very puzzling brake problem...

Discussion in '80-Series Tech' started by drexx, Jul 19, 2005.

  1. drexx

    drexx

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    I've got a very tricky brake problem that I, nor any experienced mechanic I trust, can figure out the problem. The brake pedal is soft and mushy... so I change the fluid and bleed all four plus the LSPV, right? But no, the problem still isn't solved. I check the thickness of the rotors and the pads, but they're still within specs for my 1995 FZJ-80. We've bled it at least five times already, but still no avail.

    I've had the brake master cylinder repair kit from CruiserDan: didn't get fixed. Changed the brake master cylinder itself, still no good. The thing is, the brakes are very responsive when just idling/crawling along, but gets mushy when you get up to speed. Not the best when cruising down mountain roads, so I just pump it once or twice as I approach the curves before the brakes really bite.

    But on panic stops, when I press the brake pedal hard and fast, they bite just before they hit the floor.

    Any idea what's wrong? I'm stumped.
     
  2. parabola

    parabola

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    The only thing that comes to mind based on a similiar experience with a different vehicle is that you may be overheating the brake fluid. Are all the calipers in good condition? No stuck pistons that may be overheating everything at speed? Any other signs, such as poor gas mileage?
     
  3. Bongani

    Bongani

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    Check the front left wheel to see if the wheel bearing nut is not loose which will result in the callipers getting pressure at speed. (Ok, it is a lame technical answer but have seen exactly this symptom and the problem is typically the front left wheel)
     
  4. Kalawang

    Kalawang

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    I've had pretty much the same problem, and likewise changed my MC all of 4 timwes already, looking for the solution, even using substitute parts. No joy. Currently, I'm back to the Aisin origs.

    My next effort will be to remove everything, including the brake lines, pads calipers, all of it. I intend to clean each and every part, check all and then re-assemble. I plan to do this at home with my voice activated tool, but I'll have to schedule it. Just too many things on my platter these days. A mechanic did this for my old Toyota some years ago. We did not find the problem then either, but it went away after re-assembly.

    Life Happens.


    Kalawang
     
  5. archie

    archie

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    From Kalawang's similar experience,did this happen when you were in California? It might be because of the higher ambient temps of the Metro Manila driving. This in combination of heavy stop and go traffic and low speeds(no air to cool rotors) could heat up your brake fluid. Some 70 series LC that went up Mt Pulag(Philippines) this recent 2005 had mushy brakes issues due to overheating. Ask me how I know :) .

    Just another thing to consider.
     
  6. landtank

    landtank SILVER Star

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    go out and buy yourself a set of SS braided lines from slee. Your originals might be expanding under pressure slightly. I and others have noticed a nice improvement with the pedal feel with these installed.
     
  7. drexx

    drexx

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    My thoughts exactly! I've bought those SS brake lines a month or two ago already, but my family haven't shipped them to me yet because our freight boxes aren't filled yet.

    The other things I've done so far is the LSPV adjustment to 90mm because of the 2-inch OME lift --which made the rear brakes stronger; and checked for leaks in the system --none found.
     
  8. drexx

    drexx

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    Cool! So how do you know? :)
     
  9. alia176

    alia176

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    How's your engine vacuum at speed vs idle? Are you losing vacuum somewhere and loosing boost?
     
  10. miked

    miked

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    i was going to go with vacuum issue as well but, it seems the problem is where they bite "just before the floor". a vacuum issue seems like it would change how hard or soft the pedal is, not the point in travel that it bites.
    huh.. i dunno.

    i would also agree that the fluid might be boiling.
     
  11. Kalawang

    Kalawang

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    Unless the problem were more common, I might have actually believed the heat issue. Friends have almost all pointed to heat as the culprit, but my use of the brakes, though significant, was not excessive nor prolonged when the problem arose. My only experience with brake/heat problems was with them steaming or burning from excessive use, but that was with a 50's Plymouth a long time ago. Even then, the brakes didn't fade, they smoked.

    Does anybody else experience brake fade on windy downhill rough mountain roads ?

    Drexx,

    I neglected to mention that I was able to severely reduce that mushy feeling by bleeding the MC. My brakes now feel as rubbery as the Jaguar MK10 of the mid 60's. The method we used was to replace one of the tubes with a short sealed one and do the pumping thing. When satisfied with one, we moved the sealed pipe to the other and did it again. I still have the sealed pipe if you want to try it.


    Kalawang


    Kalawang
    i
     
  12. sixsracing

    sixsracing

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    For the price of rear pads I would install new ones, whether the old ones are in spec or not. I had a Honda Accord with mushy pedal last year, rear pads looked good. Replaced them anyway because of some slight cracking and the pedal went from mush to firm for $15.00.
     
  13. sjcruiser

    sjcruiser

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    Vacuum is highest at idle and lowest at full throttle on a gasoline engine. From the symptoms you're experiencing, I think the brake booster is the main suspect; not the MC or the air in the fluid.

    Frank.
     
  14. parabola

    parabola

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    But, you may not be causing the overheating yourself. If you have a sticky caliper piston, for example, you could be building up heat in the fluid without your foot even on the brake.

    One way to see if heat is the issue is to switch to some campatible high end racing brake fluid that has a significantly higher boiling point than the stock stuff. If the problem severity reduces than you have found your issue and then can look into what is causing the excessive heat. Of course, just inspecting the brake system including calipers, lines, etc. would be the best start but may not completely rule out anything.

    On my Supra I ran ATE Super Blue. This stuff is great; I would routinely get my rotors glowing red hot during street use and it took alot of abuse to get this stuff hot. I think it has a nearly 80C higher boiling point than DOT3.
     
  15. Bear80

    Bear80

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    I have this same problem on my '86 mini truck. I've been fighting it for years. The pedal is mushy and soft but will NOT pump up and will not allow for brake lock up and will go to the floor under meduim foot effort. It really feels like there is a leak in the system, but there is none. The pedal feels this way the moment the truck is cranked, so there is no possible way it is boiling fluid.

    So far I've ruled out Master Cylinder, Booster, Calipers, Slave cylinders. Just to note, I have stainless steel lines and in NO way could the old rubber lines flexed enough to cause a soft pedal, so SS lines is no where near a fix for this problem. They only thing I can think of is that the LSPV is some how bleeding off pressure and not allowing the full use of EITHER front or rear brakes. I have taken a LSPV apart and there isn't much to them, a few rubber o-rings, one spring and thats about it. I'm down to replacing the LSPV and if that doesn't fix it I have no clue.
     
  16. Bear80

    Bear80

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    I would really like to know how you got the rotors to GLOW?!!? IF and that's HUGE IF, you would have warped them and by chance lucky they didn't catch anything on fire. AND I strongly doubt you have carbon rotors which are the ones that do glow, because under normal use....ie racing conditions they ALWAYS glow red. :flipoff2:
     
  17. parabola

    parabola

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    Hard braking from high speeds (150+ mph). And it is quite a normal phenomenon for Supra rotors to glow under these conditions.

    The Supra's rotors (metal, not carbon) were designed to wear with the brake pads and you typically changed them at the same time due to loss of thickness and/or warping, which yes, was fairly common with heavy abuse such as what I would normally do. Along with the glow you'd see sparks and such coming off the rotor as well.
     
  18. IdahoDoug

    IdahoDoug

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    That does sound like a vacuum issue. The big tip is they work when just idling along, don't when the engine's pulling.

    DougM
     
  19. wileetoyote

    wileetoyote

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    and you were driving this fast on the track or through downtown Pflugerville?


    Drexx...have you been drivng 150mph again? :rolleyes:

    That may explain your issue however, I'm placing my bet on the SS braklines

    WET
     
  20. LX_TREME

    LX_TREME

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    Also regarding the 80s 1FZ SS brake lines isnt it suppose to be 7 lines instead of 3?
     
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