how does a master cylinder self adjust?

[dieselnutjob]

When you press the (say clutch) pedal I am thinking that as soon as there is any pressure in the cylinder there can't be way for the pressure to escape, except through the pipe to the slave cylinder.

 

Therefore any way for fluid to get in from the reservoir for the self adjustment must be right at the top of the pedal movement.  It must be in that dead feeling bit when the pedal comes off of the stop but before it actually meets any resistance?

 

So lets say that there is not enough fluid in the system because it has not yet self adjusted.  What makes the pedal travel back to the top?  Surely the clutch springs will push on the slave cylinder which will push the pedal half way up but no further, so how does more fluid get in?

 

The only thing I can think is that if you then hooked your foot under the pedal and lifted it all the way up then this would suck the slave cylinder completely off of the clutch springs until a port in the top of the travel would be uncovered allowing fluid in from the reservoir and the suction would pull in a bit of fluid.  I guess that this is why they have an overcentre spring so that as the pedal gets near the top it is pushed all the way to the stop.

 

Otherwise ff you didn't lift it yourself then the only thing that would pull the pedal up would be a return spring.  I notice that brake pedals always  return up, do they just have a really big return spring?

 

Have I gotten this about right?

 

Also I have been looking at alternative clutch master cylinders for SWMBOs freelander.

 

The standard one is plastic and rubbish, but the bore is about 18mm which is pretty near to 0.7 inch which is a standard size for a Girling master as used on various classics.

 

These seem to have a much greater stroke on them than the Freelander one, so I suppose it doesn't matter if it is just adjusted so that the pedal bottoms out before the piston and so not all of the stroke is used up.  What if I got it wrong?  Presumably the thing could push more fluid into the slave than it is designed for and blow it apart?  It would sort of super extend the clutch springs and then extend too much and pee fluid all over the bell housing?  So I guess I have to be careful then...

 

Have I misunderstood anything?

[Taff]

Unless you are keeping the Freelander for ever, I wouldn't waste time trying to get an incorrect m/c to fit.

 

I've done a couple of mix'n'match brake set ups for hot rods and they aren't much fun to get "adequate". Pedal travel to m/c ratio is more important that you would imagine, so mixing up components can make for a lot of work.

[dieselnutjob]

We are planning to keep it forever. I refuse to spend £0.01 more on plastic hydraulic components which is why it got a metal slave today along with a solid flywheel. SWMBO loves the thing to bits and I believe in making cars last as long as possible.

[red5]

Right, if you're determined then...

 

Measure bore and stroke of both. Calculate fluid displaced*. Adjust stroke of 'new' to match displacement of original one (shim etc etc)

* or, fit clear plastic pipe to end,and turn upwards. Fill m/cyl with Dot4, pump some fluid into pipe (no air). Mark starting point. Pump once. Mark finish point. Do same with alternate - compare.

 

[hauserplenty]

If you Google Pascal's Law, it will explain why that is so. I'd link you to an article but CBA so soz innit.

 

 

Sent from my tin cans and string using Crapatalk

[Junkman]

No Master (or indeed slave) cylinder can possible be self adjusting in any way.

By pressing the pedal, a defined amount of fluid is ministered into a hydraulic system.

It's this quantity, and this quantity only. Not more, not less.

Any adjustment is by means of mechanical devices outside this hydraulic system.

[Barry Cade]

  On 04/12/2015 at 02:25, Junkman said:

No Master (or indeed slave) cylinder can possible be self adjusting in any way.

By pressing the pedal, a defined amount of fluid is ministered into a hydraulic system.

It's this quantity, and this quantity only. Not more, not less.

Any adjustment is by means of mechanical devices outside this hydraulic system.

Unless it's French. Then all bets are off. E= MC Hammer.

[red5]

Eeer..

 

The pedal applies pressure to fluid (liquid) which is present in the system.

 

'Extra' fluid is allowed into the system via the master cylinder bleed port to allow for wear of the (in this case) clutch material  - a self adjusting slave cylinder keeps its piston in contact with the clutch release arm (which moves 'away' from the slave cylinder as the clutch wears down)

 

You're welcome 

[purplebargeken]

I was lost at 'thinking'. Please carry on. 

[dieselnutjob]

  On 04/12/2015 at 02:50, red5 said:

'Extra' fluid is allowed into the system via the master cylinder bleed port to allow for wear of the (in this case) clutch material 

 

Is this bleed port simply a hole in the side of the cylinder?

Where is the hole?  Once the piston has passed by the hole what stops fluid from the reservoir going to the wrong side of the piston?

[dieselnutjob]

The generic Girling ones I think might work like this

https://www.youtube.com/watch?v=zd18NcTDCLU

[PhillipM]

  On 04/12/2015 at 02:25, Junkman said:

No Master (or indeed slave) cylinder can possible be self adjusting in any way.

By pressing the pedal, a defined amount of fluid is ministered into a hydraulic system.

It's this quantity, and this quantity only. Not more, not less.

Any adjustment is by means of mechanical devices outside this hydraulic system.

They're all self adjusting by the bleed port that allows more fluid in from the reservoir. There's a soft internal return spring to help when there's no hydraulic pressure.

 The initial part of the pedal push shuts this port off. Cheap master cylinders are just a drilled hole, but more expensive and race master cylinders are a machined slit which the piston stop point is hand shimmed right up to to reduce unnecessary pedal travel (which then means you can use more hydraulic leverage with bigger calipers, etc)

[castros_bro]

Gods teeth are you all fucking morons?  It's nearly lunch time so here's a quick* sketch and no words.

 

 

[PhillipM]

  On 04/12/2015 at 12:05, dieselnutjob said:

Is this bleed port simply a hole in the side of the cylinder?

Where is the hole?  Once the piston has passed by the hole what stops fluid from the reservoir going to the wrong side of the piston?

There's generally a cup seal with a backup ring which is your primary hydraulic seal, and there's an o-ring or face seal at the back that seals fluid in the res once the pedal is pressed.

 

The ones that seal on the far end like the Girling design are good as they can be used inline for handbrakes, etc, but suffer from more travel as the seals have to compress, and they need more clearance to start with as the end seal expands towards the end when the fluid warms up.

[steve_earwig]

  On 04/12/2015 at 12:12, PhillipM said:

They're all self adjusting by the bleed port that allows more fluid in from the reservoir. There's a soft internal return spring to help when there's no hydraulic pressure.

 The initial part of the pedal push shuts this port off. Cheap master cylinders are just a drilled hole, but more expensive and race master cylinders are a machined slit which the piston stop point is hand shimmed right up to to reduce unnecessary pedal travel (which then means you can use more hydraulic leverage with bigger calipers, etc)

That's just letting more fluid past to allow for friction material wear. Master cylinders (and wheel cylinders) don't have any physical adjustment, it all happens elsewhere.

 

  On 04/12/2015 at 02:25, Junkman said:

Any adjustment is by means of mechanical devices outside this hydraulic system.

 

Unless you're talking caliper pistons and, presumably, clutch slave cylinders (it's years since I had one of those apart, on a Marina)

 

Back to the OP, if we're talking clutch systems then this stuff isn't life-threatening and allows for some experimentation. 18mm, 0.7" is .7x25.4=17.78 so yeah, pretty close. Not sure about the stroke but won't the real limiting factor here be pedal travel? So the length of the cylinder is probably an elephant irrelevant.

[Dave_Q]

Yeah I think the pedal travel is more of an issue than the cylinder, yes the cylinder will move the same amount of fluid if approximately the same diameter.

 

The distance that the piston is moved by the linkages on the donor will control the amount of fluid moved.

 

You will really need to measure on the donor car how far the pedal moves the cylinder and compare to the freeloader.

 

If you're really determined you could change the m/c travel to match the donor by modifying the pivot points on the pedal linkage....

[dieselnutjob]

The Girling generic 0.7 inch master cylinders have a longer stroke than the original Landrover plastic one.  Apparently the Girling ones have a stroke of 35mm whereas the LR one was twenty something.

 

For the time being the Landrover plastic master is working with the metal slave.  "gearbox Dave" told me that the plastic slave fell apart when he tried to remove it which might indicate that it was a problem.  Or it might be that he removed it with a lump hammer....

 

My neighbour told me that he had to fit three master cylinders on his Freelander before he found one that worked, therefore if (when) the one that I have fails I will just redesign it instead.

[dieselnutjob]

  On 04/12/2015 at 12:14, castros_bro said:

Gods teeth are you all fucking morons?  It's nearly lunch time so here's a quick* sketch and no words.

 

 

Master_cylinder_diagram_svg.png

 

This is what I was thinking of using

 

[Dave_Q]

I wouldn't have thought that too much stroke would be an issue.

 

**edited to add - as long as the m/c reaches the "top" of it's travel as that's where self adjustment would occur.

 

Give it a go and let us know how you get on.

[Eddie Honda]

castros_bro, Fuckin' 'ell! A diagram of a dual circuit brake master cylinder is gonna blow their minds!

[castros_bro]

  On 04/12/2015 at 13:51, Eddie Honda said:

castros_bro, Fuckin' 'ell! A diagram of a dual circuit brake master cylinder is gonna blow their minds!

 

Mitigating circumstances

 

[PhillipM]

  On 04/12/2015 at 12:29, steve_earwig said:

That's just letting more fluid past to allow for friction material wear.

What else would you have something self adjust for?!

 

It's still adjusting

[red5]

Indeed...it's like being in class all over again...