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Master cylinder bleeding

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JParty:
Hello,

This one is really doing my head in. I’m trying to bleed my brakes after restoration (so there is no fluid inside at all). I was originally trying to bleed them assembled, but have now taken the master cylinder off to try and bleed it separately.

What I’m noticing is that on depressing the lever, fluid moves around in the reservoir, but only a mix of air with a very small amount of brake fluid comes out. I have checked that both ports in the bottom of the master cylinder is clear, and cleaned the very small orifice with guitar string. I’m also noticing that the brake rod isn’t returning after depressing the lever. If I cover the outlet with my finger whilst pulling the rod back, I feel it pull vacuum on my finger, but it isn’t pulling fluid through the reservoir hole.

I’ve pushed on the outlet side with a blunt rod and it all seems free moving.

Any ideas?

I’m inclined to buy a whole new aftermarket master cylinder as these rival the price of a rebuild kit. Would like your thoughts for any with experience!

Thanks for your time!

Johnwebley:
I personally would buy a replacement,



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K2-K6:
Sounds like the port from reservoir to the piston chamber is not clear.

That vacuum you feel shouldn't be there as when the lever is retracting it should open fully to let new fluid replenish the piston bore.

As it pulls vacuum from output line it won't bleed as is pushes fluid down there (which should stay static when released) and not pul it back. You'll go round in circles like that as you've found, until that port is completely clear.

With the mc empty, can you blow into the output and hear air coming out of the reservoir?

K2-K6:
To add, it suggests the seals are working as it's pulling that vacuum,  just that the port to reservoir should be competently clear to allow the system to ingest new fluid as it needs it and not pull the fluid back up the output line.

JParty:
Thanks for your quick replies. If I blow on the output, I don’t get flow through the reservoir.

I agree this suggests it’s not clear. I got a guitar string and can pass that through which made me think it was clear.  I can can force WD40 down the port and it flows out the outlet. This is why I’m super confused!

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