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Bleeding Drum Brake Systems

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Whether you have replaced/rebuilt your entire brake hydraulic system or merely repaired a single part, you will have to bleed the system to make it function properly.

Start at the master cylinder (MC). A new MC, especially a dual system unit, needs to be ‘bench bled’ before installing. The easiest way to do this is to get an inexpensive brake bleeding kit at Auto Zone, Kragen’s, etc. Screw the appropriate sized fittings into the outlet holes. Fit the tubing on the fittings and lead them into the reservoir. Chuck up the MC in your vise. Fill the reservoir. Pump the MC with a rod or a Phillips screwdriver. The secret is to pump slowly and don’t push all the way in. Watch for bubbles coming out of the tube in the reservoir. When the bubbles stop, you are done.

Mount the MC and hook up the brake lines securely. Use a flare nut wrench to avoid stripping the fitting or rounding off the nuts.

Adjust the brakes before you bleed. This will set the shoes to the proper clearance and make bleeding more effective. Raise each wheel. Turn the adjusting star wheel until the drum just stops turning then back off three clicks.

Bleeding the system: First, go to each wheel cylinder to open and close the bleed valve just to be sure they all work. It is most disconcerting to bleed three only to find that the fourth valve is rusted solid. Use a box end wrench or socket to prevent damage to the valve.

Prepare a bleeding jar. A small clear glass or plastic jar and a piece of clear hose 18” long that is a push fit on the bleeder valve nipple. Put ½” of brake fluid in the jar. Make sure the end of the hose is in the fluid. Start at the right rear wheel (farthest away from the MC). Find a friend with a big foot and place him behind the wheel. His instructions are to pump twice then hold pressure on the pedal until you holler. Meanwhile, you get underneath with your box or flare nut wrench and your bleeder jar with hose. Big foot holds pressure while you crack open the bleed valve to allow air and fluid out. You close the valve before he lets up on the pedal. Repeat until you see no bubbles. Refill the MC reservoir. Go to the left rear and repeat. Refill the MC. Then do the right front. Refill the MC and finish with the left front. You should now have a nice, firm pedal feel.

Refill the reservoir each time or you will be pumping air into the system and will need to start all over!

Brake fluids: DOT 4 withstands higher heat than the usual DOT 3 and is not terribly more expensive. If you have an entirely new system, use DOT 5, 'synthetic', fluid for a trouble free system for many years. Well worth the extra expense. Do not use DOT5 in any system that has older cylinders. They will most likely leak.

Systems using DOT 3 or 4 should be flushed once yearly for maximum function.
Author
Doc Stewart
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Well written easy to follow and consice (can't spell the last word)
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