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Random loss of brake pedal pressure, occasionally


ymanchik

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In your diagram the 2nd circuit rear seal is working, thus no fluid loss into the brake booster.  The other two front seals are defective and the two chambers are just passing brake fluid back and forth providing no pressure to the two circuits.

BMWCCA  Member #14493

www.2002sonly.com

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http://www.britishv8.org/articles/tandem-master-cylinders.htm

 

What Hal said.  The picture you posted is misleading- there's no mechanical link between the 2 circuits in the master on a 2002...

 

There is a 'bottom out' stop that mechanically shoves the front master piston forward if the rear one fails, but the hydraulic pressure

of the rear is what moves the front, usually.  It's not obvious at first.

 

And the 'occasional' failure is what happens when the cup seals are hardened and worn- they'll sometimes catch fluid,

but sometimes not.  Often, a 'stab' will work where a slow, gentle application will not- the fluid, when pressurized inside the master,

pushes the seal lips out against the bore, which helps them seal.

 

t

 

 

 

master-brake3.jpg

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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The picture is not meant to be misleading but for the sake of conversation reference and attempt to solve/explain the issue. As I said, willing to learn and Hal's explanation makes sense to me. I was drawing on my clutch issues where I had leaks everywhere but if you pumped the pedal you had a clutch for awhile. All good here and thanks for the explanation! Hopefully the OP get some relief.

Slowly building a $20,000 $4000 car

If it "ran when parked" you wouldn't have parked it!

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Nope, never meant to imply YOU were wrong, just that the pic you posted was of a failure in one of the circuits- it shows the first piston bottomed out on the second,

which is only a failure mode...

 

all good

 

t

 

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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No, I did not think you were. I could not get my head around the issue of no loss in fluid. I just found the first picture on the web and posted it so everyone can see what we were discussing in general. As for lack of knowledge, I thought I was fat....nope just thick skinned. 

Slowly building a $20,000 $4000 car

If it "ran when parked" you wouldn't have parked it!

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Yeah, I had to experience it once to figure out what was happening.  It's not supposed to, it makes little sense, and it's a major failure of the master.

AND it takes 3 seals failing while the 4th, outer seal holds.  It doesn't happen often.

 

It happened to me with a master that was rebuilt using cheap parts- the cup seals hardened, and I got good brakes if I stabbed it,

but gentle, easy pressure would let the pedal sink straight to the floor.  I'm ashamed to admit I drove it like that for a week... or 3...

 

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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