The common question we get here at BMP is how the hydraulic clutch system works on a BMW and how to bleed the system. So what I have put together here is kind of quick and easy demo piece just so you can see how it functions. The item above here is the clutch master cylinder. Clutch master cylinder attaches to your clutch paddle. Okay, then you have got a flex line, a section of hard line and then the clutch slave cylinder.
The way it functions is your break fluid reservoir which is shared with the break master cylinder, in this case we have just put a simple reservoir on top of the clutch slave or the clutch master is taking in a reservoir break fluid. Inside the clutch master cylinder there is a piston and a chamber that holds about two to three ounces of break fluid. That break fluid when you push your clutch paddle will depress the piston like this and it will force about two to three ounces of break fluid through this line into the clutch slave cylinder. The clutch slave cylinder in turn has a piston which is connected to a rod and that pressurized break fluid since you cannot compress liquid will pressurize the fluid, it pressurizes and pushes the piston out, pushes the rod out, depress the clutch lever. The clutch lever in turn operates the clutch.
The next part of this tech session is going to be how to identify which component has failed and how they fail. Most common failures are leaks. Each of these units has a seal on a piston. Once that seal has deteriorated, weakened or give out, you will get break fluid leakage, hydraulic fuel leakage actually. On the clutch master cylinder, you will typically get leakage through this dust boot which will eventually either travel down your clutch pedal or get into your carpeting underneath your vehicle. You will notice that it will be a wet oily film on the carpet or below the carpet that seems to never want to dry out and what you have there is the seal inside the clutch master cylinder has failed and the break fluid or hydraulic fluid is constantly leaking out.
An indication is for weeks or months you will be adding break fluid to the vehicle with no indication on where it is going, it is always just disappearing. The other component that likes to leak is the clutch slave cylinder. That mounts to your transmission bell housing right about the clutch and actually where bell housing and the engine come together. That has a tiny little hole at the bottom of the bell housing and typically when that seal gives out inside the clutch slave cylinder that will leak down out of the bell housing on your garage floor.
Now we have identified all the components, we have identified how to figure out which component has failed and now I am assuming you replaced either both or both of the components. Now the biggest problem that I get after this or I hear people complaining about is I can't bleed the system completely or I am having difficulty getting the air out of the system. Well this is the reason why.
Air wants to travel uphill; it has a tendency to want to travel uphill. Your whole system now is full of air, you have replaced both components and there is no fluid in the system, you have got air in the system. Introducing some break fluid into the master cylinder and then preceding to pump the break or the clutch pedal to bleed the air out is really just moving and also break fluid forward and then as soon as you like to go to the paddle, the air starts traveling back again. So you are basically just shuffling the air back and forth.
This demonstration and this technique I have to do this bleeding process is going to be bleeding the clutch system in the way that it naturally wants to expel the air and that is uphill; air wants to travel uphill. So what we are going to do is kind of a unique system that I discovered years ago, is we are going to take a common oil cane, a clean new oil cane, fill it with break fluid, we are going to attach that to the clutch slave cylinder bleeder screw, we will open the clutch bleeder screw and what we are going to do is pump break fluid into the system from the bottom up which then chases the air all the way up and you will see the bubbles coming right out of the break fluid reservoir.
You want to get yourself a common syringe style turkey baster and what you are going to do is suck out all the break fluid from the break fluid reservoir. You want to get the fluid out because what we are going to do is we are going to introduce new break fluid in from the clutch slave cylinder and we are going to fill it essentially from the bottom up. You are going to see the break fluid reservoir fill up from the bottom along with the air bubbles coming through.
Now to do this I just use a common pump style oil can which you can buy at any pepboys or auto zone and you are going to use a new clean on with a piece of 3 millimeter flex line, fill it with new break fluid, we use Ate type 200 and what I am going to do here is I will put my 7 millimeter wrench on top of the bleeder screw first, get it ready, then I am going to take my oil can and I am going to just bring a little bit of break fluid to the top to get as much air out of the hose as possible. So I don't introduce new air into the system. I am going to slip it over my bleeder, bring the bleeder close to the closest position and then I will open it a bit and I am going to put a little bit of break fluid into the reservoir so you can kind of see what is happening as we fill this thing up.
Now since we are going from the bottom up any pressure I put on the system is going to essentially start bring in air bubbles to the top along with fluid. You are going to see a little bit of air coming up there and our master cylinder is essentially chasing air out of the bottom and there you see it coming up. All those air bubbles you see are air bubbles that you would be normally trying to force through the bottom of the vehicle which you will never do, you will never get it all out of the bottom of the vehicle because it will take quite a bit of moving the paddle.
Once I have got all the air bubbles out, it will almost be nothing coming out and then I will slowly start closing my bleeder screw and at some point, it completely stops and at that point you have got no more in the system, you can disconnect your line and you are almost ready to go at that point.
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