I thought I'd share a little bit of info to those who are interested in learning how I ended my clutch bleed woes so that you can incorporate the fix into your existing hydraulic clutch system if you choose to.

This fix is only intended to address those situations where air is being sucked into the system via the slave cylinder. Its a simple solution which no one seems to have addressed on this forum (at least to my knowledge).

Over the years, I've changed my slave using generic parts, geniune GM parts and rebuild kits. I achieved some success in using all three techniques but eventually, air would begin to bleed into the system at the slave. It got so bad that after each 1/4 mile run at the track, I could barely make it back to the pit, as it became nearly impossible to shift gears.

After wracking my brains trying to figure out various ways to replace the slave (e.g. designing a cable clutch linkage or using an aftermarket slave produced by Wilwood), I realized that the reason why air was bleeding into the system was because the seal at the slave was collapsing everytime I released the clutch pedal. The same would happen when the car was at rest. The only difference is that air would bleed into the system at a slower rate.

I eliminated the problem in its entirety by installing a 10lbs. Wilwood residual pressure valve at the master. These little devices are normally used in brake systems to maintain postive pressure in the lines to keep the caliper pistons from retracting. Likewise, the valve keeps positive pressure in the clutch line and prevents the umbrella seal from collapsing. Its been nearly two years since I installed the valve and I have yet to bleed the system once (this is the same slave that I had to bleed after every pass at the 1/4 mile strip). The great thing about the fix is that the valve cost me less than $20!

Farknocker

Very, very COOL!!

Do you still have the return spring on the clutch pedal?

Archie

I took the return spring off of mine. Is that good?

Do you have the part number(s), fittings needed, pictures and all that stuff??? We NEED to know.

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Timothy E. Smith
1986 SE V6 BACK ON THE ROAD AGAIN!!!!
1984 Coupe
1996 Ford Probe GT
1999 Chevrolet Silverado Z71
1975 Chevrolet ElCamino SS

quote Originally posted by Archie: Do you still have the return spring on the clutch pedal? Archie

Yes. Everything in the stock clutch system (from pedal to slave) is the same except for the RPV. The only thing that's different is the length of the slave push rod. I used a shortened Ford valve push rod that I found at a junk yard because I needed a longer rod. The rod was installed long before I added the RPV.

I forgot to mention that a 2 lbs. RPV is also available and may perform equally well. Also, although one might suspect that the pressure in the system may cause leaks, I have not experienced this problem.

Farknocker.

Not sure I understand correctly, but you are maintainging 10 psi of pressure in the clutch lines and slave at all times? I would be worried that this would cause premature wear on the release bearing.

Hmmm...sounds interesting. So basically there is always 10lbs of pressure on the clutch?

Sounds like you may go through throw-out bearings prematurely since it will always be riding on the clutch. Just a thought.

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Its never going to be finished...

Visit my website www.purplefiero.com for Show Dates and Fiero Stuff!

This one?
http://www.wilwood.com/products/master_cylinders/rpv/rpv.asp

quote Originally posted by Joe Torma: Hmmm...sounds interesting. So basically there is always 10lbs of pressure on the clutch? Sounds like you may go through throw-out bearings prematurely since it will always be riding on the clutch. Just a thought.

I'm no engineer, but I think you need to take 10lb's per sq. inch (or whatever your RPV is rated at) and figure out the pressure exerted on the piston in the slave which, as you are aware, is some distance from the Master. I think the actual pressure exerted on the piston will be based upon the total surface area of the piston, etc., and some other factors.

If anybody has knowledge in fluid mechanics, please give us your thoughts.

Anyways, you can always try the 2lbs. RPV if you are afraid of prematurely wearing out your TO bearing.

Farknocker

quote Originally posted by Mr. Farknocker: Anyways, you can always try the 2lbs. RPV if you are afraid of prematurely wearing out your TO bearing.

Can you move the clutch lever on the trans at all? Just curious...this may defientely be a solution here!

I forgot to mention that if you incorporate the use of the RPV, you might as well purge the existing brake fluid from the line and use silicon. It seems to resist the absorption of moisture and prevent the formation of rust and pitting in the slave cylinder wall. The fluid at the slave has remained clear with the silicon fluid instead of turning brown or black like how the Dot 3 brake fluid usually does.

Farknocker

[This message has been edited by Mr. Farknocker (edited 05-23-2002).]

I understand what you're doing. If anyone else tries this, try the 2psi valve. It should hold enough line pressure to expand the seal without loading the TO bearing heavily. It may also let the clutch release a bit faster.

The 10psi valve is typically for a drum brake system to expand the seals and/or preload the heavy return springs a bit.

Unfortunatly I've not been able to spend any significant time recently with Fiero clutch to see how it works...

Most hydraulic clutches are "zero clearance" setups. That means the TO bearing rides the disk lightly at all times. This is part of the self adjusting nature of most Hydraulic systems. I had thought that Fiero was also a ZC setup.

A large portion of the Fiero clutch problems extend from the fact these transmissions are generally used with cable activators. The Hydraulic conversion was a required evil. Hydraulics are about the only decent method to activate a clutch that far from the padal. The Fiero system always seemed like someone at GM/Pontiac said "Geeze, we have to use parts bin stuff... so how in hell do you activate the clutch back there?"

Trivia
In factory drum brakes, you rarely use a RPV. Most factory systems I've seen have a metal expander that pushes the seal against the cylinder so the RPV isn't needed. PRV's would typically be used with older drum sets to reduce lag when front axles are converted from drum to a disk.

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11-Sept-01, The day the world as we knew it ended.

[This message has been edited by theogre (edited 02-26-2002).]

Pretty interesting..... I'd suggest the 2lb. PRV and removing the pedal return spring.

I'm gonna try this at my next opportunity.

A very interesting thread well worth bookmarking.

Archie

As I understand the Fiero system, it's got a spring in the MC and another on the pedal... Yes?

Are we sure the MC spring is strong enough to return the pedal and hold it tight? If not then you could have cruise control problems on cars with that option. Maybe a lighter return spring would be better than none? Using something like a gas pedal return spring.... you can get long soft ones of those.

The return spring itself is another point to look at... to much spring could be pulling a vacuum in the hydraulics. Such a problem could easily suck air past cup seals. The RPV may be preventing that problem by fighting the pedal a bit.

Doesn't the MC have a cup seal and an O ring? That would make the MC a bit more vacuum resistant than the slave which I believe has only the cup seal on it's piston....

I recently got around to taking some pics of my set-up. Here they are:

Farknocker

quote Does anybody have any thoughts on this? [/B]

Seems to me that any "positive" pressure, would stop air from being drawn in.
Just a wild guess. I don't know.

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Raydar

From the Department of Redundancy Department.

quote Originally posted by Raydar: Seems to me that any "positive" pressure, would stop air from being drawn in. Just a wild guess. I don't know.

It would seem that way, but its conceivable to have insufficient pressure at the seal. That is to say that there must be enough pressure to keep the umbrella seal at the slave expanded sufficiently enough to form a tight seal. I don't know what that pressure is, though.

Farknocker.

Couldn't say for sure... The things are fairly cheap so if you try a 2 and it doesn't work you aren't going bankrupt if you have to switch to 10.

Good install... kind of looks like it belongs there.

In addition to gravity and my theories about the return springs above... you've got one other issue... Vehicle deceleration. That long line under the car may be a factor when braking. The RPV is probably resisting that flow as well.

I think anything that prevents the fluid from pulling a vacuum in the slave is probably going to help.

1 item of note. I think yours is a Type 2 MC... The info I have is that MC works more like a brake MC. (1 Cup seal and 1 O ring on the piston.) The Type 1 MC may not be as resistant to pulling a vacuum as the Type 2 even if it has the same piston arangement.

If air is getting sucked into a Type 1 MC, even with an RPV, I'd say some fine tuning of the return springs is called for.

this may sound dumb, but i've heard of liquids being used as examples of electron flow so...

if you split the line and use two in parallel, or two in series on one line could you approximate 5lbs? sort of think in terms of resistors or capacitors (i don't know which, if either, it would work like).

------------------
Phil T.

start where you are,
use what you have,
do what you can.

quote Originally posted by Philphine: this may sound dumb, but i've heard of liquids being used as examples of electron flow so... if you split the line and use two in parallel, or two in series on one line could you approximate 5lbs? sort of think in terms of resistors or capacitors (i don't know which, if either, it would work like).

Don't know but the same effect can probably be accomplished more easily by placing a 5 lbs. spring on the arm that the slave rod pushes against.

Farknocker

quote Originally posted by Mr. Farknocker: Yes. Everything in the stock clutch system (from pedal to slave) is the same except for the RPV. The only thing that's different is the length of the slave push rod. I used a shortened Ford valve push rod that I found at a junk yard because I needed a longer rod. The rod was installed long before I added the RPV. I forgot to mention that a 2 lbs. RPV is also available and may perform equally well. Also, although one might suspect that the pressure in the system may cause leaks, I have not experienced this problem. Farknocker.

Hey Fartknocker!
Isn't the longer push rod alowing the slave piston to contact the bleeder valve???
And by using the pressure valve, you are forcing the piston away from the bleeder???
Wouldn't it be better to use a shorter rod, and avoid damage to the piston cup???
crash...

Great stuff,
However, how much fluid pressure is actually exerted on the clutch? Fluid displacement being what it is in the system, is there any possiblity of a clutch "not" being completely engaged? Example, driving while maintaining foot pressure on the clutch pedal? Could you concieveably drive a good clutch and slip it into oblivion with this little mod?
Just a thought from the new guy?
88Ironduke

quote Originally posted by Fierowrecker: Hey Fartknocker! Isn't the longer push rod alowing the slave piston to contact the bleeder valve??? And by using the pressure valve, you are forcing the piston away from the bleeder??? Wouldn't it be better to use a shorter rod, and avoid damage to the piston cup??? crash...

I'm not sure what you mean by contacting the bleeder valve. The piston travels freely in the cylinder from one end to the other and is only limited by the physical dimensions of the cylinder. To my knowlwedge, the piston never contacts the bleeder valve at any time since the valve does not protrude into the cylinder. Do you mean that by using a longer rod, the piston will be pushed so far into the cylinder that it goes past the bleeder? Possibly but I don't think so. I haven't look at my slave for a while so I’m not sure. I think what matters is whether the seal (as opposed to the piston)goes past the bleeder valve opening when its retracted. If it does, I think it would bleed air into the system,leak or both.

The longer push rod is necessitated by the limitation in the piston's ability to travel in the slave cylinder. For those running a V8 set-up like myself, I suspect that the problems stems from the fact that the adapter plate pushes the tranny housing away from the surface of the flywheel and clutch. Therefore, the starting point of the piston is farther into cylinder than in a normal 4 or 6 cylinder setup. I've never torn apart my tranny and taken measurements to confirm this. All I know is that many people complain that they are seeing limited movement at the slave. I was a part of that crowd.

As I mentioned, I think the problem is the starting positon of the slave piston. The piston can move no farther than the end of the slave cylinder. For example, if the length of the cylinder was 2" and the starting position of the piston at rest was already 1-1/4" from the left end of the cylinder, the piston travel would be limited to 3/4." If you wanted more travel, you would need to move the starting position of the piston closer to toward the left of the slave (i.e., closer toward the bleeder valve). If the piston started, for example, 1/4" from the left end of the cylinder, you would then have 1-3/4" inch of travel. To accomplish this, you need to install a longer push rod, hence, the ford push rod that I mentioned.

Hope this answers your questions.

Farknocker

While I can't say for sure what the effect of the RPV is... Clutch Fluid pressure isn't hard to figure. (Had to do this for the brake articles.)

I think the clutch uses a 4:1 pedal like the brakes do. Even if it was 3:1... I'll go with 4:1

The clutch is a little differant in that it's pressure is limited by available pedal travel. The clutch pedal, unlike the brake bottoms out and won't let you get any more pressure. (At least that's what is suposed to happen...)

(This design is also why it's easy to wreck the aluminum extension arm on the original style clutch pedal. If the hydraulics or transmission bind, you could put allot more pressure on the arm than it was made for. All but the most feeble drivers could easily generate 400-500 pounds of load on the arm. Even more is posible from a strong driver.)

Figure even if you are holding only 50 pounds on the pedal, the MC rod is deleivering 150-200# to the MC piston depending on the pedal ratio.

An 11/16 MC has an area of 0.371 sqin. (spec for 87 fiero clutch MC)

So if we just use the 4:1 pedal as example with 50 pounds on the pad you have 200 pounds loading the MC piston

200/.371= 539 PSI

With a fairly extreme 125 pounds from your foot generating 500 pounds at the MC pivot...

500/.371= 1348 PSI

If anyone has a clutch pedal out, we can figure the exact pedal ratio if we had the following numbers.

Dimension A. from the center of the top pivot to the center of the pad. (About where the arm and pad are wleded together)

Dimension B. From the center line of the top pivot to the center of the MC rod pivot.

After I replaced the clutch I had a constant problem with bleeding my clutch on my 5speed Isuzu trans on a 85 with the Iron Duke. I first rebuilt then replaced the master and went through slaves and the entire pipe before I decided to dig deep and reverse engineer.

I found that the slave cylinder that fit my transmission (not the same as the one on the 4speed!) was not "right". It is the same black plastic one that fits the other GM application of this transmission on 4 cylinders. It has the right parts numbers but it has 1.2 cubic inches MORE volume than the Master cylinder!! I formed a "filler" piston for the slave by cutting off a piece of the piston from an old slave, reassembling the slave with both the "filler" and its normal piston, and refilling and bleeding the system.

I used Archie's bleed method, worked first time around.

I have driven it for almost two years now. But admittedly I only drive it occasionally in the winter. No complaints, works perfect.

quote Originally posted by Tryxalon: I found that the slave cylinder that fit my transmission (not the same as the one on the 4speed!) was not "right". It is the same black plastic one that fits the other GM application of this transmission on 4 cylinders. It has the right parts numbers but it has 1.2 cubic inches MORE volume than the Master cylinder!! I formed a "filler" piston for the slave by cutting off a piece of the piston from an old slave, reassembling the slave with both the "filler" and its normal piston, and refilling and bleeding the system.

O.K. Let me get this straight. Are you saying that you solved the problem by reducing the volume of the slave cylinder by making the piston bigger?

Farknocker

[This message has been edited by Mr. Farknocker (edited 03-21-2002).]

Did the "filler" have still have its seal? The use of a second slave seal would also fight anything trying to pull a vacuum in the slave. Even a longer piston would help some. It would be the same effect as putting a sweaper O ring on the slave piston. (You'd have to turn a groove for the sweaper.)

The Slave in a clutch always has more volume than the MC. All clutch slaves have extra travel built into them. The extra slave travel allows for variations in slave position and clutch wear. A certain amount of extra travel is a required element of the system. Also keep in mind that a small amount of clutch wear can create a large change at the end of the release lever....

The end result is that the extra travel alows the system to self adjust just like the brakes do.

Hey Ogre, about the way the clutch MC works...
The return spring inside it has to be srong enough to counter the spring that keeps the seal to the reservoir closed. If it isn't then if you have pressure in the line, the seal won't ever open, and you won't be able to take in fluid from the reservoir. Not really a problem if the system isn't leaking at all, but something to watch out for. On the other hand, if the seal to the reservoir did open like it's supposed to, wouldn't the pressure in the line force fluid back into the reservoir?

Post, Sounds like you have the type 1 MC... Yes it's differant. No I remember much about the guts of it. The springs inside the MC aren't the ones I'm worried about. They shouldn't cause a problem. It's the spring, if any, on the pedal itself that may be an issue.

yes, I lowered the volume of the slave with the "helper" by making the piston longer. Any modification of the rod length would have changed the geometry. Didn't want to do that.

When I get a chance, I'll take some pictures (I think I have one or two of my first experiments and can re-construct what I did without tearing mine apart). Maybe this weekend.

Yes, Ogre, I did use the portion of the piston with the seal so would up using both seals ... stablized it in the bore and, as you said, increased the "sealing". Noted the "play" neccesary for proper operation to adjust. The "helper" is not 100% (of the difference) but more like 60% I think.

Let me see what I can put together to reconstruct and illustrate.

[This message has been edited by Tryxalon (edited 03-21-2002).]

After reading this thread (which I think is one of the more interesting and useful threads I've seen recently) I came across a page on the Wilwood website that mihgt apply to our discussion. Here is the portion that appears to apply to what we are trying to do in the clutch system.

"Brake pedals should be mounted securely. When possible, keep the master cylinder reservoir level higher than the horizontal plane of the calipers to prevent excessive fluid drain back which can result in double pumping of the pedal. If this is not possible, a two pound residual pressure valve should be plumbed into the brake line at the exit of the master cylinder to prevent fluid drain back (do not confuse the two pound valve with the ten pound version; the ten pound valve is for use with drum brakes only)."

I found this at http://www.wilwood.com/products/pedal_assem/pedaltech.asp .

Just thought I'd share.
Joe Furedy
'88 Formula (need a 5-spd parts car for my 3800 SC swap)
Phoenix, AZ

quote Originally posted by jfuredy: I found this at http://www.wilwood.com/products/pedal_assem/pedaltech.asp . Joe Furedy

Great lead!

BTW, for those of you who are interested in going through the mathematical gyrations discussed above, I was able to measure the diameter of an old slave piston out of my '87 GT. It's .933" It may be different for other cars/slaves.

Farknocker