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Mushy brakes


Slartibartfast
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Yep! Picked up a booster today from a '93 4x4 HB, which I'm pretty sure had the four-banger in it. Same diameter, different sticker, though one of the numbers matches. It bolted right in and the output rod doesn't have the 1/8" of lateral play that my old one does, so we're hopeful. Tomorrow we'll bleed the SOB (ran outta time today) and see if anything has changed.

Also to update my last post, we tried a wrecking yard caliper in place of the passenger's front reman and it didn't change anything. Guess that's the upside of throwing parts at it... you end up with lots of spares.

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My dad and I bled the brakes tonight. My dad noted that he could pump up the pedal against the air bubbles, which it wouldn't really do before. It also wasn't making nasty popping noises like it was before. It still made a little groaning noise from the check valve each time the pedal hit the floor, but the valve I got with the new booster makes the same noise if you blow through it just right, so I suspect they just do that, same as the rollover valve on the evap line from the fuel tank. We went in Nissan's bleed order, but also bled by cracking the fittings at the master, and got a little air out of each one. The pedal felt good after that but the rear brakes still weren't doing much. They'd stop the wheels, but I could push the tire with my foot and turn the wheel. My dad suggested we take it out, romp it, then try bleeding it again in case the rear circuit still had air in it. We took it out, did a few panic stops, and managed to lock the front wheels on dry pavement, which I'd never gotten this truck to do before. The next bleed got a bunch of air out of the driver's rear caliper. We tested it again with the wheels in the air and hallefrickinluyah--the rear brakes are back! :dance:

 

I tore apart the old booster (straighten the waves in the ring around the back and it all comes apart without having to cut anything) and found that it was surprisingly clean and nice inside. The rubber diaphragms (it has two, which I wasn't expecting) looked brand new. The play in the output rod is supposed to be like that, near as I can tell; nothing on the output side was obviously damaged. Given the different part #s I suspect the one I put in is just a slightly different design. Some of the boosters at the wreckers had that play and some didn't. The one glaring issue I found was the internal air valve that makes the booster work. It was dirty (the only dirty part in the whole booster) and it caught and popped in its bore when I worked it back and forth. When you press the pedal, the input rod compresses a spring, pushing this valve spool into the plastic chunk it lives in, which allows air from the cab to fill the rear chamber of the booster, pushing the diaphragms into the front chamber (which is under vacuum). The plastic chunk pushes forward against the output rod, both from the force applied to the pedal and from the force applied to the diaphragms.

 

I'm not 100% sure how this cluster unfolded but my best guess at this point is that the booster inhaled something it shouldn't have, which I pushed deeper into the valve while bleeding the brakes, which made the valve bind and catch, which limited the effect of the booster, which limited brake pressure, which wasn't high enough to activate the prop valve, so the rear end didn't get enough pressure to do much of anything... and what I thought was the master bottoming was actually the the valve catching in the plastic and trying to push the brakes directly without vacuum to back it up.

TL;DR the booster was hooped and my brakes are back! Thanks to everyone who followed this clown show and threw suggestions at me. Hopefully this thread saves someone else from spending two months pouring brake fluid on the floor.

Edited by Slartibartfast
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Well then That is where I will start with my 1990 pathfinder. I was hoping your problem did not have anything to do with ABS since my 90 does not have it. I will have to locate a Booster for my 90 and see if that fixes it.

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I'm glad it's over LOL. If my truck can stick to easy-to-diagnose breakdowns for a while, that'd be great.

 

Msavides, good luck! Hopefully yours responds to the same treatment. You'll have to remove the master entirely to get the booster out, and even then it's a squeeze. I freed up a lot of room for maneuvering the booster by removing the siren for the car alarm.

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:headwall:

 

It appears the celebration was premature. I got in the truck tonight, started it, and felt the pedal bottom out again. I jacked up the back and the brakes will stop the rears at idle (just like last night), but as soon as I tapped the gas, the bastards started spinning again.

I don't know if I was distracted last night by how well the fronts were working or if I just saw what I wanted to, but I think I'm back at square one here. The new booster solved the weird noises and the front brakes work way better now, but there's clearly still something very wrong with the rear circuit.

 

I had another look at one of my spare masters and there appears to be no way for air to leave the prop valve. All ports are below the centerline of the bore, and the bore is above the bore for the pistons. This makes me wonder if the master was just designed by gibbons and a couple bleed valves at the top of the prop would allow me to bleed the SOB properly and get the brakes back. Then again, swapping the rear line to one of the front fittings before didn't give me good pressure at the back either, so I'm not entirely convinced the problem is in the master.

 

I haven't checked the small flex lines to the rear calipers yet. I'll take the rear wheels off, have someone press the pedal, and see if they bulge.

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Dude, maybe it’s a bad caliper in the rear?!? Or like you said, the soft lines.

 

When ever you find the culprit, you should destroy it in some creative way and post pictures; like melt it in an acid and record it in a time lapse.

 

Or melt it down and make it into a knife so that you can recoup the heartache it caused with the joy of a new knife.

 

...sorry I get overly angry at inanimate objects... my dad was the same way.. it might be a genetic defect.

 

 

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I think I have the same genetic defect. It appears I'll have quite a pile of junk parts by the time this thing's back on the road.

 

A friend and I checked the ABS again today. Once again, the actuator appears to function perfectly. Messing with the isolation valve (effectively cutting off the rear system past the ABS valve) confirmed that the sponginess is not between the ABS block and the rear rotors. After cycling the ABS, we were able to bleed a little more air out it, but the pedal is still garbage and the rears still barely do anything.

We bench-bled the holy hell out of one of my spare MCs for an experiment. We bled it level with bleed plugs until air stopped rising through the res. Then we tilted it forward and got a bunch more air out. When we could no longer get air out with plugs, we switched to hoses (I have a few different bleed kits to choose from thanks to the multiple MCs this thing's gone through). More air came out. Some probably entered when we switched bleeders, but much of what came out was in tiny bubbles, like it had been stirred up during earlier bleeding. We bled that out, left it in the vise for two hours, checked it again, and then immersed the whole thing in brake fluid and took the plug off the end of the prop valve. What comes out? More bubbles.

Until this point I've figured that if everybody else manages to install these without issue, I must just be screwing something up, or something else in the system is causing problems. If this master won't even bench-bleed, though, I'm out of other things to blame. I'm also remembering my uncle (who rebuilds cars for a living) having a hell of a time bleeding the master on my '95 when he replaced it--and those brakes were never quite right again either. (I don't think they were this bad, but I never checked the rears.) The OE masters probably bleed better than this aftermarket crap, but of course this issue started (near as I can tell) when I had the system open and air found its way into my OE master, and refused to leave.

 

So, new plan. I'm trying to find a MC off something else that'll fit this booster, with no stupid built-in prop valve (I'll run an aftermarket one in line if I have to), the same 1" bore, and three M10x1 ports. Bolt spacing isn't usually listed but can likely be adapted. If I can track down the fittings to split one front port into two, I've got a couple solid contenders.

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I had a look at that one, but it looks like the reason he went for the larger bore was to match the larger pistons in his front calipers after the SAS. A larger bore on an otherwise stock system would probably just make the pedal harder and the brakes weaker. It also has a Nissan prop valve on it it, and at this point I'm inclined to avoid anything associated with those three words.

 

I'm actually looking at a Corvette-style universal master. They've got two fittings on each side (and plugs for the ones you don't end up needing), so I wouldn't need to figure out a splitter for the front hard lines, and I'd only have to re-bend one of them to reach around to the other side. I'll need some thread adapters, obviously, and I still need to see what's available there. If it needs a prop valve, I'll splice it into the rear line where it hits the firewall so I have a convenient place to mount it. The downside to the Corvette MC is that the bolt pattern to the booster is just a little off (stock is 80mm or about 3 3/16, the Chevy is 3 3/8"). Looks like the holes are slotted, though, and I can always file it if needed.

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You ever realize you're making something harder than it needs to be?

 

If a truck has an LSV, it doesn't need a prop built into the master. The RWD Hardbody master cylinder has no prop valve. This is good news--the HB master should bolt right up to my booster, the bore is right, and the ports are M10x1 so I don't need the $60 worth of brass fittings it would've taken to make the Chevy master work. It's only got two ports, so I'll have to use a tee, but I'm over that. I've got parts in the mail.

I'll post part numbers if this works.

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Bollocks.

The 2WD Hardbody master bolts up like it was meant to be there, and I managed to hook everything up (including a Wilwood adjustable prop), but the issue of a crappy pedal and no rear brakes continues. My dad and I two-man bled it tonight and the brakes are still garbage. The pedal does feel slightly better, and it'll stop the rears at idle, but that could just be because I've got the new prop set as high as it'll go. Breathe on the gas and it'll still spin the rears.

The prop wasn't the issue.

Something is holding air. Previous tests suggested it's not in the system behind the ABS block, and one of the mechanics who looked at it said that bypassing the ABS block didn't fix anything. The only things between the ABS block and the prop valve are two sections of hard line and one fitting to connect them. Guess I'm opening that up next, but hell if I know what would've gone wrong with two pipes and a union.

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So I don't really know why I never asked this before, as it's one of the things I normally check and verify myself, but a resolution post in another thread reminded me...

 

Have you pulled the drums and visually check that everything was good on both sides of the back end. Are the adjusters working property, shoes, springs, levers, etc in good shape... or pads if they are disks.

Is it possible that one of there rear cylinders/calipers seals are bad and sucking air back in when brakes are cycled?

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I have an extra MC if you want it--aftermarket for rear disk.  I bought it when I swapped to rear disks and removed it for the 300zx master when I threw a dana 44 up front. I had also replaced the booster too thinking after replacing the drum MC for the disk MC and didn't notice too much of a difference between pedal feel. I have always suspected that the aftermarket knuckleheads making disk and drum MCs used drum proportioning valves for the disks.

Also you might want to try and redo your bleed sequence.

I did ABS module first, and then LR, RR, RF, LF.

Now If that doesn't work you could try the traditional method of farthest to shortest.. LR, RR, ABS, RR, LF.
The rears do not really matter which you do first since they're T'd off from the middle above the diff.

 

 

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Thanks guys--yeah, I would not be surprised at this point if I just cracked a flare or something while doing lines/calipers on the front and it's been screwing me over since. I did some testing last night with bleed plugs (made some proper ones with flared ends and nuts, not that plastic crap) and found that the pedal's hard with the master plugged completely, and still hard with the prop installed, but it went to garbage as soon as I hooked up the front circuit. I'm starting to wonder if the front circuit is what's borked--near as I can tell the rear piston is actuated by the pressure in the front brake system (there's also a spring, but I don't imagine that spring is putting several hundred PSI to the rear circuit), so if the front system is sucking air, maybe that's why the rears don't work. Clearly I have more testing to do to try and narrow down where the issue is.

Also odd: I was trying the brakes before (rear axle raised, wheels spinning) and when I pressed the pedal, the rears didn't stop. Then the pedal kinda popped a few times, dropped, and then the rears stopped. What do you want to bet that popping was air bubbles leaving somewhere?

I haven't torn down the rear brakes yet (disks) because they worked fine right before I tore into the fronts. And Cuong, thanks for the offer, but I've got two of those SOBs on my bench already.

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I would think if the fronts have air in them  then the pressure is going to go to the path of least resistance.  ( shortest lines first) the air is going to compress easier that the fluid. 

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  • 2 weeks later...

It's a dual-circuit system, and yes, the correct calipers are on the correct sides with the bleeders up top.

 

ABS is now bypassed, no change. The line between the prop and where the ABS was has been blown out with compressed air and didn't spit up anything unusual. I didn't blow out between there and the flex line yet. I bench bled the master again, hooked it up, then my dad and I bled the hell out of it. The rear flex lines show little to no squirm when the brakes are applied, though the calipers do clamp (I removed both, put them back on, and they pushed the pads back up against the rotors). Blocking off the front brake port at the master does not change the rear brakes at all. Adjusting the prop valve does seem to have some effect, but the rears still suck no matter where the prop is set. When bleeding, the system sounds like it's pushing air through something, but no air escapes the bleeders, and the pedal does not pump up.

I'm ordering a brake pressure gauge. We'll hook that to the master, determine the pressure available to the rears from the master, then check after the prop, then work our way down until we figure out what's eating my brake pressure.

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Thanks guys--yeah, I would not be surprised at this point if I just cracked a flare or something while doing lines/calipers on the front and it's been screwing me over since. I did some testing last night with bleed plugs (made some proper ones with flared ends and nuts, not that plastic crap) and found that the pedal's hard with the master plugged completely, and still hard with the prop installed, but it went to garbage as soon as I hooked up the front circuit. I'm starting to wonder if the front circuit is what's borked--near as I can tell the rear piston is actuated by the pressure in the front brake system (there's also a spring, but I don't imagine that spring is putting several hundred PSI to the rear circuit), so if the front system is sucking air, maybe that's why the rears don't work. Clearly I have more testing to do to try and narrow down where the issue is.
Also odd: I was trying the brakes before (rear axle raised, wheels spinning) and when I pressed the pedal, the rears didn't stop. Then the pedal kinda popped a few times, dropped, and then the rears stopped. What do you want to bet that popping was air bubbles leaving somewhere?
I haven't torn down the rear brakes yet (disks) because they worked fine right before I tore into the fronts. And Cuong, thanks for the offer, but I've got two of those SOBs on my bench already.

Id check with some local auto parts stores and see if it’s possible to rent a pressure bleeder or a vacuum bleeder. I had issues with getting the clutch and brakes on these things with mushy pedals. I had a small hand vacuum pump, so, I made a hand vacuum bleeder. It worked awesome. Sounds like you’ve just got some trapped air in the system and either the vacuum or pressure bleeding process will eliminate that.


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I would hope that one of the mechanics this thing had been to would've tried that, or the gallon (at least) of brake fluid I've put through the system over the past few months would've dislodged whatever's still caught in it just by brute force, but it's not like anything else makes sense at this point. I've got plenty of spare reservoir caps so I modded one today to take a valve stem. I'm not sure how much pressure the res will take but I've got spares of those, too. I'll give it a go while I'm waiting on the pressure gauge.

 

I did try a mityvac, but the threads on my bleeders aren't what they used to be and the vacuum just sucked air around them (between the bleeders and calipers). I've read you can wrap the bleeders in teflon tape, but haven't tried that yet either.

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17hr day, not thinking straight so forgive if I am stupid :) ...Air gets by the bleeder valves?  Tried replacing them too? And yes you can use Teflon tape on them or at lease I have in the past (not the Pathfinder) and never had a problem.  If air is getting by them under vacuum, then it's reasonable that air is getting into the system because of them too.

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The bleeder threads just push the tip of the bleeder into its seat in the caliper, and the tip/seat are what holds back brake fluid with the bleeder closed. The threads themselves aren't the seal (unless you've got the caliper open and a mityvac on it). I suspect the threads in the caliper are what's wearing, not the bleed screw itself, unfortunately. Good to hear the tape works.

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Tried pressure bleeding tonight with an old inner tube connected to the res cap. I got a surprising amount of air out of the driver's rear (especially considering we two-man bled the SOB twice last weekend), and the rears seemed to grab slightly better (might've just been that I didn't have the wheels/tires mounted up), but the pedal still felt like crap and no amount of pedal pressure would hold the rears against a light tap on the gas pedal.

 

I hooked the front brakes back up (had them unhooked and the port plugged from earlier testing), pressure bled all four (few more bubbles out of the driver's rear), tried the brakes, and after one push they went to complete crap. Looks like the line to the fronts didn't seat quite perfect in the master and fluid blew past it, so now the master probably has air in it again.

 

I hate brakes.

 

I'll be out of town until next week, and the pressure gauge should be here when I get back. Hopefully that'll help me track down what the hell is wrong with the rear system.

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