wouldn't even drip fluid out of the connection for the rear brakes. I would not be surprised if my attempts at gravity-bleeding (and certainly the reverse-bleed I tried) confused the hell out of this valve, and I also don't know that the mechanic knew Nissan's ass-backwards bleed order when he installed the master that's on it now.

 

We're going to two-man bleed the SOB this weekend, in the proper order, and see if that gets us anywhere.

[adamzan]

So if you went out now and put vice grips on the rear flex hose (the one from frame to the T on the axle), you're saying it still has a soft pedal? Try at each end of the hose to see if there's an issue you can't visually see? I'm running out of ideas.

 

Couldn't hurt to grab a JY master that is OEM and try that. Not sure how you could have gotten 4 bad ones, but I got 3 bad after market coolant temp senders so anything is possible.

[Slartibartfast]

Yeah, with the rear line clamped off, the brakes still suck.

Given that the brake master in my '95 leaked in the same place, and the newest factory one I'd find at the wreckers would be the same age as that one, I'm leery of getting a used one. New from Nissan is $260. I would just rebuild the factory MC (it looks brand new inside) but Nissan wants $80 for the piston kit.

I also can't believe at this point that every master cylinder I've tried to install was trash right out of the box. It sounds like most guys just throw one in and they're good to go.

I'm running out of ideas too!

[RedPath88]

I'm sure you've already done it, may have even mentioned it her... Have you inspected every mm of the hard lines and all hardware for kinks, bents, holes, cracks, etc that may be restricting fluid flow or allowing it an exit point, however minor, under pressure?

[Slartibartfast]

I don't think there are any damaged lines (barely any rust on this truck and no obvious loss of fluid or wet spots) but I'll check again for kinks etc when I'm under it tomorrow.

[Slartibartfast]

Proper two-man bleeding didn't magically fix it. Passenger's front bleeder made a noise like it was passing lots of air but only the smallest of bubbles were coming out, at least as far as I could see. We bled that one for quite a while and it still made weird noises (along with the one or two tiny bubbles per pump). Finally I said screw it and reverse-bled that one, assuming air was caught in the line where it crosses the firewall and was rising between pumps. My dad watched the master and saw that a lot of clear fluid and a couple of very small bubbles came out. Didn't change anything, of course. Swapped the stock caliper onto the driver's front again because neither of us liked how floppy the reman was, but, again, no change. Rears still get bugger-all pressure but bleed just fine. No signs of damaged lines or swelling hoses.

 

Given the garbage that came out of the right front caliper the first few times I bled it, and the weird bleed issues with it now, and how the brakes went to crap when I replaced the front calipers, we're thinking that caliper may be junk (and the prop valve's trying to compensate). I never tried putting the old one back on, and I'd sent it off by the time I realized something was wrong with the brakes.

 

I replaced the front brake hoses shortly before doing the calipers, so for all I know, it could be one of them (though, again, I don't feel anything out of the ordinary with them). I still have the front hoses that were on it before and may test it with those quickly before laying money down for a caliper.

[onespiritbrain]

I thought I read somewhere up there that you were able to produce a stiff pedal, what condition was it?

 

It looks like this is a brute force repair so it might be good to make a list of every single thing you’ve tried so far..

 

 

Sent from my iPhone using Tapatalk

[Slartibartfast]

Looking back at it, I got a hard pedal from installing the master with all its bleed plugs in (I misremembered this later, apparently) and then again more recently with a piece of angle iron across the front of the booster in place of the master to rule that out.

 

Brute force is right. I'm tired of thinking about this mess right now but I'll see if I can get a blow-by-blow together soon if the next thing I'm going to try doesn't fix it.

Today I opened up the drum master I had on the bench (because what else was I gonna do with it) and think I figured out the prop valve. Fluid pressure from the front brake piston (teed off from where the front brake lines connect) presses a plunger, opening the prop valve and allowing pressure from the rear piston to flow into the rear brake system. The rear brake piston is not solidly linked to the front piston (there's a spring in between them), so the front brakes move with the pedal, and the pressure developed in the front circuit (not necessarily the travel of the front piston) determines how much fluid passes into the rear circuit.

 

This tells me two things. First, it tells me that a hydraulic failure in the front brake circuit would also take out the rear brakes. Second, it tells me I've got a hydraulic failure in the front brake circuit. It's building pressure, but not enough to open the prop valve. Something is compressing (air in a caliper) or ballooning (bad soft lines).

 

I ruled out the front lines before because both squirmed the same amount under brake force and neither had any obvious damage. Clamping off the right front line (which would isolate a bad caliper) has no effect on the pedal. Clamping both front lines close to the top raised the pedal about an inch! I hadn't considered the possibility that both of them were junk straight out of the box.

I'm considering it pretty hard right now.

Edited May 29, 2018 by Slartibartfast

[adamzan]

I just checked my master cylinder on both my pathfinders on the weekend. Both appear to the be the factory units. I hope I never have to change them because if this @!*% show happens to me I might burn it in the driveway.

[msavides]

Wow ,, I think I am pretty lucky then. I replaced my leaking master on my 1991 se and had just as firm petal as always. It was a discounted Master from Rock auto. I think I paid $39 for it.

[onespiritbrain]

I’m not sure if the fitting on the end of the front brake lines is pipe thread, bolt thread, or a banjo but I wonder what would happen if you plugged/capped them off right before the calipers and also have the rears plugged at the master. Then switch it to test the rear soft lines. Rule those things out..

 

This is brutal. I am always worried about something like this happening when I am changing brake pads. I actually don’t ever bleed the brakes because of this stuff.. I had a civic that never was right again after bleeding the brakes to change pads.

 

If you could get set up to where you could test one item a time or something..? It’s really tough because like you said if the master is sensing a problem somewhere and causing different conditions while your trying to track down an actual problem... sheesh!

 

I just plain old have no experience working with brakes. Dude it might be a benefit to use one of those websites where you pay to talk to someone who specializes in the exact area of your problem and is specific to Nissan. Then you could tell them everything exactly as you have already done it and if they’re worth their salt they’ll know the right path forward.

 

I will be researching and thinking too man.

 

 

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[Slartibartfast]

I've worked on brakes before, and it's usually gone alright (except when I stripped a bleeder on a friend's van), which makes it that much worse that this one's kicking my ass.

 

I swapped both front lines for the lines I had on it before. Pedal still sucks. Thinking about it, the rise I got in the pedal from clamping the lines was probably just because fluid wasn't going into the calipers.

I tried blocking off both front ports on the master, thinking that surely this would make enough pressure to make the prop valve engage the rears. It did not. I reconnected the left front and disconnected the rears (I'm bleeding the connections at the master each time I open one) in case the rear circuit was the problem and the pedal still sucks.

 

I'm back to staring at the master, booster, and pedal box. Can't find anything wrong with the pedal box, but the pedal does have an odd little pop when I'm pressing it down, like there's something wrong with the pivot, or like something's catching. I can't see anything wrong from the footwell. Master, well, it's master #4, so on that alone I'm inclined to say it's not that, even though the symptoms seem like the master's all it can be. The booster felt fine when I tested it with the angle iron but with the weird pedal feel I can't shake the suspicion that something's gone wrong inside, some plastic ball end has split or something, and it's limiting travel or something.

 

Also, disregard how I thought the prop valve works. If it worked like that, two-man bleeding wouldn't work on the rear brakes. So, yeah, I have no idea how it works at this point, which is really not helping me diagnose this pig.

[msavides]

I reread your post from the beginning trying to determine when you first experienced the soft pedal.

 

Only thing I can think of is that you may have one or both bad calipers. Since the pedal was fine before replacing those. You also said that when you release the brake pedal the calipers have quite a bit of space between the pads and the pistons. That is not right. The piston should still be resting on the pad with the pedal released. Do both front calipers do this or is it just one?

 

I have had a leaking master before and besides running out of fluid there was no performance issue with the master.

 

 

[Slartibartfast]

I pulled the booster out again tonight and had another good look at it. Got the adjuster to move, figured out how to set its length, couldn't find anything wrong with it, put it back. The guy at the parts store told my dad a bad vacuum booster check valve could somehow introduce air into the braking system (push the pedal with the engine off and the booster pushes air past the seals or something), but given that my check valve seems to work and there's no air seal between the master and the booster, I ruled that out.

 

The driver's front caliper that was letting off too far is off the truck, and the original is reinstalled in its place. I'd ruled the passenger's side caliper out after clamping the hose changed nothing, but then I pulled off the master again tonight and found a lot of air in it, which would explain why blocking various things off was not getting me anywhere. I bench-bled it again (they're a real pain to bleed, the prop valve is mounted higher than the main bore for some reason and there's no easy place for the air to escape) and installed/tested it with the plugs in. The pedal felt better. It would fall if I stayed on it, but the mess in the engine bay told me this was just fluid leaking past the bleed plugs (plastic plugs aren't meant to take full brake pressure? Who knew?). I hooked up the rear line, and the driver's front line, and it still felt about the same. Then I hooked up the passenger's front and the pedal went straight to garbage. That caliper is also the one thing I haven't tried screwing with or swapping out so far, so naturally it's the next thing I'm tearing off again.

 

(And yeah, the leaky master in my '95 never caused problems other than fluid loss, either.)