Bleed Screw problem

Gerard

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Aug 20, 2020
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Had to fit new front brake pads and polished the pistons.
So i had to bleed the system and i use a vacuum pump. Bled the RHS first as per manual and that was successful.
But when trying to bleed the LHS, it would not hold a vacuum. So i removed the bleed screw and found some corrosion at the sealing end, the most likely cause.
Did anyone else came across this problem?
 

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ocgeek

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Aug 20, 2019
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Cant' see much from the pictures but it does not seem too much rust over there
my (modest) experience with the Bleed valves / Bleeding task is that those valves once un screwed a bit DO NOT hold vacuum; air escapes between the caliper and the valve thread
Some use teflon strip (the one used by plumbers) around the thread to reduce air suction
Just keep producing vacuum and the juce comes
I used vacuum to fill the pipes and finished the work by manually bleeding the old way
 

gunslinger_006

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Yup. I had the same issue.

What happens is on the left front caliper, water will infiltrate the threads and corrode the bleed screw.

When i pulled mine i saw the samw corrosion. I got out what i could and installed the speed bleeder. Took a few tries to get it completely seated where it would hold extreme pressures without leaking (two hands on the brake as hard as possible). But i got it right.

When i installed the new bleeders, after everything was done, i used a bead of RTV to seal where the threads meet the caliper. This shouldnt happen again with my seal in place.

When i need to bleed the brakes next, i will just peel off the rtv and bleed, then reapply.


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Gerard

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Cant' see much from the pictures but it does not seem too much rust over there
I'ts the corrosion on the tip of the screw and the rough valve seat i think is the problem. Not bad enough to let the brake fluid go past, just not holding a vacuum when there is air in the system.
I might see if i can lap it out with some fine lapping paste and machine the screw, otherwise bleed it the old way.
 

Don in Lodi

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I must be not understanding here. Pulling vacuum at the bleeders won't hold a vacuum. There's a direct open connection to the master, and air will pull past the threads of the bleeder as well. The speed bleeder fittings should prevent air past the threads, but not the open connection to the rez.
Are you saying you can't pull a vacuum on a closed bleeder? I would blame a poor seal at the pump end of things. You couldn't be loosing vacuum past the seat, you would have a fluid leak as well if the seat wasn't sealed.
 

ocgeek

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... and air will pull past the threads of the bleeder ...
IMHO he meant that
....that he could not build a vacuum between the brake pipe (past bleeder thread) and the vacuum pump (attached to the bleeder)
As commented already above... that'gonna happen...but it's not "exact" vacuum... rather a depression ;)
 

Gerard

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I would blame a poor seal at the pump end of things. You couldn't be loosing vacuum past the seat, you would have a fluid leak as well if the seat wasn't sealed.
I was losing vacuum on the left caliper only with the bleed screw closed. The right caliper and rear caliper hold a perfect vacuum. So yes, i blamed the bleed screw. Tried to swap the right and left bleed screws but they are completely different. I still managed to bleed the system and the brakes are perfect. It still had a belly full of air but once that was gone, it would hold a vacuum.
No fluid leaks, probably because of the viscosity of the fluid.
 
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