Clutch bleed theory

Mxgrds

True Classic
With a new crunch between 1 and 2 I was worried that the clutch was not bled well enough after a revision of the pedal box. So i started to bleed it over and over again, but without results. It might as well be the synchromesh. But it made me think about the clutch dynamics. On the forum there are many ways how to bleed described. I was wondering were the air sits that is so difficult to get rid off.

Below a schematic of the clutch.
From left to right:
- reservoir
- supply line
- master
- pressure line
- slave
6B07C7A6-0A0A-40AF-8D80-B6C715345C5B.png


Master?
The pressure line exits in the middle of the cylinder. Some air might get trapped above that exit. If the valve reaches the end totally, the air will be pumped in the pressure line when bleeding. But does the valve completely empty the cylinder when the pedal hits the floor?

Pressure line?
Directly when the pressure line exits the master it rises 5 cm to pass the pedal box. Then it falls 30 cm to the midconsol. Then 150 cm to the rear and up 30 cm to the slave (approx).
The highest point in this line is, it think, the most likely part where air gets trapped.

Slave?
By manually pushing the valve into the slave completely, the air can be pressed out through the bleed nipple.

How to bleed then?
2 person job. One pumping the peddle and the other one operating the bleed nipple and a lot of shouting. Tried that but not always with success. A problem is the small reservoir. 3 to 4 times pumping and it needs to be refilled. Air that is just moved down in the pressure line is moving up again while the reservoir is filled. The pressure line has a large capacity, so with every pump the oil moves with the air bubble only a limited distance. In the 30 cm down and the 150 cm horizontally, the air will even rise between 2 pumps. To overcome this problem I bought 4 meter transparant silicon hose to pump the oil round fom nipple back to reservoir. Works great. You can pump 15 minutes without stopping and at the same time watching air passing by. Can be done by one person.
Still no improvement. Might be as well the synchromesh.

Bleed reverse?
Many people on this forum are very possitive of reversed bleeding. Why is that a more successfull method? probably because the trapped air in the highest point of the pressure line only needs to go 5cm down to get to the master and out in the supply line. And maybe the oil volume used is bigger. This is my last resort of hope to avoid a gearbox overhaul.
 
Without getting into any theory, all I can tell you is pressure bleeding is the only thing that works for me on an X clutch. Suction bleeding just pulls air in around the bleed screw threads and makes you think you have tons of air in the system. Pedal bleeding is a total waste of time. Again, this is what works for me.

If you are doing this because you think you don't have full disengagement of the clutch, check the pedal engagement height and also the adjustment of the threaded rod on the slave, most of us have the adjustment nut out almost to the end of the threaded rod.
 
I've found the clutch master cylinder draws from the reservoir in the middle of the cylinder and applies pressure to the slave through the opening at the end. I've noticed a big air bubble gets trapped as shown in the pictures when replacing the master. That, I think, is why reverse bleeding works so effectively for me. It allows the air bubble to escape through the reservoir. I like vacuum bleeding but acknowledge it works best with some teflon tape on the threads of the bleed.

A quick way to tell if you have clutch drag is to shift into first while parked then slip back to neutral and keep holding the clutch pedal to the floor. After a few seconds, with clutch still held to the floor slip into reverse. If it grinds the clutch isn't fully disengaging. The reason is because you fully stopped the gears spinning when you slipped into first. The only way those gears can start spinning again with the clutch fully depressed is if the clutch is not fully disengaging.
 

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Inside of the clutch master.
3653A847-7536-4666-9526-E6B6B3919D4F.jpeg
In my X the cylinder is 4 degrees up towards the front. So air is flowing to the pressure line. Found out that the valve is hollow.
EA103DC4-D46C-4506-B9F3-1B5C60517BE0.jpeg
To get even more air trapped.
In this picture with red the area where air can sit and won’t get out with bleeding by operation the pedal.
F56115F1-EA0A-4867-8C6F-1525E7B1992C.jpeg
To get this air out i think it is best to press oil from the slave nipple. The air will then leave through the reservoir. And since the pedal touches the floorboard, i assume the the valve is not pressed al the way.
 
Directly when the pressure line exits the master it rises 5 cm to pass the pedal box.
Interesting thought that air might get trapped at that highest point. Sure would be nice if everything could travel only downhill like it typically does on most vehicles. But I'm guessing there is a small enough volume in that short section of pipe to get flushed easily with bleeding. But you never know. I haven't tried it but others have suggested raising the rear of the car way up to make the slave bleeder the highest point (with normal bleeding techniques, front to rear). Or perhaps the opposite; raise the front way up, open the bleeder, and let gravity drain the system. But that would be slow so it might be difficult to make sure the reservoir remains full. And I'm not sure it would improve the trapped air issue.

In general I'm with Carl, I seem to have the best luck with pressure bleeding from the reservoir/master end to the slave/bleeder end. If you use one of those pressure bottles, it holds enough fluid that refilling the reservoir isn't an issue and there won't be any chance for air to move retrograde while refilling.

However I can also see how reverse bleeding may help if air is getting trapped at the master end - as suggested by Cratecruncher and Mxgrds (especially with the installation of a new master). So perhaps do both; start with reverse bleeding (using pressure from the slave/bleeder to the master/reservoir), then forward pressure bleed it the other direction.

On the subject of installing a new master cylinder (either clutch or brake), the idea of pre-bleeding the master on the bench before installing it is often discussed. While I agree that will help in the bleeding process, it also seems to make a huge mess when installing the unit in the car - particularly on the X with the inconvenient location of the pedal box. Anyone have some tricks to make it less messy (i.e. prevent the fluid from leaking out while connecting all the lines - both going into it and exiting it, which can be very difficult)?
 
It usually takes three to four full bleeding sessions to get my X clutch working properly when I have replaced the master cylinder.
 
I'd love to know how they did it on the assembly line!
Good question. I have no idea how Fiat did it, but I believe the common method of the times for production lines was a pressurized filling machine that attached to both ends of the hydraulic circuit and pushed fluid through the system. I imagine it had enough flow and volume to dispel the air as it filled. But for the X1/9 they may have had a particular technique to assist with making it successful. Because most vehicles of the era had the reservoir as the highest point, next the master, all lines flowing downhill, to the slave as the lowest point. So no real chance for air to get trapped as fluid flows through it.

I've never looked, what is the overall height of the slave relative to the master on the car (vehicle sitting at normal height/stance)? In the diagram Mxgrd posted the slave appears to be lower, but just thinking about the car the master is very low and the slave is rather high. If the slave is higher than the master, that may be part of the difficulty? But you would think pressure and flow would overcome any gravity effect from height differences.
 
It usually takes three to four full bleeding sessions to get my X clutch working properly when I have replaced the master cylinder.
Hi..there is a very easy way to bleed the clutch on an x1/9..i replaced my master cylinder and slave hose and cylinder today..took me alone 5 minutes to bleed the entire clutch system. Just follow this carefully. First step buy a syringe..i used a 100mil plastic one.Get some plastic tube to fit over slave bleed nipple.take lid off clutch reservoir..with the syringe and tubing attached suck about 70 mil from the new clutch fluid container.hold upright and push out any air. Attach tube and syringe to slave cylinder nipple,open nipple 1 full turn..push in brake fluid,close nipple..repeat twice..or as necessary check fluid in reservoir when reservoir is full...job done...no air...no pumping..very clean..no waste..only way to do it..cheers hope it helps many
 
That’s a novel way of doing it. What worked for me was to put the car nose down on the drive way and jack up the rear of the car so the master cylinder wasn’t tilted up trapping that bubble of air. It then bled fine by pumping the pedal.
 
I've had to use the Triple-Whammy method: A few psi of air pressure on the fluid reservoir + vacuum drawn on the calipers with a hand vacuum pump + Daughter pumping the pedal. Works every time, eventually.
 
Because the fluid reservoir for the clutch is so small, you need to fill it after 3 to 4 times pumping. During that refill the trapped air resettles at the highest points in the system again and you start all over again. To overcome that problem I bought a long hose (2 meters) and had one end on the slave and the other end into the fluid reservoir. That way I was able to pump as long and quickly as needed. Through the clear hose I could watch the fluid until no bubbles were pumped up.
 
I recently replaced my clutch master and had a devil of a time getting the air out. I tried all the various methods but nothing worked.

I finally connected an AC vacuum pump to the reservoir and drew a partial vacuum on the system. Then I reverse filled the system with fluid up through the nipple on the slave cylinder while drawing a vacuum on the reservoir. Worked great.
 
I found the other thing that will prevent a proper bleed is if the pedal return spring hook is on the wrong (back) side of the pivot bushing. I made that mistake - it prevents the pedal from going full stroke. Clutch works fine that way, but won't bleed.

Also putting the reservior up high (on the windshield) whilst bleeding also makes sure any air in that line comes up & out.
 
I will make the following observation and then shut up about this perennial issue.
It seems that there are variable results for different people. I suspect that what is happening is that some people get an air bubble on the back side of the master cylinder and others do not. The latter are able to pressure bleed, elevate the reservoir, or even pedal pump the system free of bubbles. The former requires reverse bleeding.
I seem to be in the first category, so I do the syringe pressure bleed from the slave. Last time I did that, after years of trying the other methods, I was rewarded with a satisfying gurgle at the reservoir and the pedal finally, after years of fighting with short clutch slave travel, was working like it should. A bubble apparently had been living happily on the back side of the master all those years.
 
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I pressurize the reservoir using a bicycle pump and an old mountain bike tube. The tube is cut somewhere so one end can be fit over the reservoir opening (hose clamp optional) and the other end rolled up and clamped to keep the air inside. With just a couple of PSI, it forces whatever is in the reservoir through the master, line, and out the slave. I usually jiggle the pedal slightly. I've noticed that it is sometimes necessary to get things going, particularly with the brakes. It has worked many times for me on a number of vehicles and no helper is required.
 
Because the fluid reservoir for the clutch is so small, you need to fill it after 3 to 4 times pumping. During that refill the trapped air resettles at the highest points in the system again and you start all over again. To overcome that problem I bought a long hose (2 meters) and had one end on the slave and the other end into the fluid reservoir. That way I was able to pump as long and quickly as needed. Through the clear hose I could watch the fluid until no bubbles were pumped up.
This is the way I did it as I am usually working alone. Worked great and really no mess. Happy Happy
 
I just did this by myself without any problems... I used a Motive Power Bleeder at about 10 PSI and made sure the reservoir on the car was topped off before I started the process. You have to bleed way more fluid at a pretty fast rate to make sure the air gets moved through the system. Zero issue but I tossed out about a cup of brake fluid..

I don't think you can get the brake fluid moving fast enough to carry the air bubbles with the pedal pumping method...
 
I used a Motive Power Bleeder at about 10 PSI and made sure the reservoir on the car was topped off before I started the process.
For what it's worth I've always had better luck with this type of forward pressure bleeding than suction or reverse bleeding. But I always do it without assistance, so pumping the pedal usually can not happen.

I imagine one benefit to this pressure method is also the non-stop continual flow of the fluid through the system for a duration of time. Without the pauses in between pumps of the pedal, or to refill the reservoir, there may be a better chance for difficult bits of air to be moved forward and all the way out of the system. Just a hunch. As long as it worked is all that really matters.
 
I just did this by myself without any problems... I used a Motive Power Bleeder at about 10 PSI and made sure the reservoir on the car was topped off before I started the process. You have to bleed way more fluid at a pretty fast rate to make sure the air gets moved through the system. Zero issue but I tossed out about a cup of brake fluid..

I don't think you can get the brake fluid moving fast enough to carry the air bubbles with the pedal pumping method...
I use the same technique but I keep the pressure around 3-5 psi using a mountain bike tube and a bicycle pump.
 
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