Moss Motors, Ltd

danoz,

Also, it is 7/16, and is NOT metric. Like enfro says, a flare nut wrench is like a box end wrench with just enough of a slot cut out of one spot to slide it on past the hard line. We'd be lost without them in the business...

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

The technique for bleeding a clutch slave cylinder depicted in the Moss/YouTube video will sometimes fail. In the video, the clutch pedal is depressed, then the bleeder opened and the slave cylinder's piston is pressed into the cylinder, and then the bleeder is closed. One would then lift the clutch pedal. But, with the slave piston depressed into the cylinder with the pedal down, once the bleeder is closed and the pedal is released there is no fluid to "make up the volume" as the master cylinder retracts as the pedal lifts (but see the later posts in this thread that speak to how fluid can be made up past the seal on the master). In some instances, the slave piston may be sucked further into the slave cylinder, buckling the spring that's inside the cylinder, behind the piston, letting the piston tip off true and breaking its seal, letting more air into the system (and letting fluid leak out). This latter situation may be possible if one pushes the slave piston deeply into the cylinder (which, quite frankly, might buckle the spring independent of any suction effect).

The best way I've found to bleed the slave cylinder is to disconnect it from the bell-housing and hang it vertically by its hose. The slave piston needs to be slightly pressed into the bore or it will extend down enough to break the seal. I used a couple of long zip-ties looped over the length of the cylinder to hold the piston in. Then, use an 80+ cc oil syringe (available at auto parts stores) to inject fluid into the slave cylinder through its bleeder (I use a short length of rubber hose between the tip of the syringe and the bleeder). The fluid level in the master needs to be down or you'll overflow. With this method you're back-filling the system from the slave to the master. And, with the cylinder hanging, you're filling from a low point in the system and pushing fluid and air toward a high point in the system (the master cylinder). Worked the first time for me doing it this way.

As noted in the following discussions and comment from Moss, others have used the technique depicted int he Moss video to good effect. It cost me hours of frustration, but that's just my experience. It's good that in this instance, if one technique fails, there are others that will work. You've got options, and once you get a particular technique to work, that's "the one"!

That is not how it works. If the master cylinder is kept topped off and the bleed screw is tightly closed before you release the clutch pedal, fluid from the reservoir flows into the master and to the to the slave when the piston clears the fill hole from the reservoir. If the reservoir is full and the bleed screws are tight air cannot get into the system.

 

The piston in the slave cylinder is returned by the cutch bearing fork as the clutch pedal is released. The slave cylinder cannot be sucked further into the cylinder because it has the pushrod connected to the throughout bearing fork at one end and the fluid coming in at the other end.

 

The procedure that Moss lists is what I have done on all the British and Japanese cars that I have owned since 1966 and it has worked to perfection every time. The only improvement was adding ABV valves instead on the stock bleed screws which makes the job a 1 person one.

Sorry to differ, but I don't agree. "...fluid from the reservoir flows into the master and to the to the slave when the piston clears the fill hole from the reservoir."  The situation I describe happens BEFORE the piston clears the fill hole. As the piston in the master retracts, it pulls (via the clutch fluid) the slave piston deeper into its cylinder.

"The slave cylinder cannot be sucked further into the cylinder because it has the pushrod connected to the throughout bearing fork at one end and the fluid coming in at the other end."  Not true. The push-rod on the B slave cylinder is not physically connected to the slave piston; it just rests within a conical depression within the slave piston. So, there's nothing to prevent that piston from being pulled back into the cylinder.
 

Perhaps it's a matter of degrees. If one doesn't push the slave piston in very far, then one might not collapse the spring when the pedal lifts (depressing the piston too far might also collapse that spring, apart from any other cause). But, how is one to know what is "too far"?The slave piston in the B cylinder is "thin" relative to its axial thickness, and can easily "tip" within the cylinder breaking the seal.

A big  problem with bleeding the B's slave cylinder is that it's horizontal on the bell housing, and the bleed hole isn't precisely at the high point of the cylinder; one can easily trap air in the cylinder. That's why hanging the slave vertically and back filling to the master works so well on the B.

DrC,

Sorry, that's indeed not how it works. This is a hydrostatic system. In short, it seeks its own "level" at rest. The clutch springs will always return the piston in the slave to the correct rest position. The master cylinder will always return to the rest position when the pedal is released. As it does so, it uncovers the relief port, allowing communication between the resevoir and the master cylinder barrel, and thus the rest of the system. As such, there is ambient pressure in the entire system when the pedal is at rest, and fluid may freely flow into or out of the active parts of the system. The secondary seal in the master on later master cylinders assures that there is no point that the system itself can be exposed to a negative pressure which could cause air to be drawn in past the primary seal, as it has itself passed the relief port on that same return stroke.

The fact is, however, we always use either a pressure or a suction bleeder to get things started and then finish up with the foot and pedal method, bleeding just like a brake cylinder. We don't usually find we need to use the clutch springs to assist in bleeding, as the video would suggest, but each case is just a bit different.

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

No question that it's a hydrostatic system, and no question that what you and others have described is the normal operation of the system. We're debating the Moss video's bleeding instructions, which is definitely not a normal operation of the system.  Let me explore some questions, then, to perhaps educate me and to bring out what may (or may not) be happening.

You reference a secondary seal; does it permit fluid flow past the piston in the master on withdrawal? For this to be so, it would have to act as a check valve, resisting pressure on the actuation stroke, and passing pressure on withdrawal. 

Once the system is closed by the master, and a volume of fluid is bled out following the moss video technique, where does the fluid come from to make up that volume as the master piston withdraws, but before the relief port is opened? I've seen an earlier post on B clutch problems (7292) where you refer to "the vacuum of withdrawal sucks in air." If it can suck in air, it can pull the slave piston deeper into the cylinder.

I guess my basic problem comes down to "where does the fluid come from to make up the volume bled by depressing the slave piston"? 

Good morning Gentlemen.

 

Thank you for bringing up this matter.

First we need to establish common ground. We have all known owners of these classic cars and who have spent considerable time trying to bleed their clutch.

 

Sometimes they bleed perfectly with almost no effort. Sometimes they just seem to enjoy making life difficult. So, irrespective of your preferred method, we’ve established that something more than conventional bleeding is sometimes called for.

 

Which is best? That’s easy. The best method is the one that works for you.

 

An old proverb says: ‘Insanity is doing the same thing the same way a second time, and expecting a different outcome.’ The poor guy who tries over and over again to bleed the system with no success is missing the need to try it another method.

As for the method in our video, it is not the only option, but it is an option. It has worked for many people. If somebody has another method, and it works, that’s great!

 

How does the fluid in the reservoir refill the cavity formed in front of the piston in the master cylinder? The seal in the master cylinder is of the “umbrella” variety. When you step on the pedal, the seal swells outward from the pressure and holds tightly against the wall of the cylinder. When the pressure is released (i.e. you let up on the pedal) the umbrella seal leans forward and the fluid (encouraged by the vacuum that forms in front of the piston) from the reservoir flows in to fill the void.

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