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Brake Master Cylinder Bench Bleeding
 
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Pelican Technical Article:

Brake Master Cylinder Bench Bleeding

Tom Morr

Time:

30 minutes30 mins

Tab:

$0 to $10

Talent:

*

Tools:

Phillips screwdriver, bench vise

Applicable Models:

Mercedes-Benz R170 (1998-04)

Parts Required:

Fittings, tubing

Hot Tip:

The calipers will also need to be bled

Performance Gain:

Better braking

Complementary Modification:

Replace brake booster if necessary

Removing air from the braking system is critical for safe stopping. Most SLK owners know that the brakes must be bled any time a system component is replaced or the lines are cracked open. Mushy brake pedal feel is the primary symptom of air in the lines.

Some confusion surrounds brake master cylinder bleeding. Many people assume that bleeding out air at the calipers automatically purges the air from the master cylinder's bore. This isn't necessarily the case. At normal ride height, the end of the master cylinder that faces the front bumper is angled upward. The piston pushes fluid out through the master cylinder's two ports, but air can remain trapped at the forward part of the bore.

Some people claim that this trapped air can be completely purged by jacking up the car's rear end and securing the car on jack stands or placing ramps under the rear tires, making the master cylinder's front end lower than its brake booster side. Then normal bleeding theoretically purges the master cylinder in the process of removing air from the rest of the system.

However, bench-bleeding is the traditional, time-honored way to eliminate air from the master cylinder's bore. In fact, new master cylinders often include a bleeder kit in the box: a pair of proper-sized fittings and vinyl or rubber tubing. If not, these bleeding components can be purchased individually at a good auto-parts store, as can pre-packaged kits that contain an assortment of fittings.

The process is similar to manually bleeding the calipers: stroke the piston until no more air bubbles appear from the bleed tubes. In the case of the master cylinder, brake fluid is routed back into the reservoir instead of to a catch container. Once air bubbles quit gurgling out of the tubing, the master cylinder can be mounted in the car. Have a rag available to catch corrosive brake-fluid drips when removing the bleeder fittings and reconnecting the brake lines to the master cylinder.

Please note that the procedure outlined in our article and the Cardones System are both valid ways to bench bleed the master cylinder. It's simply a matter of personal choice. 

Replacement master cylinders often come with a bleeder kit: namely a pair of fittings and length of hose.
Figure 1

Replacement master cylinders often come with a bleeder kit: namely a pair of fittings and length of hose. This kit is designed to cover a variety of applications. More cost-effective for the R170 SLK if parts aren't included with the master cylinder might be to buy two M12-1.00 barbed fittings and a couple feet of clear vinyl tubing in whatever ID fits tightly on the fittings' barbs.

The master cylinder is carefully clamped into a bench vise.
Figure 2

The master cylinder is carefully clamped into a bench vise. We protected ours with a rag. Then the fittings (red arrows) are screwed into the outlet ports.

Add new DOT 4 brake fluid to the reservoir.
Figure 3

Add new DOT 4 brake fluid to the reservoir. Fill to within 1/4 inch of the top.

Run the tubing into the reservoir, submerged in the fluid.
Figure 4

Run the tubing into the reservoir, submerged in the fluid. We used the parts-store bleeder kit's clips (red arrows) to hold the hoses securely to the reservoir's lip.

Slowly pump the piston with a large Phillips screwdriver or a similar bar-type apparatus.
Figure 5

Slowly pump the piston with a large Phillips screwdriver or a similar bar-type apparatus. Repeat until air bubbles no longer surface in the fluid. Then install the master cylinder in the car.

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Comments and Suggestions:
Duf Comments: http://www.cardone.com/tech-help/brakes/how-it-works-and-best-practices/master-cylinder-bench-bleeding-procedure

CLEAR THIS UP !!!!!!! PLEASE !!!!!!!!!!!!!!!!!.
March 20, 2015
  Followup from the Pelican Staff: I will have someone look into it. - Nick at Pelican Parts  

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