Master cylinder bleeding

[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!

[K2-K6]

Have you dismantled to clean it through ?

[Athame57]

I bought a replacement from DSS it looks a little different but it works fine.

[fogrider]

I've sometimes got things going by placing a long clear tube over the bleed nipple and sucked on it whilst working the lever.
Worth a try, minimal cost.

[McCabe-Thiele (Ted)]

With brakes I'm a big fan of either pressure bleeding where possible from the top or suction from the caliper end.

[K2-K6]

Ultimately it's about that vent/passage between the piston bore and the reservoir for any brake system.

If it's not competently open, clear and able to pass through fluid easily, then the system won't work as designed.

There's almost nothing in them, with the lever piston first closing that vent pathway then subsequent travel pushes pressure into the now sealed line to caliper to exert force on the pistons/pads.

As soon as the lever is retracted the line then has to be filled with new fluid in volume to how much the pads have been worn. It's this that keeps the lever travel equalised during use. Any impairment on that feed flow will stop the system working/bleeding etc. It just shunts the fluid down and then retracts the fluid (which it shouldn't do) because a vacuum is held in the operating line when that shouldn't be so.

Because you can feel it suck with your finger on output that shows the main seals are functioning,  but it should be able to pass backward along the master cylinder bore without doing that, and facilitated by the vent being open.

[philward]

I had similar issues years ago on my 750k2 and as K2-K6 says, it was the smaller hole blocked - it appeared clear when poking a pin down but drilled the hole with a 1mm drill and it blead instantly

[JParty]

Thanks for your help and time. Really appreciate the replies. I’ve ordered a DSS master cyl as need to get the bike on the road before the weather turns. Come winter I’ll get a rebuild kit so I can get the original back on.