Brake Master Cylinder Replacement

A faulty master cylinder, unfortunately, is not easy to diagnose. However, if you are in doubt, the relative low cost and relative ease of removal/installation makes this a no-brainer. The following are possible scenarios that may point to a faulty master cylinder.

1.	Brake pedal feels spongy and does not offer precise response based on pedal height, despite a proper brake bleeding.
2.	Brake pedal sinks towards the floor during constant braking.
3.	Car drags to one side under braking. The brake system is set as a criss cross setup, so that if one piston is faulty, it affects a front wheel (ex. left) and the opposite rear wheel (ex. right), while continuing to provide braking (albeit reduced braking power) to the front right and the rear left wheel. With a faulty cylinder, more pressure could be applied to one piston, causing uneven braking and potential veering under braking. Of course, faulty calipers, brake lines could also cause dragging to one side.

There doesn't seem to be a hard-set diagnostic procedure for a potentially worn out master cylinder. The Acura Vigor Service Manual procedure is as follows: “With the engine stopped, depress the brake pedal several times, then depress the pedal hard and hold that pressure for 15 seconds. If the pedal sinks, the master cylinder, brake line or a brake caliper is faulty.”

With the diagnosis and background out of the way, let's move on to the replacement! I have numbered the parts in the pictures so you can cross reference to pinpoint the exact part you need.

Tools/materials needed:

1.	Brake master cylinder. Purchased New Beck Arnley brake master cylinder from rockauto.com for $94.49, Part number 072-8877.
2.	Line wrench. Helps to provide better hold on the brake lines bolts. Using a open ended wrench is possible but not recommended, as the bolts are likely to strip if one is not careful, resulting in a lengthy (pun intended) replacement of the entire brake line and $40+.
3.	Assorted sockets and extensions (10,12,14mm)
4.	Jack/jack stands
5.	Aquarium hose with 7/16in and 1/2 in outer diameter.

First we need to bench bleed the new master cylinder. Just like we sometimes bleed the brakes to purge the air in the brake lines, we need to bleed the new master cylinder to purge air out of the cylinders. Without doing this, the brake pedal will feel spongy due to the compression of air in the master cylinder.

1. As shown in the photo at right, connect old brake lines (preferred) or aquarium hose to the two openings (PART #1 and PART #2) on the new master cylinder and route them back into the master cylinder. For the smaller opening, we snugly fit a 7/16in aquarium hose. For the larger opening, we used the same hose, but had to apply constant pressure or else the fit wasn't airtight. I would recommend getting larger hose for the larger opening.

2. Fill the master cylinder with new DOT3 or DOT4 fluid and secure the hoses so that they don't fall out during the bleeding process.

3. Using a block of wood on the piston area (pictured) press in on the piston to start routing the fluid. Another option is to place the block of wood against a secure surface and push on the other side of the master cylinder. Keep pumping the piston in and out until you can see fluid without bubbles coming out from both openings. If fluid isn't coming out of the hoses, the openings are not airtight. Readjust and continue. It takes about 10-15 compressions to bleed the master cylinder.

4. Take the plugs that were originally on the openings, and plug them in place of the aquarium hoses/brake lines. You will take these plugs off when you install the brakes lines on the car. Put the strainer back in if you took it out earlier, and put the cap on. Make sure the "FRONT" marking on the cap is pointing towards the front of the car.

Club Members can also read Heimonator's excellect write-up in the Files Section of the Clubhouse.

1. Remove the old master cylinder cap and either siphon out the fluid or use a turkey baster and remove as much fluid as you can. Then remove the reservoir to reveal what's in the photo at right..

2. Disconnect bolts holding the EVAP (PART #3) to clear out some space to access the master cylinder bolts. Do not remove any hoses/wires on the EVAP. Open the plastic holding (PART #8b) clip allow movement of the blocking hoses.

3. Using a 10mm line wrench, break loose and disconnect the brake lines connected to the master cylinder (PART #10 and PART #11).

4. Remove the two nuts (PART #6 and PART #7) that attach the master cylinder to the brake booster. Then remove the bolt (PART #8) that holds the black bracket (PART #9) which is between the master cylinder and the brake booster. (See photo on next page. Move the bracket out of the way and remove the master cylinder module.