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master cylinder

paul dox
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paul dox
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Joined: January 18th, 2013, 10:32 pm

March 19th, 2013, 8:07 pm #1

Hi just rebuilt my master cylinder put back the same as came apart .But still no fluid at the caliper
ANY help would be great
cheers
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mech73
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mech73
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Joined: May 19th, 2007, 8:43 pm

March 19th, 2013, 9:47 pm #2

This seems a very common problem with a rebuilt master cylinder. I did this sketch some time ago to work it out for myself when I had the same issue.



As you can see in the sketch, with the piston is in the start position. Both holes to the reservoir are uncovered. Almost as soon as the piston moves forwards, the small hole is covered by the cup seal. Therefore as the fluid is compressed (by the cup seal) it can only flow down the brake hose.

When you release the lever, the spring pushes the cup seal and piston back. This drops the pressure in the front part of the cylinder (the bit with the spring in). This will try to suck the fluid back up the hose. The idea is that gravity (and a bit of suction) resists this reverse flow. Therefore to equalise the pressure in the cylinder, fluid from the reservoir flows through the larger hole, then through the tiny bleed holes in the piston and past the cup seal. The cup seal in this case acts as a one way valve allowing fluid past towards the hose outlet, but not back towards the reservoir.

Unfortunately with a brake hose full of air and not much fluid, there isn't much effect of gravity or suction. Couple this with a new cup seal which seals too well in both directions and the fluid just yo yo's up and down the hose and it doesn't work.

A good solution is to connect a vacuum pump to the caliper bleed nipple, but most (including me) dont have that option.

I found that if you disconnect the brake hose at the caliper end and apply some human vacuum direct to the hose, the master cylinder will then start to pump. Be careful though as brake fluid tasts horrid. Once the hose is full and the 'one way valve' is operating it's normally away, so reconnect the hose and the caliper should fill and bleed normally.

Pre-filling the calliper and hose backwards using a syringe via the bleed nipple can also work and I know some succesfully use this method to get the master cylinder started.

Obviously it's important to make sure both reservoir to cylinder holes are not blocked, either by debris or piston/seals that aren't quite the correct dimensions. The small hole is usually less than 1mm dia and easily gets obstructed.
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Fs1eweb
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Fs1eweb
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Joined: June 6th, 2004, 1:11 pm

March 20th, 2013, 7:01 pm #3

Fantastic explanation and diagram Mark! Bang on! Pinning for prosperity!
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Helping the FS1E 6,500 perfect their appetite for a Mid Life Crisis since 2004!
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Mikew
The 65mph Club Member
Mikew
The 65mph Club Member
Joined: July 2nd, 2011, 6:02 pm

March 20th, 2013, 10:04 pm #4

I'm having similar problems with my AR80 brake having just bought a genuine master cylinder piston. Tried most of Mark's tricks but did not realise there were 2 holes in reservoir. Will check to see if it is blocked. Good pick Mark
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sidewayscarl
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sidewayscarl
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Joined: September 5th, 2007, 6:48 pm

March 21st, 2013, 6:38 am #5

After many many years struggling to bleed brakes I bought one of these:

http://www.ebay.co.uk/itm/Neilsen-Hand- ... 19d4335f86

It is some of the best money I have ever spent, makes the job so easy and saves so much time ..
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mech73
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mech73
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Joined: May 19th, 2007, 8:43 pm

March 21st, 2013, 7:46 am #6

sidewayscarl wrote:After many many years struggling to bleed brakes I bought one of these:

http://www.ebay.co.uk/itm/Neilsen-Hand- ... 19d4335f86

It is some of the best money I have ever spent, makes the job so easy and saves so much time ..
Cheers Carl, That looks spot on, I'm going to have to get one.
Lincolnshire exile in North Hampshire

Brighton 08/09/10/11/12
East Coast 10/12
IOW 08/09/10/11
Northern 12
Uttoxeter 08/09/10
Spa Francorchamps 11
New Forest 09/10/11/12
Billing 07/08/09/10
Eagle 08/09/10/11
Ace Cafe 09/10/11


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paul dox
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paul dox
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Joined: January 18th, 2013, 10:32 pm

March 21st, 2013, 8:31 pm #7

Thanks mark that is a very good explanation .Hope i can get it up and running this weekend .Cheers
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stevewoods04
The 65mph Club Member
stevewoods04
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Joined: December 18th, 2005, 8:37 pm

March 21st, 2013, 9:37 pm #8

How I managed it and done it quite a few times now is detach hose from caliper put finger over hose end and then slowly and I mean slowly pump the brake lever forcing air out of end so you need to ease pressure off the hose end but only slightly with your finger when you have fluid at end of hose attach it back on caliper then undo the nut on the caliper, the one attached to pipe that goes into the caliper,not the bleed screw on top,then slowly pump again,make sure you only unscrew this a tiny bit as if you were wanting it to only weep fluid out this always works for me takes a bit of patience,Just a point at no time let the fluid level drop in your reservoir or your back to square one
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paul dox
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paul dox
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Joined: January 18th, 2013, 10:32 pm

March 26th, 2013, 9:00 pm #9

Hi i brake working .I had to suck the fluid down the pipe but once it came through it worked really well .
Thanks for all the help .Paul
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looey
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looey
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Joined: October 30th, 2009, 5:56 pm

March 26th, 2013, 9:16 pm #10

That self bleed kit looks good.

I've got one of these and just connect it to the open bleed nipple and suck the fluid down first. As soon as I see fluid in the syringe pipe, I remove it then just quickly bleed in the normal way.

http://www.ebay.co.uk/itm/MOTORCYCLE-BI ... 2326749284
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