Brake Bleeding

We will start with bench bleeding the master cylinder. Most new master cylinders sold today come with 2 plugs. These plugs are to be used to block off both outlet ports. This is a foolproof bench bleeding method. The whole purpose of bench bleeding the master cylinder is to get the air out of the cylinder so that we don't push it down into the system when we start bleeding our wheel cylinders and calipers.

Mount the master cylinder in a vice. Using the plugs supplied with the master cylinder, screw them into the outlet ports and tighten. Next, fill both reservoirs with the type of fluid you are going to use. (Dot 3 or Dot 5). Using a dowel rod or large Phillips head screwdriver, stroke the piston using 3/4" to I" strokes. You will see the air bubbles coming up into the reservoirs through the com-pensating ports.

Stroke the piston until you cannot stroke it any more. When you can't stroke it any more, you are finished. This is the easiest and most foolproof way to bench bleed a master cylinder. It also happens to be a very good way to check a master cylinder if it is already mounted in the vehicle. When you plug off both outlet ports and step on the brake pedal, you should have a high hard pedal. If the pedal eases toward the floor, then you know you have a master cylinder that is either "bypassing" or leaking backwards out the pri-mary piston seal and should not be used. Leave both plugs in the outlet ports and mount the master cylin-der in the vehicle. Hook up the line going to the rear brakes and leave the other outlet plugged. A bladder type pressure bleeder is by far the best tool to use at this point, but they are very expensive and my suggestion would be for each NTBA Chapter to purchase one so it would be available for chapter members to use.

     

(use a box end bleeder wrench on the bleeder valve before you slip the hose onto the valve) and put the end of it in the glass container BELOW the surface of the fluid.

Now (and this isVERY important) have a friend pump up the pedal making sure that the reservoir is full of fluid. You MUST communicate with your friend who is pumping up the pedal for you. He/ she should tell you when the pedal is pumped up and being held. You then loosen the bleeder valve and you will be able to see the air bubbles come out of the system into the fluid that you have in your clear glass container. The person pumping the pedal should not start pumping again until you tell them to, after you retighten the bleeder valve. If your friend starts pump-ing the pedal BEFORE you have the bleeder valve completely closed, you will simply suck aerated fluid back into the system.

Repeat this procedure until there are no more bubbles coming into your glass container and you are then ready to move to the next wheel. After you have completed both rear wheels, you should have a high hard pedal. Now we can remove the plug from the outlet port for the front brakes and using the same procedure, we will begin at the right front wheel.

If we have disc brakes on the front, MAKE SURE that the bleeder valve is as high as it can be. In some cases, the caliper may need to be removed from the vehicle and use a piece of I/2" or 3/4" plywood to take the place of the rotor.

If you use this method, you should be able to get the air out of your new brake system and have a high, hard brake pedal.

As always, you can find me on the BBS most every evening.
Fat Pat

Editors note: Fat Pat "Brakeman" Anderson is a brake expert who has produced brake videos and taught courses on brake repair and service. Pat has a long history with automobiles, parts and T-buckets (though with the sale of the flaming chicken Bucket, he is currently without the best ride in the world!) Thanks Pat for this article, keep 'em coming!
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