My sis-in-law's car, a 1993 Nissan Sentra, had not been maintained by its previous owner. Since its purchase two years ago, I have been doing incremental maintenance on it. The last one I did was a brake fluid change.
Apparently, the brake fluid had not been changed in a long time. After changing it, the brake pedal became soft, mushy and the pedal could go to the floor. The car became dangerous to drive. Even with pedal on the floor, the wheel would not lock up. Repeated bleeding didn't help.
Then I stumbled on an explanation somewhere in the Internet (I forgot to note the URL). The old brake fluid could have been saturated with water. It caused the rubber seal within master cylinder to swell. Because brake fluid (DOT3&4) is hygroscopic, when the fluid was changed it pruned the seal, leaving it wrinkly like your fingers when you have been swimming for too long.
This causes the master cylinder to have an internal leak: instead of forcing the fluid along the brake lines to the slave cylinders, the seal allows the fluid to leak past them to the unpressurised region.
Symptoms of a master cylinder with an internal leak:
- You can push the pedal to the floor with light continuous pressure even without the help of brake booster.
- The pedal slowly sinks to the floor under its own weight (by itself).
Since diagnosing brake failure cannot be done with 100% certainty, if you have one of the above symptoms and you cannot attribute it to anything else (leaky slave cylinders, leaky brake lines, weak brake hoses, various other kinds of external leak), you need to replace the brake master cylinder.
Special Tools & Parts:
- Replacement brake master cylinder. I got a rebuilt/remanufactured one for $60.
- Flare-Nut wrench (see picture to see the difference).
- Strong strings (optional. to help seat the reservoir in the new unit).
- Aluminium foil / plastic food wrap / plastic bag / any barrier suitable for containing brake fluid.
- Bench vise. Bench bleeding is much easier with one. Much much much easier.
Procedures:
- Lift car on all wheels with wheels off.
- You need some space around the brake master cylinder.
- So, start by clearing up the area surrounding it.
- Clean the area around master cylinder with brake part cleaner and wiping (physically). You want to get the area as clean as possible to prevent contaminant in your brake system. Use tooth brush (not the same one you used this morning) to clean tight spaces.
Do you know that brake part cleaner (Tetrachloroethylene) is a carcinogen? Do not let it drip to the floor. Do not let it free in the environment. I find an oil drain pan suitable to contain brake part cleaner liquid run-off. It's wide and short and can fit under various areas of the car. Don't use the same one as the one you use for engine oil. The chemical in the cleaner will render the oil unsuitable for recycling back as engine oil. It will force the recyclers to use it as fuel.
- With the master cylinder is still fastened to the brake booster, loosen the brake line nuts using flare-nut wrench. The reason you need to use flare-nut wrench is because they are made of soft material and tighten pretty tight (13-20N.m or 10-15 ft/lb). Don't open them, don't let fluid drip. Just loosen them and tighten loosely back with finger.
- Remove the master cylinder from the booster.
- Now that you can manoeuvre the master cylinder slightly, put aluminium foil or fluid barrier under and to the sides of it because next you'll be removing the brake lines. Put some towels (that's the white thing on top of the aluminium foil in the picture below) too so the caught fluid is not sloshing around on the barrier. When the brake lines are separated, they won't drip fluid, but the master cylinder will. So, put it in a plastic bag as you carry it over the car.
- Empty the master cylinder and reservoir of brake fluid.
- Mount the master cylinder on the bench vise.
- Remove the reservoir by moving it left and right while pulling it up. It's going be hard but it won't break as long as you are using your hand. The bench vise allows you to use both hands for pulling.
- Pry off the filter screen in the reservoir
- Clean the filter and the inside of the reservoir with brake part cleaner. Don't forget to do it above the drain pan to increase your karma. Afterwards, rinse with fresh brake fluid. Pour in some fluid into the reservoir, put on the cap and cover the ports with your fingers. Shake shake shake and let the fluid out. Repeat as necessary. Don't install the the filter yet, but clean it as well.
Here is a picture of the old and new master cylinders with the reservoir and metering valves removed.
- Install the reservoir, metering valves and the bench bleed plugs (the green plug screwed to the metering valves and ports in the picture below). If the plug does not fit well, do not use it because it won't help in eliminating air during bench bleeding. You need to be prepared for getting messy during the bench bleeding, or do a long painful re-bleed session on all wheels (good thing you already have the car up, right?)
Putting back the reservoir was hard for me as I was not powerful enough to press it in. So, I used a long piece of string to help convert torque to pressure. Make sure that the string form a wide band so the pressure will not damage the nylon reservoir.
- Install clear tubings from the plugs back to the reservoir. My green plugs takes 3/16" ID (internal diameter) tubings.
- Fill reservoir with fluid.
- Use a long screwdriver or a socket extension to push the piston repeatedly until you see no more air bubble. If there is a port without a plug (because it does not fit well), cover the port with your finger to prevent air being sucked in before releasing the pressure on the piston.
- Installation is the reverse of removal. Do not forget to put the master cylinder in a plastic bag during transport as it will be dripping fluid. Take your time in screwing in the brake line nuts. Do not cross-thread the master cylinder (the nut is soft, but the master cylinder is softer). Do not get panicky because the fluid is dripping. That's why you put the barrier underneath, right?
If you did a good bench bleeding, you don't have to do anything else beside putting the car down. Otherwise, go bleed the brake. Yes, new master cylinder means you can push the pedal to the floor without damaging the seal.
Make sure to test drive it and check for external leak.
Keep checking for five days for any fluid level drop in the reservoir and for external leak. Some leaks are very small. I found a very small leak on the third day on one of the the brake line nuts that was not there previously.
