Clutch master cylinder

[Jimbo]

The master cylinder has started leaking, fluid is dribbling down the clutch pedal !

Should I get a brand new one (£20 for a Lucas part), get a pattern part (£10) or get the repair kit (£2) and replace the seals etc ?

I spent a while rebuilding a master cylinder on a MGB once, after doing it once it still leaked, so I did it again - still leaked, and then I found that the piston bore was scored.........so ended up buying a new one anyway  :roll:

[zulublue]

IMO Lucas part, job done

[Jimbo]

IMO Lucas part, job done

Yep, I'd fairly well convinced myself of that too - might try the local stealers/independant tomorrow, otherwise I'll have to pay for delivery from Paddocks.

[william127]

buy shiny stuff from paddocks, get the price to over £50, free delivery, everybodys happy(exept the bank manager) ;)

[Saffy]

There is a old grease monkey tale that more often than not the slave will go shortly after a new master is fitted. New master with increased pressure will expose any weakness in an old slave.  It has happened to me before. May be worth changing it out at same time so they both the same age and you'll only have to bled it just the once.

[Jimbo]

Thanks, I'll near that in mind - the trucks only done 24k, so I'm a bit surprised that the master has let go already, the one on my disco Lasted over 100k !

[phil c]

sam center on ware rd will prob be able to get hold of Lucas parts and if they have to oeder in you wouldnt have to pay postage cos you could just pick it up when ur next passing

[Jimbo]

sam center on ware rd will prob be able to get hold of Lucas parts and if they have to oeder in you wouldnt have to pay postage cos you could just pick it up when ur next passing

On this occasion, the SAM centre can't even supply a pattern part - AJD's came up with the goods though !

[william127]

ajds are good and the last 2 times ive been in there its been almost the same price as paddocks, when you take into acount delivery.

[Landy Andy]

Andy at AJD keeps them as stock item, got master and slave for mine in between Xmas and new year. I change the pair to save pizzing about later on.

Andy

[V8MoneyPit]

There is a old grease monkey tale that more often than not the slave will go shortly after a new master is fitted. New master with increased pressure will expose any weakness in an old slave.  It has happened to me before. May be worth changing it out at same time so they both the same age and you'll only have to bled it just the once.

There won't be any difference in pressure. But I do wonder if the bleeding can dislodge crud. Or maybe the stroke ends up slightly different and the slave seal comes in to contact with the crud buildup at the nd of it's stroke.

[Saffy]

There won't be any difference in pressure.

You will have to educate me on that one. Won't a leaking hydraulic system like in this case by nature provide less pressure to the slave? Or is the leak passed a seal external to the pressure side of master?

[V8MoneyPit]

If the clutch is releasing the pressure will be the same whether it's leaking or not. As the fluid then leaks past the seal the pressure will gradually reduce as the spring is released.

Basically, a certain amount of force is required to overcome the clutch spring. This will generate a specific pressure which will always be the same, directly proportional to the force.

[Saffy]

If the clutch is releasing the pressure will be the same whether it's leaking or not. As the fluid then leaks past the seal the pressure will gradually reduce as the spring is released.

Basically, a certain amount of force is required to overcome the clutch spring. This will generate a specific pressure which will always be the same, directly proportional to the force.

Think I see it now. So if the clutch spring needs (made up values) 50psi to overcome, no matter what the hydraulic system is capable above that value it will only ever reach 50psi. So even if the leaking system only can produce 60psi and the new one could produce 120psi, nether will get above the 50psi the spring releases at, hence "There won't be any difference in pressure" seen at the slave between leaking and new. That the jist?
What if the clutch is fully disengaged and there is more travel in the clutch pedal, won't the pressure increase then?

[V8MoneyPit]

Not really!

Think in terms of the pressure being a result of the force. You are applying a force with the master cylinder which acts against the slave cylinder. If the slave cylinder was solid (can't move), then the more force you apply with the master cylinder, the greater the resulting pressure. But the slave can move. It is pushing a spring (the clutch diaphragm) which has a specific rate. For example, the diaphragm might need 50 lbs to move it an inch (50lbs per inch rate). If, at full stroke of the master cylinder (pedal on the floor) the diaphragm has moved 1/2 and inch, it will be 'resisting' the slave by 25 lbs. So the force the system is seeing is 25lbs. The pressure in the system will be directly related to this force. So if, when you press the pedal, your clutch displaces by that 1/2", the pressure will be the same every time you do it regardless of whether the cylinder is leaking.

The pressure will only be less if the diaphragm is not displaced by the same amount. A leaking cylinder tends to displace pretty much the same amount of fluid at first. The, with your foot still on the floor, the fluid starts bypassing the seal, at which point, the diaphragm starts to move, reducing the spring load and hence the pressure.

I'm not sure I'm explaining it very well, but hope it makes some sense!!!

[Saffy]

I'm not sure I'm explaining it very well, but hope it makes some sense!!!

:D cheers for trying and I can picture it better now.