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  #1  
Old 10-09-2003, 09:14 PM
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master cylinder question

I'm trying to trouble shoot the spongy brakes on the car and I'm looking for signs of whether it's the master cylinder that's acting up or something else in the brake system.
the pedal never touches the floor, however it always stops about 2 inches away from it. it's really spongy. at times it's rock hard as it is supposed to be.
is there a test one can perform as a diy to figure out if it's the master cylinder that needs to be changed or not ?
thanks.
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Old 10-10-2003, 11:52 AM
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It is not clear to me whether you have spongy brakes or a low brake pedal. Spongy brakes come from compressable gasses in the hydraulic circuit. This gas is called air, and gets in when the circuit has been opened. You didn't say if someone has been working on the brakes. Also I have to assume you are talking about a car with 4 wheel discs. Loose wheel bearings can cause low pedal. A failing master cylinder is characterised by a pedal that slowly sinks toward the floor under pressure.

Peter
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Old 10-10-2003, 02:11 PM
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Sometimes you get that (low pedal, but still brakes) if fewer than all 'channels' are being actuated. Usually his means the rears are getting pressure, but the fronts are not.

Not usually intermittent with MC failure though.

Of course you checked the fluid level... Have you tried a bleed?
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Old 10-14-2003, 06:17 AM
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The brakes had been bled atleast twice by the dealer. once by a private mech. infront of me as well.
I have a low pedal as well as spongy brakes. There's no leak of fluid. It's full. four wheel disc brakes. Rotors and pads are relatively new as well.
Brake fluid has been flushed and changed as well...
The pedal is spongy, it goes close to the floor however it has never touched the floor. ever.
and it takes twice as much leg power to push the pedal and stop the car as it does in a similiar year/model other car that i test drove.
any more suggestions ?
thanks.
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Old 10-14-2003, 10:44 AM
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I would nominate the MC first. Possibly something in the assist 2nd.

I suspect you will find that only the rear channel is getting pressure.

Test it. Jack up the front, have someone hold down the pedal while you try to spin the front tires. Although they may grab initially, after a short while you may find that they spin!

I have had 2 MCs fail in this mode (note: neither was a MB).
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Old 10-14-2003, 10:47 AM
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From the added info, it is sounding more and more like a master cyl. Fluid is probably bleeding past one piston cup and alloung the other piston to apply brakes to only the front or rear wheels only. Since about 75% of braking is done by front brakes, I am guessing it is the front brake part of the MC that is failing.

Peter
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Old 10-14-2003, 04:24 PM
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Interesting...
I'll make sure I convey the info to the dealer now that I take the car back to them. And I'm sure they didn't bother to test the MC enough like you have suggested in the post.
thanks again for all the help !
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Old 10-16-2003, 01:04 AM
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well, the car's at the dealer right now. in the meanwhile, I'm wondering.. if the brakes require a heavy foot to stop doesnt that mean that the brake booster isn't doing its job right ?
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Old 10-16-2003, 11:45 AM
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That is possible, but doesn't explain the low pedal problem. Stopping the car with only one of two brake circuits working would also require more effort. If the front brakes are not functioning, you are trying to stop the car with the small rear pads.

Peter
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  #10  
Old 10-16-2003, 12:41 PM
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Possibly in the assist, but I would still nominate the MC first.

Vacuum assist is generally pretty reliable.

Cars with hydraulic assist are more likely to have issues exhibiting those symptoms.
This setup is common on many BMWs and Audis.
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