Clutch bleeding problem

[paulsco]

I think I understand the concept of how the clutch master cylinder works, but I am having problems bleeding the clutch.

As part of my restoration I had the original master cylinder re-sleeved and rebuilt; I fitted it some time ago, but have only just got around to bleeding it.

The hydraulic fluid flows to the master cylinder, but no further.
I have disconnected the pipe to the slave cylinder, but even when pumping, no fluid comes out of the port in the master cylinder.

I have taken the master cylinder to pieces and as far as I can tell, everything is free to move as it should.

For some reason the fluid does not seem to be passing through the master cylinder.

Any ideas?

Thanks,

Paul

[mgcjag]

Hi Paul....If you have no fluid in the master there must be a blockage from the reservoir to the cylinder.....If you have fluid in the master but no pressure when you press the pedal then possibly the valve in the master not isolating the reservoir ....check out the valve..Steve

[paulsco]

Thanks for your help Steve,

It looks like the reservoir is being isolated; the valve is not allowing fluid into the master even when there is no load on the pedal.

I have taken it to pieces again and can see nothing unusual,
but having re-read the description I cannot see how this is supposed to work.

Even at rest the seal sits against the inlet port from the reservoir sealing it off. The main spring only pushes the piston away from the spring support , not away from the end of the cylinder to let the fluid in from the reservoir?

My understanding is that rest, both ports should be open to each other?

Paul

[mgcjag]

Hi Paul....did this master ever work.....It should be quite easy once you have it on the bench looking at it...somehow the valve must be in the wrong position if with the pushrod fully retracted the valve is blocking the input...check it out again..Can the valve stem move in the piston or is there something in there blocking it..........or get a replacement...Steve

[paulsco]

Hi Steve,

This is the original unit rebuilt by professionals; stainless steel sleaving and new parts where necessary.

Originally and now it has never worked in my possession.

I have stripped it (twice) and checked it against the parts manual and the service manual and there is no indication that anything is in the wrong place; everything moves freely.

The thing is, the piston main spring and everything else that is attached, when not under pressure is free to slide forward and backwards as a unit, to the push rod and back to the end of the cylinder where it cuts off the flow. once it is pushed along the cylinder with the push rod and the push rod released the whole piston unit sits against the back (front) of the cylinder and stops any fluid entering from the reservoir. As far as I can see the only way it would move back the other way is if the reservoir was under pressure.

https://dl.dropboxusercontent.com/u/849 ... Clutch.pdf

Page E10 shows the components of the master cylinder.

Paul

[mgcjag]

Hi Paul..could the small valve spring be too strong and holding the valve closed.....Ideally you need to compare it with a known working unit...If you post some photos of your unit dismantled someone here may spot a problem... Steve

[PeterCrespin]

Fluid doesn't flow uphill. The line from reservoir to master probably has an air lock. Either unclamp the reservoir and hold it as far above the slave as the hose will let you, until the fluid run in and the bubble come out upwards, or blow the fluid through the hose until it seeps out of the master cylinder connection and then retry.

[paulsco]

Hi Pete,
The fluid does flow to the master cylinder. Each time I have disconnected it to check the cylinder, I have had to catch the fluid coming out of the end of the pipe.
The problem is it gets no further into the cylinder as the seal is closing off the port.
I am going to try pushing back the piston assembly through the inlet port to open the way to the slave port, not touching the pedal and gravity bleeding the slave; perhaps pressurise the reservoir and see what happens.

Paul

[Durango2k]

Re-sleeved may mean they forgot to drill the holes for the pipe ports ?


Carsten

[paulsco]

Hi Carsten,

Thanks for the idea, but I have checked and the drilling it is there and unobstructed.

Paul

[paulsco]

I just thought I would update you on my progress.

I went with a suggestion to pressurised the reservoir.
I used a section out of a bicycle inner tube with the valve in it. This just stretched over the mouth of the reservoir and was held firmly with a jubilee clip. The other end of the tube was gaffer taped over and doubled back on itself and taped again. I was able to pump this up with a bicycle pump to a small balloon. The pressure was enough to push back the piston and push the fluid down the pipe.

It now appears to be working fine.

Thank you all for your help and suggestions.

Paul

[mgcjag]

Hi Paul....This has now proved that you have a problem with the master cylinder...All that pressuriseing has done is to push back the valve to allow fluid in..so the valve spring is too strong or something else is incorrect........If you leave it like this you will have future problems, how under normal conditions will the fluid reservoir keep the system full and allow any possible air up from the slave..........Steve

[paulsco]

Steve,

I had thought of this, but the spring is not holding the piston and valve components against the port.
The piston, spring and valve and the other parts are held together as one unit which is like a slug and can be pushed about a 1/8th inch ether way along the bore.
So when I pressurise it, I am simply moving it that 1/8th inch towards the push rod, but this is enough to expose the port to the clutch slave. I am not pressurizing it to compress the spring.

I don?t fully understand this, but my theory is that after the clutch is pressed, the clutch itself when released will push back the fluid in the slave cylinder, thus pushing back the slug in the master cylinder and opening the port to the reservoir.
I would have thought that by design there should be a further spring behind this slug to start this movement, but I assume Dunlop had their reasons.

By the way, the clutch is quite heavy; is this normal?

Thanks for your suggestions.

Paul

[PeterCrespin]

Hi Paul....This has now proved that you have a problem with the master cylinder..

Beg to differ Steve. Dribbles notwithstanding, I bet he had an airlock and advised pressurising the fluid through in my first post. He did and it worked. QED.

It is a closed system in the operative section, so provided he keeps the reservoir topped up no air can enter.

[paulsco]

Pete,

I can see that I have annoyed you, but I wasn?t sure how.
I have read through this post and my one on JL and I think I now understand why.
It is clear that you feel you answered my query in the first place and with hindsight you probably did.

I have a great deal of respect for you and the other posters and would not consciously do anything to undermine this.

My sincere apologies.

Paul

[PeterCrespin]

Pete,

I can see that I have annoyed you, but I wasn?t sure how.
I have read through this post and my one on JL and I think I now understand why.
It is clear that you feel you answered my query in the first place and with hindsight you probably did.

I have a great deal of respect for you and the other posters and would not consciously do anything to undermine this.

My sincere apologies.

Paul

Sorry. Don't follow you Paul - I'm not annoyed. I generally don't get annoyed. I get stroppy sometimes and worry about my own posts. Don't worry about yours. We're all fellow-travellers and if we know a little the onus is on us to pay it forward and share.