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Thread: How to know for sure if Master Cylinder is bad?

  1. #1
    Bad Mother Flecker Turbo Mopar Staff fleckster's Avatar
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    How to know for sure if Master Cylinder is bad?

    On my '89 LeBaron GT Turbo convertible, the brake light comes on as soon as I start the car and doesn't go off. The pedal basically goes to the floor and unless it's just idling, there's barely enough brake to stop the car. I've checked the fluid level and the reservoir is completely full front and back. According to the Service Manual I have, following the diagnosis flow chart, I could either have contaminated brake fluid (it does look a little dirty, but it has been in there a long time) or the master cylinder is shot. I'm guessing the Master Cylinder. Regardless, if I replace it, the system will get flushed and new fluid.

    The next question is how do you determine exact which Master Cylinder I need? The auto part stores (Auto Zone, Advance, Rock Auto, etc.) show about 10 different ones. 4 door/2 door, non-turbo/turbo, etc. I can figure out but they list "Fluid Level Window", which I don't remember any of my cars ever having, 21mm, 24mm, and 7/8", and several that say rear disc brakes 15" or 16" wheels but they even have several of them.

    My car's Master Cylinder has " 7/8" " cast into it but it is rear non-vented discs and came originally with 15" wheels (now wearing factory 17" wheels).

    Why the hell are there so many applications listed? I know Chrysler and they wouldn't have more than three or so for ALL the FWD cars they made let alone the ten+ that the part stores show.

    Anyone have any ideas on confirming and knowing which one I need? (Part number maybe?)

    John M. Fleck

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    Supporting Member Turbo Mopar Contributor 2.216VTurbo's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    I'd reccomend the larger 24 mm unit over the 21 you have, pedal will be a bit harder but hard is good right I always tell the parts guys 90 Lebaron with 4 wheel disc and usually that comes up as the larger MC There will be a "4" cast into the unit where yours says 7/8. If it says "1" there, it's the same as what you have (21mm = 7/8")

    Careful, don't round off those impossible tight fluid line fittings... Even a crows foot wants to round them off. Count on a bit of heat to loosen them up.

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    Bad Mother Flecker Turbo Mopar Staff fleckster's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Thanks, Alan! Despite me "keeping a close eye on you and your friends" you still offer your advice to help me! What a guy!!!

    I like your idea as I generally try to "Fix it Faster" or in this case "Fix it Better." Why stay the same when you can increase the performance for basically the same price, right? And when it comes to Brakes, yes, harder is better!

    Doesn't everyone want a firm and confident feeling brake pedal? I want to press it and have it give enough feedback and basically stop like I threw out an anchor!

    I'll check it out although I may have to wait until after SDAC-17 as I need to make sure the Spirit R/T (and the tow vehicle Hemi Ram and trailer) is ready to go.

    John M. Fleck

    '83 Dodge Rampage 2.2 w/ 2.5L Turbo 14.79@94
    '87 Shelby Lancer #213 14.99@94
    '89 Chrysler LeBaron GT Turbo 'vert 15.88@92
    '92 Dodge Spirit R/T 13.50@104 ; 13.34@102 on slicks
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    Supporting Member Turbo Mopar Contributor 2.216VTurbo's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Want me to bring you a 24mm to SDAC? When I find them in the yards and they are 'dry' AND I have $40. to spend on one (stinkin CA yard prices! ) I grab them. I have a small stack in the garage...


    Quote Originally Posted by fleckster View Post
    Thanks, Alan! Despite me "keeping a close eye on you and your friends" you still offer your advice to help me! What a guy!!!

    I like your idea as I generally try to "Fix it Faster" or in this case "Fix it Better." Why stay the same when you can increase the performance for basically the same price, right? And when it comes to Brakes, yes, harder is better!

    Doesn't everyone want a firm and confident feeling brake pedal? I want to press it and have it give enough feedback and basically stop like I threw out an anchor!

    I'll check it out although I may have to wait until after SDAC-17 as I need to make sure the Spirit R/T (and the tow vehicle Hemi Ram and trailer) is ready to go.

    AJ (no More Alan) 84 Rampage RT TIII/568 Quaife 87 GLHS dealer optioned Red 16V Masi/568/Quaife
    90 Masi 16V White/Ginger/Black
    89 TC Masi 16V Red/Ginger/Black
    86 GLHS #110 RoadRace Built 89 CSX-VNT Recaro Car
    89 Turbo Mini 'Woody' 85 GLHT 'RedBox'
    2014 Explorer DD'r 3.5Twin Turbo Ecoboost AWD and 500HP
    My profile page has over 20,000 views, I'm somebody LOL

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    Buy my stuff!!!!!!!!!!! :O) Turbo Mopar Vendor turbovanmanČ's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    If you have no leaks, your MC is bad. DON'T use a used one, Russian Roullette. Buy a new one and get one for a 89/90 Minivan, there the 24mm jobbies.
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    Bad Mother Flecker Turbo Mopar Staff fleckster's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    If minivans have them, then I have one in the driveway I could use! (I guess the larger vans have them even without rear discs, huh?)

    Looks like I can get a NEW one from the auto parts store for only $29.52 and they don't even have a core charge! Thanks, Alan, but for that price I think I'd go new!

    John M. Fleck

    '83 Dodge Rampage 2.2 w/ 2.5L Turbo 14.79@94
    '87 Shelby Lancer #213 14.99@94
    '89 Chrysler LeBaron GT Turbo 'vert 15.88@92
    '92 Dodge Spirit R/T 13.50@104 ; 13.34@102 on slicks
    '98 Dodge Neon R/T 15.66@89
    '03 Dodge SRT-4 13.91@98

  7. #7
    Hybrid booster
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    Re: How to know for sure if Master Cylinder is bad?

    Your m/c is leaking internally and going new is probably the best. My experience has been that rebuild kits cost more than a brand new m/c

  8. #8
    boostaholic
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    Re: How to know for sure if Master Cylinder is bad?

    A bad master cyl. will have a geiser in the resivoir when pedal pres. is applied. Normally a good M/C will have a momentary small rise of the fluid level when pedal pres. is applied then drop when pedal is released. Make sure you buy a NEW one and not a reman. I've seen alot of bad remans. lately.

  9. #9
    Hybrid booster
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    Re: How to know for sure if Master Cylinder is bad?

    when in doubt....turn in the old one as a core at the local auto parts store!

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    Hybrid booster Turbo Mopar Contributor CSX321's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Since we're talking brakes...

    The pedal in my '87 CSX goes to the floor (I haven't driven it for quite a while), and there's a hissing-air-like sound as you press it. Is this likely the booster leaking, master cylinder, or what? I've only looked at it a little, but the vacuum lines under the dash seem okay on first examination.

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    See me ride out of the sunset... Turbo Mopar Staff BadAssPerformance's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Fleck, I'd go with the '89 Shelby Daytona big brake car master cylinder. Stimac and/or Still told me that was a good one a while back.

    Dave, you got brake fluid in the master? If the brakes are good even with a bad booster the pedal should not go to the florr. The hissing is possibly just so much vacuum being displaced cuz of the extra pedal travel.

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    Buy my stuff!!!!!!!!!!! :O) Turbo Mopar Vendor turbovanmanČ's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Quote Originally Posted by CSX321 View Post
    Since we're talking brakes...

    The pedal in my '87 CSX goes to the floor (I haven't driven it for quite a while), and there's a hissing-air-like sound as you press it. Is this likely the booster leaking, master cylinder, or what? I've only looked at it a little, but the vacuum lines under the dash seem okay on first examination.
    If it goes to the floor and no leaks, MC is bad.

    Hissing is a sign of the booster leaking, it shouldn't hiss unless your pumping the brakes.
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    See me ride out of the sunset... Turbo Mopar Staff BadAssPerformance's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Quote Originally Posted by turbovanman View Post
    If it goes to the floor and no leaks, MC is bad.

    Hissing is a sign of the booster leaking, it shouldn't hiss unless your pumping the brakes.
    But if it just the booster then the pedal should not go to the floor, right?

    JT
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    Re: How to know for sure if Master Cylinder is bad?

    Yeah if its just the booster pedal effort will be increased followed by the annoying hiss. The SC I just got has been sitting for a long time and the fluid level is full but the pedal goes to the floor. I think its from water though so a flush may bring it back to life. Either way if a car sits you should ALWAYS flush the brake fluid. I'm going to do that to the ram charger after it gets put back on the road to be safe. Any Dot 3,4, and super dot 4 or 5.1 will work. And after I read the article supercrackerbox posted on brake fluid I don't think I will ever use silicone in anything.
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    Hybrid booster Turbo Mopar Contributor CSX321's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    It's been a while since I looked at it, but if I remember, it was somewhat low on fluid. I need to dig into it and work the whole brake system over.

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    Re: How to know for sure if Master Cylinder is bad?

    Quote Originally Posted by BadAssPerformance View Post
    But if it just the booster then the pedal should not go to the floor, right?
    Yes, this is what I said,

    Bad booster equals hard pedal that doesn't sink. Bad MC equals pedal goes to the floor if the system has no leaks.
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  17. #17

    Re: How to know for sure if Master Cylinder is bad?

    Speaking of brakes. i put a new master in my minvan " brake warning light" still on . Brakes work great. How do i get rid of the light

  18. #18
    Buy my stuff!!!!!!!!!!! :O) Turbo Mopar Vendor turbovanmanČ's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Hows the brake pedal? it could be the prop valve is sticking. If thats causing the light to stay on, have a look at it, a metal pin will be sticking out of one end, carefully push/tap it back in, you might have to bleed the brakes some more.
    1989 FWD Turbo Caravan-2.5 TIII, GT35R, auto, a/c, cruise, pwr windows/locks, fully loaded with interior and ran with full exhaust. RETIRED FOR A FEW YEARS! 12.57@104 :O)
    1984 Chev Getaway van, 6.2 Diesel with a remote mounted turbo setup burning WMO-For sale.
    2003 GSW 2.0L TDI, auto, fully loaded, modified, 360K-wife's.
    2004 GSW TDI, 5 speed, fully loaded, modified.

    Aurora ignition wires for sale. Link to info

    Super60 roller cams or custom/billet cams. Link to info

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    Bad Mother Flecker Turbo Mopar Staff fleckster's Avatar
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    Re: How to know for sure if Master Cylinder is bad?

    Just an update for everyone...I have a Dorman new 24mm master cylinder coming from Rock Auto. The price was great, only $32.71 including the shipping using a 5% code I found. Should be here in a day or so and I'll again update when I get it and install it.

    John M. Fleck

    '83 Dodge Rampage 2.2 w/ 2.5L Turbo 14.79@94
    '87 Shelby Lancer #213 14.99@94
    '89 Chrysler LeBaron GT Turbo 'vert 15.88@92
    '92 Dodge Spirit R/T 13.50@104 ; 13.34@102 on slicks
    '98 Dodge Neon R/T 15.66@89
    '03 Dodge SRT-4 13.91@98

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