A few weeks ago I bled the brakes because the peddle was getting low and I wanted to switch to silicon fluid. The only snag I ran into was that I broke off one of the (too small) rear bleeders. After the break it didn't leak at that spot and the peddle was firm so I left it, although I did purchase a new bleeder screw since then and plan to install it soon.
Anyway the other day I found out that the peddle goes down to the floor! I had driven it at least a few hundred miles after the bleed without any braking problems, so it would seem that the bleed was lost while the car sat for a week or so.
Upon inspection I found all was dry and there is no fluid missing. My attention turns to the brake master cylinder. Other than the occasional replacing of pads and bleeding the system I haven't had to pay too much attention to the brakes for the past seven years, so I'm not sure how old the master cylinder is. It looks OK, although I know that means nothing.
I guess my question has to do with the master cylinder. Can they just poop out without even leaking?
I'm tempted just to replace it since, after all, we are talkin about brakes. But is there a way to trouble shoot it as it sits in the car?
Thanks.

How did you bleed the system originally? Did you use the brake pedal pumping method? If you did and bleeding the system again doesn't improve things, then its possible that the seals in the master cylinder have failed. This can happen on older master cylinders that have corroded. Under normal use all is well but if you empty the system and refill it with new fluid then the greater pedal travel while bleeding causes rusted parts of the piston/cylinder bore to make contact with seals that normally wouldn't move that far. They get torn and fail sooner or later.

I've read somewhere that you're better off bleeding silicone fluid systems with some sort of brake bleeder that doesn't involve pedal pumping - the pedal pumping action can cause micro-aeration that makes a firm pedal much harder to achieve.

I'd be inclined to think that if you haven't lost any brake fluid then either you've air trapped in the system that's moved (maybe possible with an unbled corner? - I'm not sure) or else the master cylinder seals have failed. You'll know if the master cylinder seals are ok if you can get a firm pedal that lasts.

Aidan
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1967 131, 1969 131, 1973 Triumph GT6

pearl;

John Mc has it pretty well covered (and I approve of his posting too!).

It is not possible for the "bleed to be lost while the car sat for a week or so." What may have happened is some micro-bubbles lodged in the rear brake valve got into the system and made a big bubble (notorious for this), or some other effect having to do with the adjustment...

It would sound like you could stand to do more of a flush to get rid of the old fluid, since there is likely still old fluid at the corner with the broken bleeder. I usually pump one whole reserviors worth through each corner while bleeding.

Try to elaborate about the "feel" of the pedal when it finally has some resistance...is it soft (suggests air in system) or nice and firm (rear adjustments need closing up). Is there a vac booster involved?

Cheers

>>(and I approve of his posting too!)

I guess the passing of the election makes that a sort of lame signature. Off with its head!

PS - some sort of forum software error when I tried to remove the sig setting, so I guess I'm stuck with it for the time being...
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I'm JohnMc, and I approved this message.

I changed my sig a while back and it had the same problem... Was quite annoying going through some of the posts and changing it to be what I wanted... The thing that fixed it for me was deleting all the cookies and clearing everything that was stored for the brickboard site on my computer, and then just logging back in and it went over to the sig I had put in... Weird yes, worked though... Hopefully you understand what I'm trying to say and it works for ya...

OT - Dingoes ate my sig... and welcome to Turbobricks ;)
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new sig in progress...

>>OT - Dingoes ate my sig... and welcome to Turbobricks ;)<<
Soaking up all the MSnS info I can for my (somewhat) soon to be injected PV...
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Dingoes ate my sig!

...obviously, but I still think it's kinda cool...plus, it lets readers know that "you reeeeaaaallly mean it"...which has to be worth something...

A 'low' pedal is not an indication that the brakes need to be bled. The feel when the brakes finally engage (high, low, or pumped) indicates if they need to be bled. A firm, solid feel means there is little or no compressible air bubbles in the system. A soft, springy sort of feel indicates that compressible air is making 'slack' between the master cylinder and the brakes on each wheel.

A 'low' pedal - where the brakes only engage when the pedal is further down, or after 'pumped' means there is an excess of mechanical slack between the brake shoes and the brake drum. (Of course a pedal can be 'low' and soft at the same time if the brakes are out of adjustment AND need bleeding). The discs on the front are self adjusting in the manner they operate - but not the rear brakes. Well, some of them have auto adjusters but they are prone to sticking. And older cars (like my '63 PV) have manual adjusters that must be adjusted every 10K miles or so. If pulling the hand brake seems to temporarily cure the 'low' pedal its a good indication that the rears need to be adjusted.

AFAIK if you are going to put silicon in you need to clean the system very well to rid it of all the old style brake fluid, and since you broke a bleeder off (soak those daily with PBlaster for a solid week before you try to get them off!) I can't imagine you got the old fluid out.

Fluid can't just disappear. It has to leak out somewhere. If you don't see it dripping out under the car or under the hood two more sneaky options remain. First is leaking out inside the car from the end of the master cylinder, trickling down the floorboards under the mats. Another possiblity is that if you have power brakes the motor could be sucking fluid through the vacuum connection with a couple of failures in the booster and master cylinder.
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I'm JohnMc, and I approved this message.

John,
I was under the impression that silicon fluid was a no-no for cars of this vintage, but since you , nor anyone else, referred to that, I guess I was wrong, right?
Bob S.

Bob;

It's not a no-no "for cars of this vintage" by any means...as far as I'm concerned, it's the BEST THING in my opinion (and Walrus's, last time the subect came up, but I'll let him chime in with his own comments and experience)...besides, how does a hydraulic fluid know, or care what vintage of vehicle it's in?...in the first place, the only thing it's in contact with is metal and seals, so the ONLY issue which is critical with respect to particular vintage vehicle is seal compatibility, and in order to receive a DOT rating one would presume the DOT did the exhaustive research necessary to pass hydraulic, thermal, deterioration and compatibility requirements...not that I give the US DOT blind faith...after all, they still haven't seen fit to standardize bumper heights for ALL vehicles in the US, and it wasn't until into the 80' that they finally approved lighting other than the sealed beam junk, and they are the same bureaucratic brainiacs which assigned the completely illogical and downright deceiving 5.1 designation to non DOT5 compatible glcol based fluid (DO NOT MIX THE TWO)!...not exactly shining examples of our tax$ at work...

I have done my own compatibility studies with Girling seals and have satisfied myself that no excessive swelling or deterioration of the Girling seals occurs (besides do you know of ANYTHING better for rubber and synthetics than silicon?...Armour-All et all is making millions on that!)...further, I have better than DECADES of nothing but excellent first-hand experience on a number of vehicles (122, 1800, 1800ES, 140) and systems (brake and cluth hydraulic). Nothing bad to report yet...NOTHING!...clutch masters are still full after a year of being parked, and cluth and brake rebuild times more than quadruple!...show me that with LMA!!

...but don't take my word for it!...make up your own mind...I periodically search on the subject to see if there's anything new and enlightening out there...here is my compilation of the most interesting items which I found up to now in my research, complete with more of my insightful thoughts and (WA) comments: http://www.intelab.com/swem/Amazoning%20with%20Silicon%20Brake%20Fluid.htm ....enjoy!

Cheers

>>Armour-All et all is making millions on that!<<

From my personal experience - don't ever put that junk on your old Volvo dash. A 30 year old dryish looking old dash with maybe a crack or two will soon turn into bacon if you Armor All it.
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I'm JohnMc, and I approved this message.

I wouldn't know in particular, I've never used it. What I've been using in my old cars is Dot 5.1 - which is compatible with the older style brake fluids but has the moisture resistance and temperature range of silicon. Why they gave it such a confusing name that makes it sound just like Dot 5 silicon I'll never know.
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I'm JohnMc, and I approved this message.

The DOT number rating system pertains ONLY to physical properties (heat soak, water absorbtion, performance, etc.) At the time of developing the "5" level silicone was the only material that could perform at that level. Well, now glycol based stuff can, so in order to minimize confusion for us consumers they coined "5.1" to clue you in that it meets DOT 5 specs (except for water absorbtion) but ISN'T REALLY THE SAME STUFF. For bureaucrats I thought it was a pretty good solution.

By the bye, I also use silicone DOT 5 and like it immensely.

I disagree slightly on the every bit of old fluid out. The old fluid retains water so any pockets of old fluid will lead to corrosion localized at those points. System will still operate fine but boiling point of fluid will degrade wherever old, water soaked fluid remains.

So, it needs to come out but the system will still operate in the meantime.

Sounds like a bad MC to me.

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Mike!

Thanks guys. Looks like that pretty well covers the subject. I don't know where I picked up the notion -- one of these days I'll probably stumble into the source and give it the old Bronx cheer. Glad to have one thing less to worry about in this glorious new year ;-)
Bob S.

Hi John,
Guess I forgot to mention that I checked the inside part of the MC, where there is no leaking, and I adjusted the rear pads by way of inserting something flat and sturdy to move those star things through the backing plates.
I'll repalce the broken bleeder and flush and bleed the entire system, and see what I get.
Thanks,
Stan