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Slartibartfast

Mushy brakes

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Man,  You got some patience.  If I were this far into it I would have said F it and replaced all the soft lines on the entire system, Maybe even with braided Stainless steel lines. 

 

I probably will be going through this with my 91.  In fact I already have the Braided stainless steel lines.  Just got my rebuilt master.  The 91 has rear drums and I have already replaced the Wheel cylinders. just need to replace front calipers and I will have everything replaced.  That is worst case. 

 

 

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I have replaced all the soft lines! I had braided steel on for a while but the more I researched them, the less I trusted them. They're uncoated and apparently those ones tend to abrade themselves to death under the braid, and I never noticed a difference in braking performance vs the rubber lines anyway. Switching between rubber and braided lines didn't get the rears working, either.

 

At this point I'll probably end up replacing hard lines.

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talk about puzzling. Might as well replace the booster check valve too lol.

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On 7/18/2018 at 10:45 PM, Slartibartfast said:

I have replaced all the soft lines! I had braided steel on for a while but the more I researched them, the less I trusted them. They're uncoated and apparently those ones tend to abrade themselves to death under the braid, and I never noticed a difference in braking performance vs the rubber lines anyway. Switching between rubber and braided lines didn't get the rears working, either.

 

At this point I'll probably end up replacing hard lines.

been running braided SS lines on my 91 for over 10 years now. no issues so far. I did not feel a difference between the rubber and ss either.  

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I finally got a brake pressure gauge. It's a little fiddly to set up in each location to test (disconnect the line, connect it to the gauge, bleed it, set up the phone to record the gauge, then get in and press the pedal) but it's nice to have some actual data to work with for a change. I started at the master and got as far as where the hard line from the frame meets the soft line to the axle so far. I'm getting good pressure there (I can max out the 1500 psi gauge if I stand on it, and that's with the engine off), so the problem must be downstream of that. Hopefully tomorrow I can narrow that down and find the component that's been eating my brake pressure.
 

I also verified that my prop valve works. I've got it turned all the way up for testing (seems to be passing full pressure), but with it turned all the way down, I couldn't get the pressure over 700 psi. Kinda weird to think about having too much pressure in the rear circuit at this point. Seems like a nice problem to have.

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Posted (edited)

Tested at both sides of the rear tee tonight (one side connected, then the other). I got 1500psi out of both sides. :huh: Maybe all this power bleeding finally shifted a stubborn air pocket? I was hoping to find a smoking gun here.

 

I hooked everything back up, started the engine, spun up the rear end (wheels still off), and tried the brakes. The hubs stop spinning nice and quick, but turn again if I breathe on the gas. I have no idea how well they're supposed to hold against the engine/trans, but after watching some guy on the Facebook group do some kind of brake stand drift around a parking lot, I'm guessing they don't do an awful lot at the best of times. I just expected more than "adequate" from them with the prop turned all the way up.

 

I guess the test is whether the pedal feels right. It feels like crap right now, but I think that's the air that got into the front circuit before (figured I'd be replacing something so I didn't bother rebleeding the front). I'll deal with that and then see if the pedal feels any better.

 

 

Edited by Slartibartfast

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Finally got rid of my mushy pedal on Sunday. New front pads, turned rotors and what really made the most difference was properly adjusting the rears. So nice to finally have a high, firm brake pedal again. It's been years!!

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That must be nice.

 

I bench-bled the master again, reinstalled, and power-bled all four corners. The rear brakes do appear to work (haven't tried it back on the ground yet), but I still get a thump halfway through pedal travel where the pedal builds a lot more resistance suddenly, but only when the engine is running. My theory when the rears didn't work was that this was due to the secondary piston bottoming out in the master. Now that the rears appear to be working, and the pedal is still doing the same thing, I don't know what the hell is going on. Maybe it's the front circuit, but that doesn't make much sense given how easily it was locking up the front brakes before.

I'm confused and I'm sick of this.

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Just to be clear, brakes are still mushy with the engine running?

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I think so? It's been long enough that my frame of reference is kinda shot. It doesn't knock against the floor but it's definitely softer than I'd like. It feels about right after it hits where it goes bump.

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I checked fluid pressures at the passenger's front and driver's rear tonight. Both are getting good pressure (I can max out the 1500 psi gauge on either). The bump persists. If the bump was due to one of the pistons bottoming out, I'd expect to still have a dead circuit, or at least see the pressure in one circuit stay flat when I push the pedal beyond the bump point, and neither circuit does that.

 

I'm starting to think this isn't a hydraulic issue.

 

My suspicion at this point is that what I'm feeling is the input and output rods in the booster bumping together. When I first push the pedal, the input rod moves, moving the air valve that dumps air from the footwell into the rear chamber of the booster and moves the output rod. The vacuum can only push so hard and so far, though, after which the input and output rods meet (bump) and I can push the brakes a little more manually. This would explain why it only bumps when the engine is running; with the engine off, the bump just feels like play in the linkage at the start of the pedal's travel.

WD21 guys: if you press your brake pedal, slowly, as far as you can (engine running), do you get a bump towards the end? I don't think mine used to do this but I'm not sure I ever jumped on the brakes hard enough to find out... I didn't pay that much attention to the pedal feel until something went wrong.

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Just tried it on mine. I feel something there for sure. 

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I just tested 91 SE.  with the engine running  (i must have tried this about 50 times) I slowly pressed the brake pedal, it goes down completely smoothly then just firms up and stops. I even tried to push the pedal harder  still nothing.  I even tried to push my pedal quickly, still no bump. 

 

are there any adjustments on the brake pedal push rod that may have been adjusted?

 

 

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Thanks guys. Yeah, there's an adjustment on the pushrod. I think I've got it about right but will look into it again--I've messed with it quite a bit over the course of this mess so it's entirely possible I've got it set wrong. Even if it was loose, though, I'd expect it to take up the slack before the brakes engaged, not halfway through the throw.

 

My dad noted that the master cylinder itself moves a fair bit under braking force. It doesn't look like the firewall's moving, it looks like the booster is flexing. Having torn the old booster apart I know there's nothing inside that would stop this. I doubt this has anything to do with the weird thump but I do wonder if the pedal would firm up any if I made some kind of brace for it. I did a little research and found a bunch of Miata guys bolting little brackets to their strut mounts with feet pushing back against the end of the master. Kinda funny looking but apparently it improves brake feel. I might have to try something like that at some point... you know, for all the autocrossing I do with this truck.

 

Also I know for sure now that my rear brakes are working! I can lock up all four on dry pavement. The rear end likes to step sideways when it locks, so I'll definitely be dialing that prop valve back down a bit. I'm still unclear as to what was wrong with the rear circuit originally. Best I can work out is that I had a bubble holed up in a crevice of one of the fittings I took apart when I was pressure testing the rear circuit.

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So at least you can drive it now?

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Everything's back to normal now... minus the slightly over powered rear brakes...?

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It's more drivable than it was, but that ain't saying much. The pedal thump is still there, but it doesn't seem to be impacting braking performance, so it's low on my list for the moment. The tires are shot (sidewall cracks), but I don't want to put the new ones on until I've got it aligned, and I can't get it aligned until I figure out why the steering's been bound up like the wheels are hub deep in wet concrete since I installed the Grassroots link.
 

The front end refresh that was supposed to take a weekend has pretty well killed my summer.

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It will be stiffer to turn if you’re just sitting there. That’s kind of normal. It doesn’t steer like my stock 95. But that’s the trade off for not having it wear out every month.

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I think I found the binding in my front end, and it's not the centerlink. I removed the centerlink to check if it was bent or something, turned the steering, and found that it still bound up like crazy towards the middle even with nothing attached to it. Then I remembered that I read a Beer Garage writeup on adjusting that box and had a go at it at the same time as I put the centerlink in. I didn't adjust it very far, and if felt alright when I tested it then, but apparently the box didn't appreciate it. Backing it off to where it started doesn't help, and even backing it way off only slightly lessens the binding, which is not a good sign. After looking at the exploded diagrams in the manual, I'm not sure that adjuster even had anything to do with steering slop in the first place.

 

So, yeah, apparently I'm a dumbass and I roached my steering box. Anyone recommend one reman over another?

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I remember that write up. I think I remember it cautioning against tightening it up  when the wheels were straight.   or was it when the wheels were all the way right or left.   There was a specific warning that if over tightened when the wheels were in the wrong position and you then turned the wheel it would fork it up really good. 

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I had the wheels turned when I set it. I think what might've screwed me is how tough it is to keep the setscrew where it should be while tightening the locknut around it. The setscrew has a screwdriver slot in it, so you're trying to hold a flat screwdriver in place while tightening a nut around it, which of course wants to turn the setscrew with it and screw up the adjustment. I thought I'd figured out how to work around that, but apparently not.

Whatever the reason, mine's right and proper borked now. The replacement should be here Thursday.

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Seems like I remember 88pathoffroad warning to never touch the screw, because the odds of screwing it up were way higher than any minimal improvement you may make.  I'm thinking that his warning came from personal experience, but I may be wrong.  I'm also not sure if it was here or on his old/long gone forum or both :shrug:

/tonsofhelp

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I adjusted mine on my old 95 with the help of Mr. 510 when I was out west years ago. We turned the wheels one turn (don't think the direction mattered). Then tightened the screw in 1/8 turns, then test driving afterwards till it felt good. I never did the adjustment on my 94 but it has tons of slop lol. Having a body lift makes it way easier.

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Posted (edited)

I adjusted mine ages ago. I believe mine was not a screwdriver it was a stud sticking out of the nut to lock it in place, I used a (I think) 4 or 5 mm open end and a 12 mm boxen for the lock nut. I could not tell much difference in adjusting it. 

Edited by msavides

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