Bleeding/flushing the brake and clutch fluid
Difficulty: 2/5
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Disclaimer: Before you attempt any brake work on your car, refer to the factory service manual and follow all precautions. Any and all information presented on this website is superseded by the official service manual and is not a substitute for the services or advice of a certified professional mechanic.
This article shows a few different ways to bleed the front/rear brakes and clutch fluid. The pictures are from a Mk4 VW, other TDIs are similar. The procedure can be also be used on non VWs except that many cars have separate clutch and brake fluid reservoirs, your TDI uses the same reservoir for the clutch and brake. If you have an automatic transmission, you don't have a clutch. The recommended method in the service manual is to bleed under pressure but I prefer to bleed under vacuum since I already have a good vacuum pump and I don't empty the system and let air into the ABS pump. If you would like to make your own pressure bleeder, see 1000q: DIY brake bleeder. For general tips on braking and why you have to flush the brake system, see 1000q: general brake FAQ. To do a brake job on your a4 body TDI, see 1000q: a4 brake job.
To make a long story short, the DOT 4 brake fluid used on your TDI absorbs mositure so that water will not concentrate in the corners and form rust. Note that although most VW fluid reservoirs don't have a weep hole, many types of reservoirs do have a weep hole. And every time the system heats up and cools down, some air and moisture enters the system. If you don't change the brake fluid then corrosion may occur in the brake calipers, clutch slave cylinder, or master cylinder, causing a leak or poor action. In addition, under repeated heavy braking, the brake fluid could boil, causing loss of braking authority.
Brake/clutch fluid is highly corrosive to paint! If a drop falls onto your paint, stop and dab the drop off, then wipe it clean with a damp towel. Follow up with soapy water. If you don't wipe it clean now, it will eat a hole in the paint down to the metal. Always wear gloves and comply with all safety precautions as specified in your factory service manual when handling the fluid. I suggest putting some paper towels around the master cylinder reservoir and using a funnel to add fluid. Don't be lazy and not use a funnel.
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Some common issues that can be solved by bleeding the brakes are soft or spongy brake pedal or poor braking action. If you need two pumps to firmly actuate the calipers, then you probably have an air bubble in the brake lines. The air bubble prevents all of the force of the pedal from going to the slave cylinder and fully releasing the clutch. Since air bubbles can be compressed whereas hydraulic brake fluid cannot, even small air bubbles will degrade pedal feel and prevent smooth brake or clutch action. If you just replaced the brake lines or let the fluid level on the fluid reservoir go below "min" you will also need to thoroughly bleed the brakes because it may have sucked in air.
You should also always use the parking brake when parking. This is good practice in any car, but in your VW, it resets the rear caliper self adjusting mechanism. Not applying the parking brake can result in poor braking.
VW brake and clutch fluid: DOT 4 fluid only
Procedure
You may or may not have to jack up the car and remove the wheels to get to the bleeder nipple on
the brakes. If you have ramps or a lift, it's easier to get to the brake
bleeder nipple. Here is a picture of the bleeder nipples (nut in green below,
rubber boot circled in red). Pictured below is the front caliper.
Below that is
the rear caliper. The brakes are rusty on the outside but this doesn't affect
braking action. All calipers have the bleeder nipple on the top of the
caliper. This is because it helps let out air bubbles and helps prevent
air from getting in. I still try to avoid opening the bleeder unless fluid is moving out, either through vacuum or pressure.
Remove the rubber cap and check the size of the wrench around the bleeder. Aftermarket brakes sometimes have differently sized bleeders. It is normally 10mm or 11mm. I use PB Blaster penetrating lubricatn on almost all rusty threads but not on the brake bleeder nipple. The penetrating lubricant is designed to flow into threads and it's possible that it could get into the brake fluid. If the bleeding nipple nut gets damaged, a new one can be purchased at autozone, napa, or any other auto parts store.
There are a few ways to bleed the brakes, here they are.
First method to change fluid (manually)
If you do not have a vacuum or pressure bleeder, have another person pump the brake pedal a few times then press and hold the brake pedal down while you open the bleeder screw to let the fluid out. This will relieve the pedal pressure. Just keep the pedal down until you close the bleeder. Note that pumping the brakes when bleeding is okay, pumping the clutch is not recommended, more in the next section. Once fluid almost stops flowing out, close the bleeder screw so that they can raise the brake pedal (or else it could suck in air). Repeat. See how using a vacuum or pressure bleeder is better?
When you press the brake pedal down, a piston inside the metal cylinder (pictured below) gets pushed forward and pressurizes the fluid in the brake lines. If air gets down to the level of the cylinder opening, you will hear a sucking sound from air entering the cylinder. If this happens, I suggest bleeding the entire system again since air probably got in the lines!
To prevent this while bleeding, keep the fluid level in the brake fluid reservoir above the
"min" line. About every 2-3 full pedal bleeds is a conservative
estimate for the fluid level to go from max to min. If the fluid does not go down
easily then the filter on the
reservoir is clogged - STOP, remove the filter and clean it before going any
further.
With non ABS cars, I recommend rear brakes, then front brakes, then clutch.
With ABS cars, the factory service manual recommends bleeding calipers in the following order (mk4 cars).
1998-2001 cars
Right Rear, Left Rear, Right Front, Left Front
2002-2005 cars
Left Front, Right Front, Left Rear, Right Rear
Another way you can do it manually is to use speedbleeders (which I don't
like due to possible failure of the springs or getting jammed/clogged), or with a hose
loop. With a loop, you tie a clear hose in a loop and SLOWLY pump the
pedal. If you pump the pedal quickly, any air bubbles won't "slide"
back. Pictured below is an example of a hose loop.
Bleeding the clutch fluid
Since the brakes and clutch share the same fluid reservoir, you should bleed the clutch at the same time (manual transmission only). However, it should be bled slightly differently if you are bleeding it manually, and this is why. When you pump the brake pedal, the brake calipers press against a rotor and don't go any further. The fluid in the lines is building pressure but not moving too much. When you press the clutch pedal, the clutch piston goes out and comes back in when you release the pedal. Pumping the clutch pedal does not build pressure like pumping the brake pedal and will cause large air bubbles to become small air bubbles that are harder to get out and notice.
So don't pump the clutch pedal when bleeding, have a helper push and HOLD down the clutch pedal, then you can open the bleeder (circled below in red) to draw out the fluid. This will relieve the pedal pressure, close the bleeder nipple and then the helper can raise the clutch pedal. Otherwise, it can suck in the old fluid or any air bubbles. If the pedal drops to the floor, just pull it up. Again, have your helper press and hold the clutch pedal, open the bleeder, let drain, close the bleeder, and then they can raise the clutch pedal. On your VW, the bleeder nipple is near the top of the clutch slave cylinder but on many cars it's on the bottom of the transmission.
This is why I prefer to use the other methods of changing the fluid - they will be faster and with less chance for air to get in.
Pictured below is the slave cylinder in green and the bleeder in red.
This is on top of the transmission.
Second method of changing fluid (vacuum)
By using a vacuum pump, bleeding the clutch and brakes is much easier than bleeding manually. The main drawback of using a hand vacuum pump is that it may not be forceful enough to scrape every last bit of old fluid out of the lines. The factory service manual also recommends pressure bleeding because it can help get air out of the ABS pump. I use an electric vacuum pump with enough force to quickly suck out the fluid. Either way, it is very similar to the manual method, only you don't need a helper to pump the pedals.
First, apply suction to the bleeder nipple. Make sure you have suction before you open the bleeder! Loosen the nipple until fluid comes out.
Keep suction applied whenever the bleeder nipple is opened so there is no backflow. Check the fluid level in the master cylinder reservoir to make sure no air gets into the lines. If you have a helper, have them top off the fluid reservoir. When the fluid is fresh and clean, close the nipple.
If you didn't see it above, the factory service manual recommends bleeding calipers in the following order (mk4 cars).
1998-2001 cars
Right Rear, Left Rear, Right Front, Left Front
2002-2005 cars
Left Front, Right Front, Left Rear, Right Rear
Third method of changing fluid (pressure)
The last method involves using a power bleeder that applies fluid at the reservoir under pressure. This is pretty much the same idea as the last two methods, but the instructions for your specific model of power bleeder supercede any tips here. The factory service manual states that you should not exceed 14.5 psi of pressure due to the proportioning valve inside the ABS system which prevents a good flushing. I would limit the pressure to about 5-7 psi. The biggest advantage of this method is that unlike a vacuum pump, you won't see any air bubbles in the line unless they came out of the lines.
There are commercially available brake pressure bleeders but if you want to make your own, see 1000q: DIY brake pressure bleeder.
If you did get an air bubble in the lines while bleeding and have fresh fluid and just want to purge any air out, try a re-circulating purge.
Caution: this method is ONLY if your fluid is clean. If you just flushed the entire system but got an air bubble in the lines during the last pump, or if you have a track car with brake fluid that is only 3 days old, you can use this method. If your fluid is dirty or old and not new, do not use this method since it can put dirt, rust, and contaminates into the brake reservoir, damage the master cylinder seals, etc. You MUST have all clean fluid in your system before you try a re-circulate purge.
Some cars have the clutch slave cylinder on the bottom of the transmission, making it easy to get air into the system. In these cases, getting tiny air bubbles out is very important or else you will lose clutch pedal feel and authority.
Always use a clean tube used only for brake fluid! If the tube is not clean, the brake fluid must be discarded! Also make sure that the bleeder is clean and no dirt is in the fluid that comes out of the hose. The first few pumps should always be discarded.
Again, first drain all of the old fluid using one of the methods above. Then connect soft silicone tubing from the bleeder and route the other end back to the brake fluid reservoir. Attach one end of tube to the cleaned bleeder, and the other to the reservoir. With the bleeder loosened, slowly pump. Move to all four bleeders and the clutch bleeder to ensure clean and air-free fluid to all corners.
As always, you must keep the fluid level above "min".
I also suggest taping a paper towel near the tubing ends to catch any drips. Again, brake fluid is a powerful paint stripper and will eat your paint if not quickly cleaned. If your hands or tubing have any fluid on them, wrap the tubing in paper towels as well.
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