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Or the best kept secret in automobile maintenance? I'm referring to the dire warnings seen often here, similar to: "...if you drive the pedal to the floor while bleeding the brakes, you will badly screw up your master cylinder."
I've questioned this precaution since seeing it here on the BrickBoard, but nowhere else in my fairly extensive auto repair library. This includes the Volvo Green Books for 140 and 240—both of which use the dual-diagonal brake system with the double piston master cylinder, and permit pedal bleeding in lieu of the pressure method.
The 1971 140 manual says: "Check to make sure there is a full return on the pedal and that neither carpets nor suchlike prevent full travel (about 5 1/2") from being utilized during bleeding." And later: "Slowly press the brake pedal down to the bottom".
From the 1975 on 240 Manual: "Unscrew the bleed screws and gently pump the pedal 5 times. Keep the pedal depressed and re-tighten the screws."
The 700/900 Brake Manual does not mention pedal bleeding, but when replacing calipers, advises: "...the brake pedal must be depressed throughout the operation to ensure that the system is not drained of fluid." It also shows a drawing of a blocked-down pedal, saying: "Lock brake pedal in depressed position."
It implies full pedal depression and, like the 140/240 manuals, offers no warnings or precautions otherwise.
I'll admit the peril seems so plausible that I'm wary of going all the way myself. But why is the BB the only place it's mentioned, given the world of Master Cylinders out there that have been getting pedal-bled for decades?
My guess it that once upon a time, someone here on the 'Board found rust and pits in exceptionally bad MC and posted about it. Then after being repeated and embellished, it became BrickBoard lore to be applied to all Master Cylinders forever.
• So does anyone know of a reputable, documented source that warns against the heartbreak of "full pedal travel"? I have yet to find it described anywhere else — not it in some 45 years of DIY car repair.
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Bruce Young '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day
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Very interesting. Lots of things get repeated until they are "common knowledge" here. Tight lugnuts warp rotors, oil must be changed every 3000 miles, 240s are prewired for extra gauges and more. I would not doubt you have exposed a popular BB myth.
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In a well-maintained brake system with regular fluid changes and low moisture in the fluid, the master cylinder will not corrode and pushing the pedal all the way down is not likely to ruin the seals. Most cars suffer, however, from rarely-changed fluid and internal brake system corrosion. In this instance, the seals can in fact be ruined if dirt and rust line the unused part of the master cylinder bore.
Anecdotes are worth something. I just rebuilt the brake system in my father-in-law's old Pontiac 6000 prior to giving it to a refugee family. He never flushed the fluid because his idiot mechanic claimed it was "lifetime" fluid. Both front calipers and both rear wheel cylinders were seizing. I replaced these, then started to bleed the system. I slipped while holding the steering wheel, pressing the brake pedal all the way down. That one press did it: it immediately started leaking, which it had not done before. I replaced the mc and the problem was solved. The old brake fluid was pitch black.
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See the 700/900 FAQ under 'Select Link' button on the top right.
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Thanks Steve. I don't dispute anything in your response, but my basic question remains...
If grungy Master Cylinders are as prevalent as the warnings here seem to suggest, then why can't I find similar warnings anywhere else?
I'd think they should be just as common in other car populations and every repair manual. But so far I haven't found the problem even mentioned anywhere except here on the BrickBoard. That strikes me as extremely strange, so I changed the Subject line.
The only conclusion I can draw is that the problem has perhaps been more emphasized here (due to the nature of the forum?) than in the general car repair world — which goes back into the 1950s and earlier in my books.
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Bruce Young '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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You've identified a valid point: distinguishing urban legend from documented fact can be tough. This warning does seem to be in all the old Chiltons and Motor manuals but hasn't been seen in OEM shop manuals, at least to my knowledge. And any master cylinder subject to this problem likely (as mine was) should have been changed long before.
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See the 700/900 FAQ under 'Select Link' button on the top right.
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"This warning does seem to be in all the old Chiltons and Motor manuals..."
My '82 Clymer, '83 and '89 Chilton's all have the Full Pedal procedure but no warnings.
The '52 Motor manual comes closest to acknowledging a potential hazard in their pedal bleeding, saying to push the pedal "about half way down", but no reason given, nor warning about full pedal.
Chris Mooney makes a good point about the age of our cars (compared to other/older matques) making MC rust more likely and possibly prevalent. That, coupled with such a large band of dedicated care-givers, and a forum to "spread the word" may account for precautions here that aren't found elsewhere.
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Bruce Young '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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Bruce,
I've not seen it documented in other manuals or resource material. However one must bear in mind that those sources all make the assumption that regular maintenance has been performed, including brake fluid flushes. Thus the problem of water-saturated fluid would not be an issue and so corrosion would not occur in the bore of the cylinder. HOWEVER, since so many of our Volvos are outlasting the majority of vehicles on the road, and since in reality brake fluid flushes aren't ever done on most cars until there's a problem (rusted line for example, here in NS a car with steel lines gets a full flush every 5-8 yrs since that's the life of the lines) there's a very good chance that the fluid does have a lot of moisture and the end of the bore where the seals never travel will have corrosion. I have had master cylinders fail on 3 cars where I've bled the fluid and pushed the pedal all the way to the floor. Chances are it was marginal and would have failed soon anyhow, but the rust did in the seals. I took the first 2 apart to find out what happened and indeed the seal lips were torn and there was rust in the bore.
So I'd say it's not a myth but a probability unless you know the fluid has been changed on schedule and not neglected. Why our cars? As I mentioned the cupro-nickel lines mean that in many cases these cars never get a full fluid flush because only the flex hoses get changed and the m/c topped up with a little fresh fluid. And the master cylinders do last pretty well indefinitely until they're disturbed, so many of those seals are 20+ years old and don't have the flexibility to tolerate grit.
I took one apart one time, before installing it on another car and attempting to bleed it, and I honed the rust out of the bore then reassembled it with the original seals (foolish yes but I was a starving student at the time). Worked like a charm for the 2 years I had the car.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina
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Chris,
I'm not sure the common repair manuals assume a well-maintained anything. At least I don't think they should, given the range of possible conditions (age, environment, owner diligence, etc) prior to any car needing a given repair.
But I do think your theory about our cars lasting so long, may be focussing on what I'm now calling a "paradox"*. See my latest reply to Steve, above, where I hope I paraphrased you correctly (or close enough).
*I never really thought it was totally myth. My real question is/was why does no one else show any concern about this full pedal thing — when master cylinders have been around, basically unchanged, for so many decades?
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Bruce Young '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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Hi Bruce,
I know you and I have discussed this lack of mention in the literature a few times before, so I will join the fray just to restate, as concisely as I am able, the conclusions I had drawn based on examination of several dead masters in my posession, and some opinions I have of the scope intended by technical publishers.
1. I had not found any rust. Seals were not supple though, and looked as if they had added some of the "glop" found in the disused portion of the cylinder to their outer dimension. The glop was not evenly distributed, so it may have lifted a portion of the un-supple cup seal adjacent to the glop, forming a leak. 240 masters are cheap enough to keep me from attempting a re-seal and clean to test this theory.
2. Though there is a lot of repeating on a forum like this, or info passed without the benefit of personal experience, I still believe we as a group of enthusiasts may be able to come up with some new and useful information not presented in the service literature. The paradox, to me, was not being able to find mention of the same syndrome among enthusiast lists and forums for saab, mg, porsche, and other Euro marques that use similar braking components (i.e. open systems). - Sorry minidave, I had not read your post before writing. - Pressure bleeding is gaining a lot of shadetree support of late. But again, the Brickboard could be the leader in the old-Euro car maintenance arena.
3. I don't expect the publishers of Haynes, Bentley, Clymer, Motor, Chilton, etc. to address all the issues that surface as a result of age and high mileage - these subjects being better covered by enthusiasts and restorers, who have the real experience. Much more to the point, I would never expect Volvo's factory service information to be written based on experience with 20-year old cars; they are written before a model is even released, and TSBs are carefully issued only after legal has OK'd.
So to sum up, I don't really know the root cause of the master's demise after pedal bleeding, but the explanations given seem plausible to me. And, that in my opinion, this syndrome, or a warning about pedal bleeding, is not documented in commercial repair manuals, doesn't detract from its credibility.
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Art Benstein near Baltimore
Q. What is the only food that doesn't spoil?
A. Honey
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I think you've said it all. Couldn't have said it weller myself.
I'd elaborate, but am too eager and excited about tackling the trailing arm bushings on Ted's '87 that arrived from NJ last night, dripping icy, salty slush thanks to our latest dose of "lake effect".
Thanks for another one of your always insightful (never inciteful) responses.
Bruce
P.S. How about the myth of PB Blaster, huh?
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Bruce Young '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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Hi Bruce,
I can tell, the lure of physical maintenance vs. this virtual stuff, has removed you to more interesting places, as it should me as well.
Sort of on-topic, given the tendency for remanufacturers of brake parts to assemble things haphazardly, the Brickboard may have been the foremost place to learn about mismatched caliper halves. Not myth, but frustrating reality.
You posted http://www.brickboard.com/RWD/index.htm?id=926014 some time ago, also under a heading of "Slightly Off Topic". I promised to look at some of my parts pieces and since offered some pics of the Girling innards I examined to be better able to explain the difficulty bleeding mismatched calipers. Photos don't explain it, so I finally sketched what I saw schematically here, as you suggested.
I thought your description in that post represented the height of eloquence in transferring a visual concept into language.


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Art Benstein near Baltimore
"Wisdom comes with age, but sometimes age comes alone."
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There is some truth to it, it can kill a bad master cylinder that might have otherwise limped along for a while longer. But really, if it was that abused and that bad, you probably should be swapping it anyhow.
Drum brake cars probably don't have this problem, at least ones with manual adjusters. Unless you are uncommonly ontop of adjustment, you regularly get to 'exercise' those lower parts of the MC.
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I'm JohnMc, and I approved this message.
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Just last night I rebuilt the master cylinder on a friends saab 900. I had read up a bit on Saabnet, and there were all sorts of warnings there about pressing the pedal to the bottom as well. Maybe it's just a swedish thing...
Anyways, I had the MC apart, the seals were moderately worn, but the interior of the MC was prisitne-no marks, no scoring, no discolouration anywhere.
Thinking about the process of rust in the MC, would not all of the interior of the MC be exposed to the contaminated fluid, and not just the 'unused' lower portion of the bore? Maybe the piston could wear part of the bore larger, and pressing the pedal beyond this lip could damage the seal? Have to be in pretty bad shape to begin with and need rebuilding anyways...
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The entire bore would be exposed to it but the constant movement of the seals back and forth along their travel within the cylinder would keep the walls relatively clear.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina
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