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  • March 28, 2024, 10:39:58 AM

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Author Topic: mysterious brake issue  (Read 14881 times)

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Offline packotobacco

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mysterious brake issue
« on: May 14, 2016, 12:27:42 PM »
Hi there fellow eagle fans. I must apologize if I'm posting in the wrong place however this was the only place i could find the option to post a question. In any case i have had an ongoing issue with the brake system on my 87 eagle lmt. It all started when i decided to replace/repair all of the hardware for both my front and rear brakes. I ordered new hardware kits for the drums and had my drums and rotors remachined. Afterwards i installed my new and next to new components an did a few runs back and forth to set the tensioner in the rear hardware by braking sharply in reverse. Once the brake seemed to be working properly i took her for a spin around the block. For about three days of short test drives my brakes worked better than ever, and my engine even seemed to perform better. But on one fateful press of the pedal i suddenly lost all hydraulic pressure and had to struggle to stop with mechanical pressure only. At first i suspected that i had messed up with the hardware but everything still seems AOK in that department. After that i pressed the brakes many many times with the engine running to make any potential leaks visible. However after a year, yes a whole year, of troubleshooting i can't find any sign of leakage in the lines or pressure vessels in front or rear. At this point i had to replace my calipers due to stuck bleed screws. Of course the rebuilt calipers leak around the cups, really wish they would check that before selling them but they probably don't care. I also replaced my master cylinder and brake booster just to be sure. So to sum up everything in my brake sytem is new except the rear pistons and they are in great shape for their age, the new pistons leak a bit but i think i ought to get some pressure after a fresh bleed and i still get mechanical pressure only. I will continue swapping calipers until i get a pair that doesn't leak and will update as soon as i can but i need to know if I'm over reacting or if i have bigger problems than slightly leaky calipers. Any suggestions might help, and feel free to ask about what I've tried already.

Sincerely
Packotobacco

Offline mudkicker715

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Re: mysterious brake issue
« Reply #1 on: May 14, 2016, 05:08:54 PM »
Have you checked to see that vacuum is actually getting to the booster.



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Offline johnbendik

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Re: mysterious brake issue
« Reply #2 on: May 14, 2016, 08:42:34 PM »
packotobacco,

In case it wasn't clear from mudkicker's reply, there is no distinction between hydraulic and 'mechanical' pressure; they are one and the same, in that the only way you can apply 'mechanical' pressure is through the hydraulic system.  If your brakes suddenly lost hydraulic pressure, the symptom would be a very mushy pedal, maybe even going all the way to the floorboard.

On the other hand, if your symptom is that your brakes suddenly lost effectiveness, forcing you to have to press extra hard on the pedal to stop, then you have a booster problem, not a hydraulic problem.  And the 'brake booster' on an Eagle is powered by engine vacuum.  The brake booster is the big black cylindrical thing between the brake master cylinder and the firewall, and if it's not hooked up to engine vacuum, you won't have any boost and the brake pedal will be very 'hard'.

Good luck with it,
JB
« Last Edit: May 15, 2016, 10:12:50 AM by johnbendik »
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Offline amcfool1

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Re: mysterious brake issue
« Reply #3 on: May 15, 2016, 09:46:26 AM »
hi, the pp is right, sounds like a booster problem. if you replaced the master cylinder and the booster individually, they may not be adjusted to work properly together. The last time I did this, over ten years ago, I purchased a booster/master cylinder assembly together, from Autozone, to avoid any adjustment problems. good luck, gz

Offline AMC1

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Re: mysterious brake issue
« Reply #4 on: June 16, 2016, 09:59:31 AM »
just a couple of thoughts that may seem even silly to you, seeing as how it went for a year with no problem. I initially thought you may have reversed the rear linings. I had a hard time solving a similar problem until I found a weak spring which wouldn't let the automatic adjuster work. But you did say you used all new hardware. With the engine running you can pull off the vacuum line at the booster & put your finger over it. You'll know immediately if the line is working.
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Offline packotobacco

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Re: mysterious brake issue
« Reply #5 on: September 02, 2016, 09:28:52 PM »
Hello again first of all thank you all for your help and sorry for the length of time between posts. I did misspeak  when i said no hydraulic pressure. I have measured the vacuum at the booster but not recently. I got 17 psi ,or negative psi as it were. I know thats lower than the prescribed range but just barely, unless I'm mistaken. I've replaced many vacuum lines since then and figured this slight lack of vacuum could not cause my problem. Since my last post i have repaired the leaking calipers and still find no other issues with the lines. My master cylinder on the other hand seems to have bad seals as i can hear a swishing sound as the pedal is depressed. And it started leaking from the rear about two months after installation. Fortunately a company has started building new mc's from scratch so i don't have to trust a rebuilt one. I've been through three rebuilt units with no luck. Just to be clear i don't know for sure if the original  failure was vacuum or hydraulic. With my vacuum at 17 to booster i thought mc or booster b
f membrane. After replacing both components my symptoms remain unchanged,  but mc is bad for sure. Can the mc fail instantly and completely without leaking? Could i be bench bleeding incorrectly. And any info on the adjustments mentioned by amcfool 1 would be greatly appreciated.

Offline packotobacco

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Re: mysterious brake issue
« Reply #6 on: September 03, 2016, 09:26:18 PM »
after looking into the pushrod adjustment i realized that when i set it in the first place all i did was ensure that it protruded from the booster by the same amount as the old unit. I then realized that since the booster is new and slightly different shape there is no guarantee that the overall lenth of the assembly was correct. just purchased a truly new master cylinder and set my pushrod to be .020 thousanths short of contact as recomended by info i found online. I owe a big thank you to amcfool1 as the difference was nearly a full inch, and I would never have thought of that on my own. I will install as soon as my helper has time to assist in the bleeding process and post the results asap.

Offline amcfool1

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Re: mysterious brake issue
« Reply #7 on: September 04, 2016, 12:04:35 PM »
hey, good deal! glad you got it sorted out.good luck, gz

Offline packotobacco

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Re: mysterious brake issue
« Reply #8 on: December 09, 2016, 09:16:33 PM »
Hey everyone ,back again with updates and, of course, more questions. Been two months since we bled the system but i couldn't post since my laptop got fried in a power outage. After setting the pushrod we bled the lines and i got full motion in the calipers and pistons and was ecstatic. Put the wheels on and test drove and felt great when the car stopped easily just as it should kept testing for a few blocks and had no issues. Had to park it again to tap two tranny pan bolt holes and refill .afterwards i tested the brakes again and all still seemed well. I parked it again to straiten out some vacuum lines and wait for my tabs to be worth buying as they would have expired after a single month. just renewed and took it for a spin and in perfect line with the luck I've had with this car, my easy powerful stopping ability slowly diminished and disappeared as i limped home with red face and white knuckles. I suspect the rear pistons have finally failed after receiving full pressure for the first time in years. I also need to adjust my idle settings since it wants to go even with out touching the pedal. Looking for new pistons now and after the caliper fiasco i want the first set i buy to actually work so any advice there would be awesome. Also i saw a post about the drum adjustment where someone stated that there could be a clog in a rubber hose before the drum. Been awhile since i checked but i thought the rear lines were all metal. Is there a hose hiding somewhere or was he thinking of the front hoses?

Offline amcfool1

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Re: mysterious brake issue
« Reply #9 on: December 10, 2016, 12:40:27 AM »
hi, the rear brake lines are steel as they run along the axle, but, in the middle, towards pass side, is a flexible rubber line that connects to the steel line going to the front of the car. It is ALWAYS a good idea to replace the soft lines, both fronts and the one rear, if they are over 30 years old. They can deteriorate from the inside, and let little chunks of rubber clog your calipers/pistons, causing your brakes to lock up. This happened to me once , years ago, though not on an AMC.  As for rear pistons, get a name brand, don't go cheap. Personally, I like NAPA parts, but others are ok as well.
As for the MC failing altogether, yes it can, though it's designed not to. Years ago, I had a MC fail  COMPLETELY in an 82 Concord. Just by pure luck, the car was parked at the time in a Home Depot parking lot. Drove there fine, got back in, started out, and NO BRAKES whatsoever, had to walk home and get a tow. Could have been a lot worse, 5 minutes later I would have been on the interstate.
As for your high idle, your choke is sticking, before you adjust idle screws, make sure choke pull off is good.
good luck, gz

Offline amcfool1

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Re: mysterious brake issue
« Reply #10 on: December 10, 2016, 01:10:58 AM »
hi again, just as an fyi, it's a good idea to have your brake fluid replaced every 3 years or so, even on new cars. This is a PITA to do by yourself, but most small good shops can do this for small $$. Whenever I buy an old car, AMC, or even a late model used car for my wife, the FIRST thing I do, before I let her drive it, is have the brake system completely gone through by a professional, and the fluid changed. Good brakes are your LIFE!
Along the same lines, it's also a good idea to change your Power Steering fluid. This is neglected by most people, Just top it up, and all good, right? Not really. The AMC, (Saginaw) PS system is excellent, and can live for decades, but contaminants still get in, and cause wear, and eventual steering box/pump failure. Changing your PS fluid is easy, if a little messy, and you WILL need a helper.
Disconnect the low pressure line at the steering gear. This is the crimped line, NOT the screw in one. Point it at a bucket. Run car. Have helper keep pouring in fresh fluid as it drains, when it comes out looking new, you are done, good for another 5-7 years! You will be surprised how ugly that old fluid is. And, while there, change the lines. Aren't Eagles fun?!
good luck, gz

Offline packotobacco

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Re: mysterious brake issue
« Reply #11 on: December 17, 2016, 04:41:49 PM »
Thanks for the info i'm still looking for leaks because i have just replaced both wheel cylinders and after a fresh bleed i thought i was good because all the brake slave pistons moved with the pedal and the pedal was high and firm. But as soon as i started it the pedal sank and even after adjusting the drums i'm getting no pedal. i keep bleeding this thing all the time so i can't imagine the fluid is old.  The front flex lines were the first thing i tried but when i changed the cylinders i still couldn't find a rubber section in the rear just that corrugated plastic covering. And just so no one worries i am not road driving until the brakes test good beyond doubt. I'm fortunate enough to have a virtually abandoned parking lot at the end of my driveway for testing. Anyway i had really hoped to sort this out myself ,at this point I've put so much into into it I'm on a mission.

Offline vangremlin

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Re: mysterious brake issue
« Reply #12 on: December 17, 2016, 05:26:36 PM »
Man, this has got to be driving you nuts!  I can't offer any other suggestions, but I can point out that the rear brake rubber hose will connect to the rear steel brake lines on top of the rear axle on the passenger side.  Follow the steel line that leaves the passenger brake cylinder and you should find it with no problems. There is a block T-fitting on the end of the hose that sits on the axle, and the 2 steel lines screw into it.  I looked but there isn't a real good diagram in the service manual.  Good luck!
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Offline mudkicker715

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Re: mysterious brake issue
« Reply #13 on: December 17, 2016, 10:01:57 PM »
That is sounding alot like you are leaking from the master into the booster. I mean that really really sounds that way. I did not reread , but that is my 100 to 1 bet. Even if you replaced it. Wow that has to be it. But what do i know. I am just guessing based on experience.



Manitowoc WI

Offline eagleman

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Re: mysterious brake issue
« Reply #14 on: December 18, 2016, 11:49:53 AM »
Just what mudkicker said. Your master cylinder is leaking into your booster and would imagine the fluid has eaten it way through the diaphragm. Replace the booster and master cylinder. Had the very same exact problem with my 86 wagon. After replacing the entire braking system still had the same problem you are describing. Changed both the master and booster and all is good. Brakes work perfect now.
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