Brake Master cylinder

Tiny
COREY H JENKINS
  • MEMBER
  • 1996 NISSAN 200SX
  • 1.6L
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 253,000 MILES
Just mounted new brake master cylinder but have not bench bleed it.
Can I just hook up the brake lines and bleed master cylinder and brake lines at the same time?
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Sunday, April 30th, 2017 AT 10:55 AM

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Tiny
CARADIODOC
  • EXPERT
You're attempting to make this job as difficult and time-consuming as possible.

Here's a trick that works on Chrysler products to prevent the need to bleed at the wheels. Your car should be similar. This description starts with the removal of your old master cylinder, so you can skip over that part.

Loosen the two line nuts just a little, THEN unbolt the master cylinder from the power booster. Pay very close attention to the steel lines as you turn the nuts to be sure they're not twisting. If the nuts don't spin freely, the lines will twist and snap off. Then you have another repair to do that involves new fittings and making double flares. Double flares are hard enough for a professional. They're harder to do in the confines of the engine compartment. Pull the master cylinder forward off the mounting bolts, then use it as a handle to twist the two lines upward just enough so those first two inches are not parallel to the ground. Then you can remove the lines. This will prevent the brake fluid from running out of them. Be careful to not drip brake fluid on the car's paint.

After bench bleeding the new master cylinder, leave the reservoir at least half full of fluid. When you install it, you will have to tilt it to line up the ports with the line nuts. Remove one plastic bench-bleeding fitting, then install the steel line nut hand tight. While you do this, fluid will be dripping out, keeping the line and port full. If the nut doesn't thread in easily by hand, it's cross-threaded. Start over and try again until the nut goes in by hand, typically four or five revolutions. If you damage the threads in the aluminum ports, there will be no warranty and you will have to buy another master cylinder.

Once the first line is connected hand tight, do the same thing with the second line. When both fittings are hand tight, twist the master cylinder and lines back down to their normal position, and bolt it to the booster. You'll need a helper for the next step. There will be a little air in the ports and tops of the lines. Tighten the front nut, then loosen the REAR nut about a quarter turn, then have your helper push the brake pedal down very slowly. It should take him about 15 seconds to push it half way to the floor. Any faster, and the air might still get forced down the lines. While the pedal is moving down, you will see air bubbles coming out around the nut's threads. When the bubbles stop appearing, tighten the nut. Tell your helper to not allow the pedal to move back up until you tell him the nut is tight. Raising the pedal too soon will allow air to be drawn back in through the nut's threads.

When the nut is tight, tell your helper to allow the pedal to come back up QUICKLY. The brake fluid rushing back to the reservoir will wash any air bubbles back too. Do this procedure a second or third time until you don't see any air bubbles, then do the same thing to the front brake line.

As long as the brake pedal is pushed down very slowly, no air will be pushed down the steel lines. The fluid will go down, and the air bubbles will float back up. When the pedal is released quickly, any air bubbles that stick to the lines will be drawn back up to the reservoir.

Fill the reservoir with clean brake fluid from a sealed container to the same level as was found in the old master cylinder. If the level was fairly low, that's what happens when the front brake pads are worn close to the end of their life. Installing new pads results in the fluid returning to the reservoir and the level will go back up to "full". If you fill it before that, the fluid will spill over and make a mess when new pads are installed.

It's good practice to never push the brake pedal further than half way to the floor. This isn't a big deal with your new master cylinder, but with any master cylinder that is more than about a year old, it can be damaged by doing so. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal over half way runs the rubber lip seals over that crud and can rip them. The typical symptom is a slowly-sinking brake pedal, and that often doesn't show up until two or three days later. This damage often occurs when pedal-bleeding with a helper, and after the driver was surprised by a sudden, unexpected leak.
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Monday, May 1st, 2017 AT 3:06 PM

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