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Bleeding brakes Posted by JimBlake [Email]  (#141)  [Profile/Gallery]
(more from JimBlake) on Tue, 23 Sep 2003 13:38:54
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search-key: 9.3.g1_tech
Background:
Brake fluid is hygroscopic, meaning it can absorb moisture from the air. As it picks up moisture the boiling point gets lower, but more importantly it becomes corrosive.
I like to change brake fluid at least every 2 years regardless of mileage. Bleeding brakes on the 9³ is pretty straightforward if you've done this on any other car.
The order which you bleed is based on which circuit is likely to push air bubbles into the others. In the 'old days' most brake systems were bled in the same order, beginning with the farthest wheel away from the master cylinder. With ABS the bleeding order is not so simple. I believe if you bleed the wrong order, you simply have to go around a couple times before all the bubbles are gone.
If you have a pressure bleeder (Motive, etc...) it'll have instructions. All you need is the bleeding order. LF -> RR -> RF -> LR
Special Tools & Supplies:
Sealed bottle(s) of DOT 4 brake fluid (at least a liter). I've learned that it's real inconvenient to run out so I usually have at least 2 liters on hand.
Clear tubing that fits over the bleed valve, and an empty bottle.
Turkey baster. (you'll never put it back into the kitchen)
A helper to pump the brake pedal.
Procedure:
With a manual transmission, bleed the clutch first. It has to be reverse-bled, so that will push old fluid into the reservoir. See the FAQ for bleeding the clutch.
See if you can reach the bleed valves with the car on the ground. Otherwise lift & support the car. Ramps are OK, you don't have to remove the wheels. Open the windows, you'll be talking with your helper.
The reservoir cap has a vent, that's where it picks up moisture. So empty the reservoir with the baster. Optional: push the caliper pistons to push a little fluid back into the reservoir. Remove this with the baster. Now fill the reservoir nearly full. You could use another helper to keep the reservoir filled.
Many people like to put a small block under the brake pedal so it doesn't stroke all the way down (more about this below*). Your helper sits in the car, you attach the hose to the bleed valve on the left-front caliper. Drop the hose into the bottom of the empty bottle.
Helper pumps brake a couple times then applies steady pressure. You open the valve (1/8 to 1/4 turn) and fluid comes out as the pedal sinks towards the floor. Ideally you close the valve just before the pedal stops at the block/floor. Each stroke is maybe a teaspoon of fluid.
AFTER you close the valve, helper can lift the pedal, pump, and apply steady pressure again. Repeat this until you get several ounces. Check & refill the reservoir. Pump at least a cup (250 ml) of fluid, or more, until it comes out clean & free of bubbles. Then move to the next wheel in the following order:
1: left-front
2: right-rear
3: right-front
4: left-rear
If you've replaced a caliper or somehow gotten air into the system, you can start with that corner, then do the complete sequence. Rapping on the caliper can help dislodge bubbles that would like to stick to the walls of the cylinders.
Notes & questions:
* Dean recommended that the pedal should not be stroked all the way to the floor. That portion of the master cylinder never gets used and it would kick up dirt, or damage the piston seals. I've bled brakes by pushing the pedal all the way down for years before I heard this, with no problems. But my success is on cars that have always had regular brake fluid changes. So I guess this suggestions makes sense.
Speed Bleeders:
These are replacements for your bleed valves, with a small check valve. You leave it open and simply pump the pedal. When I got them I thought they were wonderful. Then 2 years later they were corroded and they didn't work so hot. Guess for yourself based on climate and road salt.
MityVac brake bleeder:
This is a kit of tubing and a collection jar, so you can suck fluid out of the bleed valve without anyone pumping the pedal. I've found it mostly sucks air in through the bleeder threads. So I put a generous gob of grease around the threads, and it works fairly well. I still get some bubbles, so I just have to trust that they're coming from the threads. I've had to finish up the traditional way (above). If you can only get a helper for a short time, use the MityVac first.
Bleeding order thanks to Doug Adams who has a Mitchell CD-ROM manual... Andy S found a different order for NG900 cars in a Haynes manual (RF-LF-RR-LR).
posted by 68.73.7...
, Tue, 23 Sep 2003 13:38:54 <-- Viewing This Message
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