Help bleeding hydraulic clutch line

I am having trouble bleeding the line on my home-brewed hydraulic clutch setup for my '72 Dart.

Pedal modified for correct ratio/connection to Wilwood master cylinder, #4 Teflon braided line (had it and fittings handy, or I would have used #3 line), a Howe pull slave cylinder actuating a stock clutch fork.

But due to the "busy" left side on a big-block A-body with underchassis headers and a scattershield, I routed the clutch braided line up and over the base of the brake master. So it's about 3-4" higher at the apex than the clutch master. I am sure that there is a large air bubble in there, at the highest point of the system.
(Yes, I bench bled the master)

Attempts to bleed it with a container connected to the slave bleed screw (and the end of the tubing under the fluid level!) have not been successful. In fact, I made it worse!
There are a few ways to bleed I can think of:

• pressure bleed from master
• vacuum bleed at slave
• reverse pressure bleed from slave

Due to the master's output port location (on top of the barrel, angled toward the firewall), it will be difficult to make a line that is always below the level of the fluid in the reservoir.
I could dismount the master, hold it as high as possible and pump the piston manually while a helper cracks the bleed screw on the downstroke. But that's a pain.
Any thoughts?
thanks
Charles

First thing I would do is reroute the hose then get a pressure bleeder that goes on the master, that will push any air out of the system

I just realized that I put this in the wrong forum Oh well.

Anyhow do you think I'd be able to shove the air bubble down and out with a pressure bleeder and the hose as-is? That'd be a lot simpler

You could do that with a residual pressure valve which you would need anyway as the line will bleed the fluid back down to the master, damn gravity. I'm not sure how well a residual valve will perform on a hydraulic clutch though. The best way to fix this would be to reroute the line. Maybe you could use a mix of flex and hard line to get the routing you need in tight areas.

To save yourself a lot of future problems reroute the line correctly below the master.

Hmm... I hadn't considered that. Thanks for the tip.
But if the system is full of fluid with no air, how could it run back down the 3" slope into the master?

I routed the braided line up to a small bracket secured to the firewall, where a bulkhead fitting provides a connection to the short "humped" line going from there to the master.
It does sound like replacing the (upper) line with a piece of steel brake line (that can make tighter bends) is the first thing to try. Then, of course, I have to buy an adapter to go from #4 AN to a 3/8-24 inverted flare... or maybe just run hardline all the way down! The slave doesn't move since one end is fastened to the torsion bar crossmember bolts.

Also I don't have a pressure bleeder. Considering the Motive Power 0103 which fits my brake master, but also need to find a cap that fits the Wilwood clutch master...

The real question is how much fluid displaces during a full stroke.

Sounds like rerouting may be the best solution. Not just for now but air bubbles will always accumulate at the highest points, which needs to be the master's reservoir.

What are the bore sizes on your master and slave?

- EM

Reroute if you can, but sometimes your stuck with the application as it it, I have a mid engine vehicle that the slave cylinder/lines are higher than the pedal's master and it's a nightmare when servicing, the only thing that has worked for me is a Mightvac hand bleeder to draw the fluid/air thru the system...

I removed the #4 braided line between the bracket-mounted bulkhead union and the master, and replaced it with a creatively bent section of 3/16" brake line. The 3/8-24IF to 4AN adapter came off the master and screwed on the free end of the #4 line going to the slave instead. Then the end of the steel line from the master connected right up to that adapter.

There's a couple of inches of pedal near the floor where I can feel the pressure plate spring load, even without bleeding it

Now I'm waiting for a friend to lend me his left leg to help bleed. I've never had much luck with "one-man bleeders" since air always seems to intrude around the threads when the bleed screw is loosened. I suspect I can just gravity-bleed it (i.e. open the slave bleeder and let it run until a big pocket of air comes out).

Somebody asked about master and slave sizes? I have a 7/8" master and a Howe 829 pull slave (not sure of the equivalent piston diameter, especially since the rod occupies some of the area). One comment on Summit says that his works with a 3/4" master but I wasn't getting quite enough plate departure when I initially tried a smaller one (couldn't get into gear when sitting still, grind reverse, etc.) and the fork was only moving about 3/4". Of course, maybe there was still an air bubble in that high spot, too!

Anyway once I get it bled I'll be able to measure the fork travel at the tip, which should be about 1". (I can't directly measure the plate departure since I have a Lakewood with no inspection cover).

The 7/8 master and 3/4 slave (which is what the Howe #829 is) should be perfect for you and your slave also has 1-3/8" of travel. I have the same set-up on mine (although Wilwood brand master and slave) The only issue you may have is that often the dirt track parts manufacturers (like Howe) won't warranty their cylinders on street cars because the duty-cycle is a lot different. You may very well be fine as many other guys have done the same as you. Best of luck on the bleeding process.

- EM

Today I gravity-bled the system, which took a while but I didn't have a helper. Fluid, air, fluid came out,.
Then I had no pedal at all

Next step was to disconnect the line and "firewall bleed" the master again. Big air bubbles just kept coming out of the tube into the reservoir, no matter how much I pumped the pedal, AND the fluid level wasn't dropping as it displaced air.

I took it off the car and was able to bench bleed it. But after a few minutes, big air bubbles back into the reservoir!
The brand-new (since last year) RPC 7/8" master must be admitting air somewhere, although there aren't any external leaks.

SO I put my used-once 3/4" Wilwood back on. Bench bleeding took only one or two strokes,using the same loop of brake tubing as before. And without even re-bleeding at the slave (the line was still full of fluid), I have a hard pedal. There is a little bit of "softness" beyond the free play which may be an air bubble in the line where I disconnected it.

But even with the slave rod still in the hole I'd drilled one inch closer to the pivot (than the center of the original A-body fork hole) there's only about 7/8" travel measured at the original hole center. That may not be enough which is why I tried a 7/8" master. The bigger master would fully disengage the clutch, but made it really stiff though. I thought diaphragm clutches were supposed to be easy on the leg

I'm wondering if my Centerforce DF sat so long that some material (or rust) is stuck to the flywheel and reducing the needed clearance. I may have to make a hole in the bottom of the Lakewood so I can see what is actually going on in there... doesn't NHRA allow that? The SFI sticker is waaay expired anyhow.

Will try bleeding and measuring travel again before anything else, though.

Yep, I went through all of these same issues with my similar set-up although I didn't have the problem of excessive pedal pressure with the 7/8 master due to my pedal ratio. One thing you really need to be sure of is slave alignment both vertically and horizontally in relation to the clutch fork hole. The slave must be close to 90 degrees to the clutch arm when the arm is mid-way in it's arc of travel. Otherwise you'll have binding and additional force and ultimately a quickly-worn and leaking slave. I also added a tiny dab of lube at the fulcrum/pivot point of the clutch arm which reduced pedal effort a bunch.

On the bleeding, after using gravity, manual, vacuum and reverse bleeding methods, the only way I could get ALL of the air out of the slave on my set-up was to remove it from my truck and hang the cylinder and line from my garage door so the cylinder was on top. Then I reverse bled with a syringe. It took about 2 minutes. Hooking that back up to an already bench bled master, all I needed to do was "burp" the line where I made the connection to the master.

- EM

Thanks for the tips.

I read my old HPDesign "Clutch and Flywheel Handbook" (the New Revised Edition... from 1991) and I think there must be excessive friction somewhere in the system.

According to the book, a diaphragm pressure plate (like my Centerforce) typically has a lever ratio of 3.5:1.
Fork is about 5:1, master/slave 1:1, pedal 6:1. So approximately 100:1 total ratio from foot to pressure plate. So far so good.

Which is reasonable since a typical plate departure is .060", translating to 6" of pedal movement. Note that this also implies 1" of fork movement.
And assuming 3000# clamping pressure, through 100:1 ratio, means only 30# pedal force at the start of the stroke. Belleville springs are supposed to require less force as compressed. My new pedal connection is made on the pedal arm, so the ratio should stay constant. Over-center spring is removed.

Slave is lined up as close as I can get it... certainly not way off to where the angle would cause problems. Even with 10 degree misalignment, cos(10)=0.985 so there wouldn't be much wasted effort. I'll see if I can measure the pedal force somehow (bathroom scale between my foot and the pedal?) but I estimate it's at least 60# or more!
More like what I'd expect from a B&B plate, not a diaphragm... and it feels smooth, not jumpy or sticky. Just really heavy.

My pedal resistance also felt heavy until I lubed the fulcrum. I think it "halved" the force afterward.

- EM

Pedal pressure cut in half?? That is a huge difference! It must have been really binding.
How often do you have to relubricate it?

I hope I can reach in through the fork hole in the Lakewood and get some hi-temp grease to the pivot area, maybe with a long swab...
Since you experienced such a dramatic change, I certainly want to try it

I've never had to re-lube it. I used the "Honda" high temp Urea grease on mine. https://www.amazon.com/Genuine-Honda-08798-9002-Urea-Grease/dp/B00BFDFYKW

- EM

I had another thought... could the ~2 feet of #4 Teflon braided line be expanding enough to cause a noticeable loss of pedal travel?

There is very little relative motion between the master on the firewall and the pull slave (with its fixed end connected to the torsion bar crossmember). Could I use a hard line all the way to the slave? Or isn't it worth the extra effort?

Mine is hard line from the master but the last 12-16" (before the slave) is braided line. Due to transmission movement and vibration, I wouldn't run a 100% hard line to the slave.

- EM

I haven't had a chance to fire up the engine yet (pan still off as in other threads).
But I'm still not sure I have enough fork travel. With the line bled, I have just about 1" travel.

I contacted Centerforce and they told me the required plate departure for full disengagement is .030", which is achieved by a 3/8" depression of the fingers at the throwout bearing, requiring 500 lbs.of force.

So now I can work backwards from the throwout.
3/8" at the bearing = 15/16" at the fork if it's a 2.5:1 ratio. My slave cylinder is pulling on a hole I drilled about 1" inboard of the original large fork hole, and the 1" measured travel is at the original hole.

Obviously with larger ratios I'd need more travel. Even though that would make for a fairly stiff pedal. As it stands (and I'm just guessing without re-measuring), 1:1 in the hydraulics and 5:1 at the pedal gives an overall 12.5:1, so 500/12.5 is 40 lbs at the pedal.

Does anyone have measurements for a CF702 fork handy? I've emailed Brewer's (where I bought it) but figure someone might have a tape measure and a fork sitting there.
If I yank it out of the Lakewood, it'll never go back on the bearing without pulling the trans and I am not in the mood for that right now.

Also, what's an easy way to actually measure the force at the pedal? I don't have a bathroom scale and even if I did it's hard to read while keeping it steady between the pedal and foot

Finally, I looked at the reviews of a Wilwood pull cylinder on Summit and they are almost all bad (premature wear and leaks)... hmm. Maybe I'll stick with the Howe I have.

Yes, the Wilwood reviews are mostly bad but remember, the people that have good results rarely comment. Mostly people that are upset leave their comments. As well, a good portion of the premature wear can be related to the slave not being in the correct position and having a side-load applied. That's the main issue that causes leaking (and that also relates to additional force).

- EM

would using a fish scale work to measure the force needed on the clutch ?

The fish scales I know of work in tension (one end is held in the hand, the other end has the fish hung from it. Won't work in compression between my foot and the pedal.

I heard back from Brewer's already He says the CF702 is 6.5" from hole to pivot, 3.25" from pivot to pads, so a 2:1 fork ratio and 250 lbs force.

That means something is wrong if it still drags - because the throwout bearing only has to depress the fingers 3/8" per Centerforce to disengage, which means 3/4" at the original fork hole. As noted, I have at least that much travel. And there is very little free play, just enough to keep the TB off the fingers. No huge slop in my linkage and I am fairly sure I got the air bubble out. Maybe not.
I'll be home this weekend and can mess with it some more... including starting the engine this time

Good point on the reviews. The Wilwood can also use silicone DOT 5 fluid (I asked them), whereas I found out the hard way that the Howe seals don't handle it. And Howe hammered me on shipping too, $17 postage for a UPS box with two thumb-sized O-rings in it

I just looked at the spec sheets for the Wilwood and the Howe cylinders:

https://www.wilwood.com/PDF/DataSheets/ds751.pdf
https://howeracing.com/index.php/clutch-slave-cylinder-pull.html

It's interesting to note that the Wilwood shows the cylinder body pulling on the clutch fork, and the piston connected via heim joint to the stationary mounting bracket. Whereas the Howe is the other way around (cylinder body connects through heim joint to bracket, and piston pulls on the fork).

I'm not sure why Wilwood would recommend it that way, because the cylinder will move back and forth every time the pedal is depressed, also flexing the hydraulic line.
Maybe that causes more alignment/side-loading problems?

Also the Wilwood sheet says not to use DOT 5 silicone fluid for racing or performance driving... which makes little sense since the boiling point is higher with silicone than glycol, and the slight compressibility of silicone shouldn't be important as long as the air is carefully bled. Who knows.