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Thread: brake master cylinder question

  1. #1
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    Question brake master cylinder question

    well i have just started having this problem and i was wondering if Characteristic of a brake master cylinder going out....i am stopping my car at any speed and it will slow down however the petal will slowly go down to the floor with a little pressure then it will eventually hit the floor and i have no brakes, unless i pump it then it will get pressure and then loose it all if i keep constant pressure on the brake.... even while stopped on a hill it will have pressure enough to keep me stopped uphill but the petal will slowly go to the floor with minimal pressure and will start rolling back ...i have blead(sp?) my front right brake a little yesterday( the rest of the bleeders were rounded off : ) and it seemed to get alot better however this morning it did it again. the fluid, although dirty, is very full. if somebody can help with some info so i dont kill myself with a car with no brakes hehehehe, i would appreciate it....

  2. #2
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    would air bubbles in the line cause that bad of a mushy petal?

  3. #3
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    That is most definitely a defective master cylinder. Most likely there is a defective check valve in the cylinder. If you don't have ABS, you can pick up a new MC for around $90-$100 at Pelican Parts or mesaperformance.com. Make sure you get new bolts. They are one-time bolts and should not be re-used. Line wrenches (also called flare-end wrenches) will help remove the brake lines from the cylinder.

    You should also pony up and buy three new bleeder valves. You'll have to replace the brake fluid - then bleed the system completely. So you should pick up a liter or two of DOT3 or DOT4 fluid. ATE supleblue is well thought-of.

    I did this job a few weeks ago, and also upgraded the flexible rubber hoses with stainless steel braided hoses. The brakes are *extremely* firm now.

    One other thing that might happen is you won't be able to bleed a caliper, even with the bleeder valve completely removed. You'll have to fish around in the hole with a piece of stiff wire. There's some kind of check valve inside the bleeder valves. I don't know what it is, but it confused me for quite a while.

  4. #4
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    thanks for the info i am going to order one today, i kinda figured that is what it was and i am going to buy new bleeder valves the only problem is getting them off in the first place i guess its time for my dremel to make new corners =)..... do they have kits for the braided lines or did you have a custom lines done?

  5. #5
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    i have a 84 eta so im sure it doesnt have abs..... ill look and make sure though

  6. #6
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    You are kind of Shooting off the hip...

    My 85 eta has nearly 500,000 miles on it with the original master cylinder. Have you checked each brake cylinder at the wheel, the master clutch cylinder, at the floorboards, the clutch slave cylinder at the transmission for leaks? It could be any of the above which are far less than the master cylinder. In general, the brakes suck on the E30's, and unless everything is tight your pedal pressure will suffer. Make sure you flush and pressure bleed the brake system after you find out what needs to be repaired.

    Hank
    1995 M3 Alpine White 4/94 production date. Clear Corners, E46 metal pedals, X-brace, Euro Floating Rotors, Motorsport Strut Brace, H&R Coilover Suspension, Kmac Camber Plates, Ground Control Adjustable Sway Bar End Links, UUC TME's, UUC TSE, JC Euro HFM, ZKW's + DDE's, Powerflex RTAB (thanks Mrob Gang) Clear Projector Indicators, New window molding and sill rubber. Bavarian Silver Powder Coated Original Motorsport 10 spokes. Benfer Performance Black Powder Coated OBD-I Valve Cover.

  7. #7
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    well i dont notice any fluid anywhere under my car, i checked each of the break bleeders, and my fluid in my brake reservoir is full i havent noticed it getting lower at all. i ordered a break master cylinder from one of my boys at pepboys and he hooked it up for very cheap =).......if that doesnt fix it ill be back on here bitching hehehe...thanks everyone

  8. #8
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    Well, it seems like no one is 100% sure... I think what I'm going to do is order the part that is suggested for my car. That way, if it doesn't fit, hopefully they will at least let me return it without penalty.

  9. #9
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    No e30s, again :(
    you have to do some checks to eliminate other stuff. just from the description, you can not be 100% of master but it is pretty dang good chance
    No e30s again.

  10. #10
    richardodn's Avatar
    richardodn is offline Old Guy BMW CCA Member
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    If your pedal slowly moves to the floor with light to moderate pressure, there is no question. It is the master cylinder. Usually you aren't losing fluid. Instead it's leaking past the internal seals. If you are also losing fluid, then either it's leaking into power booster or you have additional problems. If you're leaking into the booster, that will fail prematurely.
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