Thursday Techspecs - Bleed your brakes

Why & when bleed your brake (line) system...

Most (USA) Classic Car manufacturers recommend using Dot 3

( or later Dot 4 ) brake fluid and refreshing it every 18 to 24 months.

The fluid is hygroscopic, which means that it absorbs water from the atmosphere over a period of time, and its boiling point is lowered. Friction caused by heavy braking heats the fluid in the disc calipers or wheel cylinders, and its water content turns into steam.

The brake pedal will feels 'spongy' when you press it, and in severe cases the brakes will fail completely. Also... air leaking into the hydraulic system makes the pedals spongy because, unlike brake fluid, air can be compressed.

If your pedal feels spongy before the fluid is due for replacement, check the system for leaks. Air can be drawn in through leaking seals or faulty brake pipes. You can also have a leaking Caliper that causes failure to the system . If you find a leak, first replace the faulty components and then renew the brake fluid by bleeding the complete hydraulic system.

The fluid is drained from the system by opening bleed nipples, which are small valves, located on each caliper or wheel cylinder
( For exact location see image below )

The air / fluid can be pushed out by pumping the brake pedal.

How to bleed your brake (line) system...

Remove dirt and corrosion from the bleed nipple with a wire brush, take off the dust cap (if there is one) and put a ring spanner of exactly the right size over the nipple.

Leave the spanner in place and attach a length of clear plastic tubing to the nipple. The tubing should be about 2 ft (600 mm) long, and an airtight fit on the nipple.

Place a clean glass container, such as a jam jar, under the nipple, and put the free end of the tube into it. Pour enough fresh brake fluid into the jar to cover the end of the tube.

Fit the correct size of ring spanner over the nipple & attach the bleed tube. Loosen the bleed nipple about half a turn, and leave the spanner in place.Brake fluid should begin to ooze from the nipple and flow down the tube into the jar. Make sure that the end of the tube is below the fluid in the jar.

When the brake fluid is flowing, ask the second person to pump the brake pedal while you watch the fluid run through the clear tubing. If you see bubbles (they may be quite small ), there is air in the system. More than a few pumping strokes may empty the master-cylinder reservoir, so remember to keep checking it and top it up if necessary. Continue pumping (and topping up the master cyllinder) until there are no more bubbles.

Let the second person pump twice more, then keep the pedal depressed.

Close the nipple by tightening it with the spanner. Be sure to retighten the nipple before removing the bleed tube, then remove the tubing and the spanner and move to the next nipple.

When you have bled all the nipples, apply hard foot pressure to the brake pedal. It should no longer feel spongy. If it does, there is still air in the system and you will have to repeat the bleeding process, possibly in a different sequence, until the pedal feels firm.

It should remain equally firm under hard and prolonged pressure, without sinking to the floor. The car should pull up quickly and in a straight line, and the pedal should not feel spongy.

Bleeding Sequence of a Single Circuit Brake Line System

Start at the wheel furthest from the master cylinder,  and end at the nearest one.

Bleeding Sequence of a Dual Circuit Brake Line System

The front and rear brakes of a dual-brake system have separate hydraulic circuits. Because the two circuits are separate, bleeding is two separate operations. Start with the wheel nearest to the master cylinder in each case.

This usual is at the front-wheel brake, which is nearest to the master cylinder ( Most cases the one on the driver's side ).  Bleed the other front wheel and then the rear wheels, ending with the one furthest from the master cylinder.

At the same time the master-cylinder reservoir has to be topped up with new fluid at frequent intervals, because its level drops as the old fluid is drained off. If the reservoir empties, air will get into the system and the whole process will have to be repeated until all air is expelled. Keep it topped up as near as possible to the level mark on the side. 

Note(s):
Take care when opening the bleed nipple. They are often difficult to turn, and fragile enough to fracture if excessive force is used.

The bleeding method for drum brakes, disc brakes and combination systems is the same. Different brake types on the same car may have different nipple sizes, and you must use the correct size of ring spanner for each one.

Bleeding brakes using a tube with a non-return valve involves exactly the same procedure, but you don’t need a second person to help.

Avoid spilling brake fluid on paint-work. It will start damage your paint within a few seconds. Hose it off immediately and wipe the area with a clean rag.