Brake Bleeding

(Difference between revisions)

Revision as of 06:22, 27 February 2016

When changing hydraulic components of the brake system, the air needs to be bled out of the brake lines. This also needs to be done when replacing the brake fluid per the periodic maintenance schedule. Also be sure to bleed the brake master cylinder if it runs dry.

Overview

The hydraulic system must be bled whenever any line has been disconnected or air has in some way entered the system. "Spongy" pedal action is an indication that air has entered the system -- if the brakes "pump up" (get stiffer after you press the pedal rapidly a few times) that is another tell-tale sign of air in the lines . If however the pedal is "soft" (has excessive travel), then adjust the drum brakes, then afterward re-evaluate the pedal action. If it's still spongy, proceed with bleeding.

1. Bleed the master cylinder (sometimes called "bench bleeding" but can be done while installed in the car)
2. Bleed the front wheel cylinders
3. Bleed the rear wheel cylinders

Master Cylinder Bleeding

When replacing the master cylinder, bleed it before attaching to car.

1. Insert clean vinyl hose into front Outlet. Loop the hose into the reservoir
2. Insert clean vinyl hoses into REAR outlet. Loop the hose into the rear reservoir
3. Top up reservoirs with new brake fluid of recommended type
4. Work pushrod back and forth until air bubbles cease to appear in reservoir

If an installed master cylinder runs dry, bleed it as follows BEFORE bleeding individual wheel cylinders.

1. Top up reservoirs with new brake fluid of recommended type
2. Have assistant pump pedal three times, then hold pressure
3. Crack the fitting slightly for front outlet of master cylinder until fluid comes out
4. Tighten fitting
5. Repeat steps until only fluid comes out, and no air spurts out with the fluid.
6. Repeat for rear outlet port of master cylinder

If necessary, top up reservoir with new fluid. DO NOT REUSE BRAKE FLUID.

Method 1

The fluid prevents air from going back up into the cylinder, and can be done by one person (no assistant needed).

1. Put about 1" new fluid in a clear glass
2. put a hose from the bleeder to the bottom of the glass, making sure the new fluid covers end of hose
3. Crack the bleeder slightly
4. pump the brake pedal while you watch the glass. When air bubbles stop coming out, it's done. Often this only takes two or three pumps.
5. tighten bleeder
6. top-up the master cylinder. DO NOT REUSE BRAKE FLUID
7. bleed the next wheel cylinder

Any spilled brake fluid should be rinsed off painted surfaces with lots of water.

Method 2

Discussion: Bleeding the brakes.....on your own

photo  photo  photo  photo  photo  photo 

Traditional Method

The traditional method of brake bleeding requires an assistant to work the pedal, while the mechanic tends to the wheel cylinders.

The Datsun 1200 Factory Service Manual recommends a modification of this by using a hose from the bleeder screw into a jar with a bit of brake fluid in it covering the hose end.

1. have assistant pump the brake pedal three times and then hold pressure
2. loosen bleeder (the pedal will travel down under assistant's foot pressure)
3. have assistant keep pushing on brake pedal (do not let up)
4. tighten bleeder
5. let brake pedal up
6. repeat until a clean, steady stream of brake fluid comes out. Often only two pedal cycles are needed
7. repeat for the other three wheels