Bleeding Brake Problem

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Re: Bleeding Brake Problem

Post by warpc0il »

It's very easy for braided hoses to be assembled with leaking joints. Even if they were pressure-tested, which is very rare, then it's also common for the ends to be attached at the wrong orientation, such that there's a twist in the line when fitted, that can cause a leak.

Sometimes the leaking fluid runs back between the outside of the liner and the inside of the external braid, making it very difficult to spot.

I'm also assuming that you're using DOT 4 or DOT 5.1 fluid and not DOT 5.
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Re: Bleeding Brake Problem

Post by Ashman »

It does firm up after pumping the pedal and once it's firm you can hold the pedal down and it'll stay firm but if you leave it for 10 seconds it'll go soft again. Also the front left wheel will lock once the brakes are pumped up a bit.
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Re: Bleeding Brake Problem

Post by qwakers »

i would suspect the new hoses. well worth putting the old ones back on if possible...
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Re: Bleeding Brake Problem

Post by PeteH »

I'm going to take a punt.

Is your front left caliper seized? It might explain most of your symptoms. If the pads are not clamping the disc then the flex will cause a soft pedal. You would be able to pump the pedal until the pads were pushed against the disc, and your pedal would be firm. But the pads would stick on, and lock the wheel in a way that the ABS couldn't do anything about it. Then if you release the pedal the caliper would eventually pull the pads back off the disc.

It's a total guess, because your symptoms are weird!

Does your handbrake feel normal? (I'm just trying to exclude the rear calipers).
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Re: Bleeding Brake Problem

Post by Ashman »

Handbrake is fine, grabs and releases perfectly. I did think about changing out the calipers although they have just had new slider pins and they were greased before fitting, the pads were difficult to fit into the calipers though. We also already tried the original hoses too and we got the same results.
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Re: Bleeding Brake Problem

Post by warpc0il »

The pads jamming in the caliper may explain the brake seizing on and could even explain the (apparent) loss on pressure.

The pads must move freely for the brake to work correctly.

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Re: Bleeding Brake Problem

Post by Ashman »

Does anyone know the size of the nuts on the brake pipes going to and from the abs module? They're two different sizes.

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Re: Bleeding Brake Problem

Post by ChrisHolmes »

Surely you just select the relevant open ended spanner from your tool kit, or measure across the flats of the Union. Sorry if this seems a bit curt.
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Re: Bleeding Brake Problem

Post by 13Black »

Maybe thread size? BSP etc
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Re: Bleeding Brake Problem

Post by Ashman »

Sorry I should have been more specific. I need to replace the brake lines and I have the flaring kit with the copper lines and nuts for the pipes going to the abs module but the ones on the lines leading from the module to the calipers is a different size which I don't have. I figured I'd ask here so I get accurate info, I've already been given the wrong info elsewhere.
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Re: Bleeding Brake Problem

Post by ChrisHolmes »

Why not simply use the old ones. Also what us the reason for replacing the lines as they are not usually subject to any corrosion?
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Re: Bleeding Brake Problem

Post by Plan3tBob »

Ashman, did you ever get to the bottom of the issue?!
:shock:
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Re: Bleeding Brake Problem

Post by Ashman »

Sorry i've only just seen your post, no i haven't so far. I bought yet another master cylinder and bled it before fitting and it's not going to the floor so far but the battery is dead so i can't start it to test. I think after having fitted 4 separate MC's i can rule it out (if this doesn't work). I've bought a caliper rebuild kit so i've still got to that but the dreaded health issues have put a hold on the situation for now. Once i get those done and back on if it's still no good then i'm going to take it somewhere to get all the lines replaced, it's the only thing i haven't changed. The MC i took off today was actually leaking fluid back through the booster so that one's had it. I'd buy a new one but Mazda wan't nearly £400 and 3 months.
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Re: Bleeding Brake Problem

Post by Ashman »

The calipers have now been rebuilt and the problem is still there. The only things left are the brake servo and brake lines.
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Re: Bleeding Brake Problem

Post by Plan3tBob »

:shock:
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Re: Bleeding Brake Problem

Post by ChrisHolmes »

This may well be a totally stupid suggestion but you have got the pipes on the master cylinder, ABS unit etc: all plumbed correctly, if indeed it is possible to fit them into the wrong places.

I cannot see what replacing the entire brake pipe system is expected to achieve if they are not leaking at all.
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Re: Bleeding Brake Problem

Post by warpc0il »

Could there be an internal seal leak in the brake booster (vacuum power assist)?
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Re: Bleeding Brake Problem

Post by ChrisHolmes »

But strangely it seems that fluid is not leaking any where. It just is not pressurising which really focussed me into a master cylinder issue
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Re: Bleeding Brake Problem

Post by warpc0il »

It could be leaking into the vacuum reservoir, not outside.
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Re: Bleeding Brake Problem

Post by Liam22 »

Or just a vacuum leak?

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Re: Bleeding Brake Problem

Post by qwakers »

that would cause a hard pedal, not a soft one.

i personally am intrigued by this fault and would quite like a look at it myself...

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Re: Bleeding Brake Problem

Post by Harrizone »

Random thought. :-k Without reading the whole thread again did you perchance change the rear calipers, or refurb them :?: and if so did you fit them back on the right sides :?: If fitted to the wrong side the bleed nipple will be pointing downward and you will never get a pedal :-k . Please don't ask how I know :oops: .
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Re: Bleeding Brake Problem

Post by Ashman »

All bleed screws are on top. Also when pulling the vacuum line off there is a whoosh sound so i don't think there's a leak.
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Re: Bleeding Brake Problem

Post by warpc0il »

There can be an internal hydraulic fluid leak in the brake booster, without there being a vacuum leak.

I had a very minor such leak on my Alfa, which caused the pedal to be a bit soft and the fluid level to drop just a bit quicker than the pads were wearing.

It was 20 years before I removed the brake booster and found a pint of fluid sloshing around inside.

Replacing the booster not only made the brakes sharper but also cured the random puffs of smoke from the exhaust, which were probably gulps of fluid being drawn into the intake through the vacuum pipe.

You've already disproved all the likely causes of the problem so now it's time to check the unlikely ones.

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Re: Bleeding Brake Problem

Post by boosted »

This is weird....there is some logic in the vac line drawing fluid into booster. But that means all master cylinders would have bad seals....
Do you get a pedal bleeding up then when engine running it goes to floor?


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