Follow by Email

Sunday, 10 April 2011

Cleaning Up the Front Brake Master Cylinder

Opened up the brake master cylinder cover and the rubber diaphragm was damaged. It was totally stretched out and was sagging into the fluid itself. Forgot to take a picture of that.

Here's what it looked like on the inside....looks like the fluid hasn't been changed for decades...

I drained out a little of the fluid so that I could clean up the inside with a ball of crumpled aluminum foil. Look at all the corrosion. It's actually crytalized old brake fluid I think.

I begin scrubbing with my ball of aluminum foil.

Another view...


Stopped halfway through to take some pictures of the progress...

Looks better, but still not very clean

When I started out, I didn't want to drain all the fluid out of the hydraulic system, because I wasn't looking forward to bleeding the system. But things don't always go according to plan. I realized if I wanted it really clean on the bottom, I'm going to have to drain out all the fluid so I can see what I'm doing.

Another view..

After more scrubbing, it's finally starting to clean up. I used a sharp knife to carefully slice off the old, gummed up brake fluid from the top surface of the master cylinder.

Looking pretty clean now...

Fill it up with fresh brake fluid, DOT 4 in this case.

Now it's time to bleed all the air out of the system. The bleed nipple screw looks rounded, so I thought I'd better not risk it with a 12-point wrench. Used a 6-point socket with a ratchet (an old Snap-On F830 in case you're wondering) to break the screw loose.

A close up view of the bleed nipple...

Now it's time to bleed the air out. I spent a really long time bleeding the system. For the first 5 - 10 mins, there was hardly any pressure at the brake lever at all, because air had entered the system from the brake master clyinder at the top.

From trial and error, I discovered that the correct way (or rather, a faster way) to do this is to pump the brake lever multiple times, say 20 times, before you crack the bleed nipple open once.

Here's what the basic setup looks like. Cover all areas where the brake fluid may drip on with rags. Brake fluid is corrosive and will strip paint.

The wrench goes over the bleed nipple first, then the clear tubing goes onto the bleed nipple through the ring end of the combination wrench. Stick the other end of the clear tubing into a jar or empty container.

By right you should submerge the end of the tubing going into the jar or bottle with some clean brake fluid. This is to prevent air from getting sucked in from the other end. However, since the hose is already full of air to begin with, I never thought it's much use to do that. Besides, I'm a tightwad and I don't want to waste good fluid if I don't have to.

Here's the procedure:
1) Have someone "pump" the brake lever five times, squeezing and holding the lever down for half a second each time you pump it. So it's squeeze, hold half a second, let go, squeeze, hold half a second, let go, and so on. After the fifth time, hold the lever down and DO NOT LET GO. If you do, and the bleed nipple is open, air will enter the system from the nipple.

2) Open the bleed nipple a quarter to a half turn. The old brake fluid in the system will be seen flowing out of the clear tubing. Look at the fluid coming out and spot any air bubbles coming out of the system. This is what you want to get rid of. Air compresses, and makes the brakes feel "spongy."

As you open the nipple, the brake fluid will go "soft" and can be depressed all the way in. DO NOT LET GO YET.

3) Tighten the bleed nipple. Now the person holding the brake lever can let go, and go back to step one.

Here's another view of the setup.


  1. Wow, truly ancient fluid. Nice job cleaning out the bowl, looks new.

    For brake bleeding I use the MityVac MV8000 hand pump kit. Got mine at AutoZone under the OEM label for $30. Worked fine on the NH too.

  2. Awesome cleaning job. I have been thinking about getting one of those replacement fittings that allow you to loosen the fitting, pump out the old fluid, but close before air gets in. I replaces the stock fitting. Think I'd like to replace the brake line with stainless steel line--I have heard it makes a great difference.

  3. i need to do this tomorrow- im afraid mine might be in the same state. thanks for the walkthrough!