Do I need a larger master cylinder?

tpiini · #1 (**permalink**) 02-27-2005, 08:47 AM

My clutch is not fully disengaging. I've blead the system already, but no improvement.

I have a 3/4" bore master cylinder and a pull-type slave cylinder, the bore of which I have not checked yet.

I've run through catalogs and found that 3/4" for the master cylinder is pretty standard. Am I missing something here? I've adjusted everything for max-travel but there is still a bit of drag on the clutch. If it matters, it's a Ford Motorsports clutch between a 289 and a Tremec 3550, and in a stock Ford bellhousing with a stock Ford fork.

My first instinct was to figure the hydraulic displacements and just go with a larger master cylinder to move the slave cylinder a bit further. But the more I think about it, I feel that my FIRST step should really be to tap the vast knowlege of the Club Cobra crowd. What I NEED is for the slave cylinder to travel a bit further. Probably 1/4" would do it.

Help???

Rick Parker · #2 (**permalink**) 02-27-2005, 09:57 AM

You are correct in assuming that 3/4" is pretty standard for the Master Cyl. I believe that 's what is specified most commonly. A 7/8" slave is what I have with a 3/4" master. The lever in my application moves rearward to disengage the clutch. It is a '65 vintage application with a "Long" style pressure plate. With those two cylinders sizes there is ample movement for proper clutch function. Pedal pressure is quite manageable. Pictures in my gallery.

Have you bled the system thouroughly?? Take all of the free play out of the pedal, bleed it and then readjust it.

Rick

trularin · #3 (**permalink**) 02-27-2005, 12:10 PM

This is a matter of volumes.

If you have a 3/4" master and a 7/8" slave, you will have to move the master further than the slave will travel.

The 3/4" master gives you a big advantage for leverage in that you will output more force on the 7/8" slave for every pound you put into the 3/4" master.

Is the pedal easy to push?? Can you live with an increase in pedal pressure? If yes, than move to a 7/8" master or reduce the size of the slave to 3/4" That will give you a 1 to 1 pedal ratio.

Hope this helps.

relaxinrob · #4 (**permalink**) 02-27-2005, 12:18 PM

do you have the ability to move the slave in farther on the clutch fork, thus moving the clutch more?

tpiini · #5 (**permalink**) 02-27-2005, 01:38 PM

Quote:

Originally posted by relaxinrob

do you have the ability to move the slave in farther on the clutch fork, thus moving the clutch more?

Unfortunately I do not. I checked things out a little more today and it looks like the slave cylinder is going full-stroke. I'm starting to wonder if I can remount it and use a bell crank type of assembly to add a little travel to the fork in relation to the cylinder stroke.

Cobra20646 · #6 (**permalink**) 02-28-2005, 06:55 AM

I went the bell crank or lever route to get some additional travel. My clutch fork needs to move forward to disengage. Only draw back is the additional pedal effort.

tpiini · #7 (**permalink**) 02-28-2005, 07:08 AM

20646,

That's exactly what I was thinking of doing! Do you have a few more pix of your setup?

Cobra20646 · #8 (**permalink**) 02-28-2005, 07:21 AM

That's the only one I have on my computer here at work..... let me check at home later today..... I think I may have some there.
The job requires some fabrication to locate mounting points on the tranny (or where ever you choose to mount it) so the thing will pull or push in a straight line. Also the slave has to pull straight back with as little movement as possible. These slaves have been know to fail if they pull at an extreem angle. Mine has been service for over four years and 10K miles without issues.