Hi guys,

Trying to sort out my hydraulic clutch and struggling.
So I have bled almost a hole bottle of brake fluid through it but yet it doesn't feel right. It doesn't have the flick feel at the end.
From what I can measure on my own, it seems that the space cylinder is only moving about 5 - 10mm which seems way to little?
The master cylinder is a 0.70, slave is a 22mm (7/8".
The three hydraulic master clinders on my pedal box are

Front brakes 0.625
Rear brakes 0.70
Clutch 0.70


Cheers!

A pressure bleeder often helps , another way I have used on several occasions is to connect the clutch slave cylinder bleed nipple to a brake calliper bleed nipple using a suitable length of hose then slowly pump the brake pedal pushing fluid into the clutch system and bleeding in reverse. If that doesn’t give enough throw possibly an adjustable pushrod if external slave or spacer if concentric

Hi ya,

The push rod is adjustable, just doesn't seem the slave is moving far enough. So could be wrong size? Need a 0.625 rather than the 0.7p? Not bleeding properly, its a bayonet fitting so pressurised bleeders don't fit, not sure if the vacuum ones that go on the bleed nipple are any good? Or the master cylinder is faulty and fluid leaking around the seals internally?

is the blead nipple at the top of the slave cylinder?

Tell you what, you just got me. I fell for this before!!!!
I forgot about it and turned the cylinder around!!!!!! Thank you, what a prat lol

I've found these to be helpful and good value- https://www.compbrake.com/product-category/brake-accessories/master-cylinders/

Thats the ones I have. I will move the slave cylinder the right way around lol bleed it and try again. If there isn't enough throw I guess swapping from 0.70 to 0.625 would help

Just think of it as ratios, currently your master cylinder is 0.8 the size of the slave, so 10mm movement on the master will give 8mm on the slave. If you change to .625 master then the slave will only move about 7mm.

Hi guys,

All bled correctly lol but still not moving enough.
Is there a bigger master cylinder size I can use?

how is the master cylinder connected to the pedal?
could you connect it so it moves further as the pedal moves?

can you do a photo of the entire pedal and linkage please?

Hi

I Had a similar problem with a standard Alfa Romeo clutch slave cylinder on the V6 and the kit clutch master cylinder,
The ratios were not right and no matter how much I adjusted the rod or bled the system I just couldn’t quite get the right length of stroke on the master cylinder.

However, after thinking about it for weeks i decided to remove the pedal and weld over the existing hole for the clevis pin and place a new hole about 5mm further up the pedal shaft, this gave me about a 5mm longer stroke and from that point all worked perfectly, couldn’t believe how different it was with just small adjustment to the pen hole position

Not sure if this will help but it sound very similar the the problem i had

Hi ya,

A bit harder with mine as its a pedal box so all laser cut and mounted. Would have to see if the cylinder would be OK with a lower angle than.straight on as it is today?

Can you just use a smaller slave?

Thats my thoughts, or a bigger master.
RWD, have just recommended a 0.75 master which will give 10% more travel. So I guess 0.75 or even 0.825 one could solve it?

Hi Guys,

I thought I had a calculation sheet we use at work for hydraulic systems. It suggest, as does online searching, that decreasing the master cylinder size, increases the slave cylinder pressure.
For example, our spreadsheet suggests that a master cylinder of 0.70" and slave of 0.875" , the slave has a stroke of 57.94 mm
Where as a master of 0.625" has a slave stroke of 72.68 mm. So in theory, if I change my master from 0.70" to 0.625", I gain an extra 14.74mm stroke of the slave cylinder.
Or a 0.500" master will have the slave stroke of 113.56mm so a gain of 55.62mm.

What do you think?

Think you have things in reverse! The bigger bore will move more fluid for any given stroke

I thought but not what the hydraulic guys say and the calculations.
Look at this too.

"Logic would tell you that a larger bore size in a master cylinder should produce more pressure but that is actually not true. Because of fluid velocity and fluid movement in most cases the smaller the bore of the master cylinder means the more pressure you are going to have at the caliper assembly or the wheel cylinder assembly."

https://www.onallcylinders.com/2013/10/29/video-master-cylinder-bore-size-affects-braking/

One of the most common misconceptions is that a larger master cylinder will create more pressure. While a larger master cylinder creates a larger displacement, it takes more force to create the same pressure as a smaller bore. While a larger master cylinder will take up system slack with less pedal stroke, it will take more force to create the same system pressure. The result after adding the larger master cylinder is a harder pedal which needs much more pedal pressure to create the same amount of braking force. For instance, moving from a 3/4" master cylinder to a 1" requires 77.7% more force on the push rod.

[Edited on 19/10/20 by the_big_1]

Rusty is right, you are mixing up pressure and volume, at this time you are not worried about force but you need to move more volume of fluid to get a longer stroke. Either a smaller bore slave or a bigger master are what you need.

Hi ya,

Yeah I am going to try a bigger master first.
Current is a 0.70, RWD recommend 0.75 but I have ordered 0.875.
If this works all good, to much travel can enter a bump stop or go a size smaller.
The slave s more difficult as they are not as common with same measurements but different volumes like the masters are.