Sunday, 20 January 2019

F10: Clutch Bleeding Guide... the whole nightmare! [Manual transmission / gearbox 520d, 530d, F10, F11, G07, E90, E92 3er etc.]

You will need:

  • Short / stubby ... wrench.
  • One-way valve type bleeding-kit.
  • DOT4 regular brake/clutch fluid.

Clutch-bleeding on the F10, F11, G07, E90, E92, G20 and a lot of other contemporary BMWs can be a right pain, but if you are replacing the master-cylinder / slave-cylinder, the pipework or removing your CDV, then it is necessary. If you clamp the clutch-hose by the slave-cylinder and don't allow a lot of air back into the system, then bleeding should be pretty straight forward, but if a fair bit of air does get back into the system or you are removing the master-cylinder then it can't be helped and a full bleed is needed. The trouble with a full bleed is that these systems are notorious for air locks, primarily in the master-cylinder, which can be tricky to dislodge and get fluid into.
The main drawback with a regular bleeding process, where the bleed-nipple is opened as the clutch is pumped, is that the bleed-screw is located half way up the transmission tunnel and very difficult to access with a wrench. This makes it difficult to quickly shut the bleed-valve, which is kind of essential for this process and air can get back in. If you are able to access the bleed-screw without difficulty or have some special tool designed for the job then regular clutch bleeding should work a charm with air locks and, though it may take a while, you will be on your way.
A pressure-bleeder may give the time needed to tighten the fiddly bleed-screw, but does not work to get rid of air locks and seems to bypass the master-cylinder completely, so I would avoid using one of these kits at all if there is a lot of air in the system. Some trapped air at the top of the system seems to want to exit through the reservoir anyway, so it's better not to cap it with a pressure-bleeder as it just does not seem to want to go with the flow. If you clamped the hose and only the slave-cylinder needs bleeding then a pressure-bleeder should do that in no time.
I found that a one-way valve bleed kit was the way forward. It prevents the return of air/fluid while the bleed-screw is left open and gives time to shut it when the process is complete. It leaves the reservoir open to be manually topped up, which shows precisely how much fluid is entering the system. This was the key, for me, to finally shifting the stuck air from the master-cylinder.

I have detailed the various techniques and problems below:


Location of Brake + Clutch Reservoir:

The brake/clutch reservoir is not very apparent in the engine-bay. It is located under the black panel in front of the windscreen on the driver's side of the car. Looking from the front, left-side for RHD cars and right-side for LHD. The opposite panel covers the climate-control entry-ducts.
The panel is held in place by 4 plastic clips, which can be removed by prising up the centre of the clip and then prising the base of the clip out with a panel-popper / trim-tool or a couple of flat-screwdrivers.
The fill-neck for the reservoir is right under the bonnet-hinge and pouring DOT4 in without spilling it can be tricky, so a funnel is a good idea when topping-up. I would recommend putting a towel around the reservoir also, as brake-fluid is not a nice thing to get everywhere and it is easy to over-fill. Special care should be taken when replacing the cap - the large plug that hangs under the cap will surely displace some fluid over the top of the reservoir as the cap is dropped in.

Bleed Nipple + Regular Bleeding:

Unlike BMWs of old, the bleed-nipple on the slave-cylinder is way up in the trans-tunnel and difficult to access with a regular spanner. Getting the spanner onto the bleed-screw is not a problem, but if the spanner has any length to it then no more than a few degrees of turning is possible and the valve cannot open enough. A short spanner, the one I used was about 4.5 inches long, has just about enough room to get the bleed-valve open a half-turn or more, but it is still a fiddly process, particularly with the car on low ramps and quickly shutting the valve can be tricky. This can allow air to get sucked back in before the valve is closed fully, as below.

Air Getting Sucked Back / One-Way Valve:

With only a few seconds to shut the bleed-nipple before air gets back in, I found the best solution to be a one-way valve bleed system. These are basic, widely available kits and just have a valve with a ball-bearing in the end, which sucks back and blocks the tube so no to air or fluid re-enters the bleed-nipple after each clutch stroke. At the least it makes the return of air and fluid very slow and that gives the vital time needed to shut the bleed-screw.
I used the Vizilbleed branded kit, left in the pic, and it worked brilliantly, with a soft rubber hose to fit over the nipple and a plastic collar that slides over it to make a tight seal. It also has a built-in bottle to catch fluid and, though the tube could do with being a bit longer, the kit is a bargain at £7 from Halfords. The kit on the right in the picture is even more basic, just a tube with a one-way valve at the end, but I am sure it works just as well. This Halfords branded kit was dearer at £10, but kits like these can be bought from eBay and Amazon for less.
The clutch takes a lot of pumping to get out air stuck in the master-cylinder next to the pedal and the fluid level in the reservoir needs topping-up constantly to ensure it properly fills. The section of reservoir dedicated to the clutch fills from quite high up in the bottle, so when topping up make sure you fill into the neck of the reservoir.
The one-way valve will prevent air getting back in, so the bleed-nipple can be left open and I found the best technique was to pump the clutch-pedal about 20 times, then top-up the reservoir. The level in the reservoir after the first 20 pumps should drop significantly and continue to draw more fluid after it is first filled, so make sure it is topped right up before pumping again. This should not take more than 2 or 3 goes and I found the amount of fluid that bled out of the slave-cylinder was less than I had poured in, meaning after a couple of failed bleed attempts I had finally got that stuck air out of the system.

Pressure Bleed Kit + Air Locks:

If you have clamped the clutch-hose and don't have a lot of air in the system or any air locks, then a pressure-bleed kit should work as normal. These kits, such as the Gunson Eezi-Bleed, work by pumping fluid in using air-pressure from one of the tyres. Problem is, if you do have air locks, particularly air in the master-cylinder by the pedal, then a pressure-bleed will not work to get rid of them. I found that the kit would just pump neat fluid directly through the system, giving the impression of an air-free system, but still leaving the master-cylinder full of air and no clutch-pedal. Even pumping the clutch while pressure-bleeding did not seem to clear the trapped air, I think because it wants to exit the system from the open reservoir and has been pushing against the incoming fluid pressure.
A 'reverse' pressure-bleed kit that builds up a vacuum and sucks the fluid through bleed-nipple as it is topped-up from the reservoir might be effective in getting rid of air locks, as the reservoir remains open and the fluid/air will be pulled down from the master-cylinder, rather than pumped straight past it, but I did not have chance to test one.

Back Filling + Clutch Delay-Valve (CDV):

I have seen many articles discussing back-filling of the BMW clutches to be the easiest way to clear all air from the system. This works by injecting/pumping fluid in through the bleed-nipple using a syringe or suitable pump and ejecting the air out the top of the reservoir. While this may work great for older models, modern BMWs have a slow-return valve fitted before the slave-cylinder [pictured]. This is known as the 'CDV' or Clutch Delay Valve and works by slowing the return of fluid to the master-cylinder when the clutch is engaged. This makes it impossible to dump the clutch too hard, as it ensures a nice soft engage every time. Flow from the master-cylinder to the slave-cylinder is unaffected, so the clutch releases as quickly as you like, it is just when fluid returns that it is slowed down. This is all wonderful technology, but throws its hand in when trying to back-fill the clutch to bleed it as you are pushing against the CDV.

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