Chicagoland MG Club
A Plug for Calipers
by Dan Herman, Lake Carol, IL (formerly Naperville, IL)
When rebuilding disc brake calipers, your shop manual or other service instructions probably recommend removal of the pistons by using pressure from the hydraulic system. This method is not only messy, but inhibits the work by allowing removal of only one piston at a time. It also forces you to work on the caliper under the confines of the fender while it is tethered to the chassis by the brake hose.
However, there is an alternative mechanical method which permits removal of both pistons at once, while allowing you to do the work off the car. Remove the caliper assembly from the car, disconnecting the hydraulic hose at the caliper. Determine the inside diameter of the caliper piston, then visit the hardware store and find a plumber's "test plug", or try the auto store for the rubber plug used to replace the steel expansion plug in a cylinder block core opening. Both are rubber sleeves with concave washers at each end and a center bolt used to compress and swell the rubber sleeve. Choose one of a diameter as close as possible to the piston bore.
Now cut a piece of 1/4" x 1" steel strap about 6" long and drill a hole in the center that will fit the bolt in the expansion plug. Install the strap on the center bolt with the nut finger tight and cut off the bolt flush with the nut.
Insert the plug in the piston bore, install the strap and tighten the nut securely to cause the plug to grip the inside of the bore. If the plug should prove too small to expand enough to grip the piston, securely wrap a few turns of rubber tape around it until it is large enough to grip as needed. Rubber tape is far better than plastic or electrical tape because it will expand with the plug, having good friction characteristics.
Once the plug is tight in the bore, grasp the caliper and tool assembly with both hands, placing the thumbs on the back of the opposite cylinder and the fingers around the strap. Squeezing the hands closed will extract the piston from the bore. Then repeat for the opposite side. Either this tool or a C-clamp can be used to install the pistons following the rework.
Notice this will not work for origianl type MGA calipers with the center guide pin in the piston. It will work for MG Midget and MGB. It will also work for MGA after you remove the center pin and install (cheaper) MGB pistons.
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