Have I bought a duff front master cylinder / reservoir?

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Asian Boss
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Have I bought a duff front master cylinder / reservoir?

Post by Asian Boss »

I'm trying to improve one of my old Honda XLs, the one with the weak front brake.

I suspected the front master cylinder seals had failed as the lever came all the way back to the bar without putting much pressure on the brake pads - and bleeding didn't help.

I bought a new master cylinder / reservoir combined unit. A patters one from the states. It looked good. I fitted it, filled it and couldn't get it to bleed or even begin to push fluid or build pressure. I realised I probably needed a new hose too so left it.

I bought a new hose and went to fit it today. I disconnected the old hose from the new master cylinder. I was expecting some fluid to come out of the master cylinder - but nothing came. I looked into the master cylinder connector where the hose joined and it was dry. I tried pumping the lever a little - nothing. I tried pumping it more - still nothing.

Could it be a duff master cylinder? There isn't a cap or something to remove before use is there?

With the top off the reservoir I can see the master cylinder moving through the apertures, which from (distant) memory looks right.
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Asian Boss »

FWIW a pic from above - I took it off the bike.
DSC_0551 (Medium).JPG
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Le_Fromage_Grande »

You've got an air lock, pump it slowly with the bleed nipple open, could take an hour or more
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Asian Boss »

How come none comes out of the master cylinder itself with the hose not connected? I'd have thought I'd see it coming out of the port where the hose screws on.
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Re: Have I bought a duff front master cylinder / reservoir?

Post by KungFooBob »

For £500 plus expenses....
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Hairybiker84 »

I've had it before where a master cylinder won't draw in the fluid, whether it's an air bubble or assembly lubricant, who knows? If you've got a vacuum bleeder, try and draw some fluid through it first. If not a small syringe with clean fluid and try and force fluid back past the piston into the reservoir. Once the master cylinder has actually got some fluid around the piston and internal seals they've worked ok for me.
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Asian Boss »

Hairybiker84 wrote: Wed Oct 13, 2021 2:20 pm I've had it before where a master cylinder won't draw in the fluid, whether it's an air bubble or assembly lubricant, who knows? If you've got a vacuum bleeder, try and draw some fluid through it first. If not a small syringe with clean fluid and try and force fluid back past the piston into the reservoir. Once the master cylinder has actually got some fluid around the piston and internal seals they've worked ok for me.
Thank you.

I don't have a vacuum bleeder. How would you connect the small syringe? I assume to the caliper bleed nipple with some hose?
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Re: Have I bought a duff front master cylinder / reservoir?

Post by David »

Dissconnect the outlet pipe. Apply finger tip to hole. Pump steadily , release finger a little...repeat until you get brake fluid in your eye. Assemble.Bleed as normal.
BE SURE BLEED NIPPLE IS AT THE TOP OF THE CALIPER.
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Wull »

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Re: Have I bought a duff front master cylinder / reservoir?

Post by DefTrap »

I always prime these old skool systems by sucking the fluid through with good old fashioned lung power.
You need a longish bit of tube on the caliper nipple, just in case, because brake fluid is foul.
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Potter »

DefTrap wrote: Thu Oct 14, 2021 7:46 am I always prime these old skool systems by sucking the fluid through with good old fashioned lung power.
You need a longish bit of tube on the caliper nipple, just in case, because brake fluid is foul.
It's better to blow.

Connect everything up dry, then fill a big syringe with brake fluid and connect it via a tube to the bleed nipple on the caliper and squeeze the fluid up the line and into the MC. It pushes air upwards and out, so you're much less likely to get air locks.
Then bleed like normal to ensure it's all good.
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Mr. Dazzle »

I was gonna say the same.

Put your gob over that 'lid' opening and blow as hard as you can ;) Or use a hose as above, your choice.

The absolute best way to do this is to build everything 'underwater'. A mate of mine used to work at an Ohlins centre, they had massive tanks of hydraulic fluid with the 'rubber gloves through a window' thing. It means you can build suspension units, callipers etc. fully submerged in oil. Absolutely brilliant for avoiding air locks.

Don't have access to that any more :D
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Re: Have I bought a duff front master cylinder / reservoir?

Post by Asian Boss »

Cheers chaps, I'm going to have a go at a range of these techniques. Once the correct length brake hose arrives. The one I ordered was the same length as the old, incorrect one!

I have a big syringe somewhere in the garage to use rather than my gob.
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