I 'm not sure how it got started, calling a single-chamber, '65-66 Mustang master cylinder a "fruit jar". I think it was because the lid is large, and screws on like one. After 30 years, this is a good safety point we should all know about. And as street rodders, we should try to encourage others who still use any single-chamber master to replace it! If you lose brakes in the front or rear with this fruit jar, you've lost all braking capability. With a dual-chamber master, the rear line coming out of the master cylinder (next to the pushrod - the larger reservoir) goes to the front wheels (usually discs), while the front line (farthest from the pushrod - the smaller reservoir) goes to the rear wheels (usually drum). Now, if one of the lines or hoses breaks, you'll still have front or rear brakes to stop the car. Late-model cars have gone one step further, using a four-port master cylinder. Two ports from each chamber (one with a residual check valve for rear drums) help split the brake system diagonally. One front brake and rear brake are fed by each chamber (check the photo). Now, if you lose any one wheel, you'll still have a front and rear wheel that will keep the car relatively straight as you stop.
If you are replacing the Mustang fruit jar, which has an integral brake light switch (the threaded hole in the casting is for the switch - check photos), you'll need to add a brake light switch, T'd into either the front or rear lines coming out of the master cylinder. At the parts house, you can ask for a hydraulically actuated, electronic stoplight switch (Borg/Warner #S-194). Or, your old '65-66 Mustang brake light switch can be used in this T, if it's in good working order. Installation can be accomplished with a threeway T (Everco #652-B) that has l/8-inch pipe threads on top (for the switch), and 3/8-24 thread for 3/l6-inch brake lines on each end.
You'll also need to separate the front and rear lines where they are T'd together under the chassis. This means you'll need to buy another length of 3/16-inch brake line with ends, to go from the T to the master cylinder. There are now two different sizes on the dual-chamber master; if you don't have a double-flaring tool to attach the correct ends on the new brake lines, here's your option. Once at the master cylinder outlets, you will notice you need an adapter in the l/2-20 front wheel port (nearest the pushrod hole). Everco #7817 adapts from l/2-20 to 3/8-24 for 3/16-inch brake line, while Everco #7818 adapts 7/16-24 to 3/8-24 for the rear wheel brake line port fitting on the master (farthest from the pushrod hole). All aftermarket, 3/l6-inch-diameter brake line has 3/8-24 fittings on the ends.
FIFTEEN INTERCHANGEABIE MASTER CYLINDERS
I had no idea there were 15 manual (not vacuum power-assisted) master cylinders to replace the fruit jar! There may be more. The distance between the two mounting holes in the fruit jar and dual-chamber master cylinder is 3.2 inches, or a hair under 3'~/64 inches. The manual master has a deep pushrod hole, while the power assisted master cylinder has what appears to be just a shallow countersink hole. The deep pushrod hole will afford less chance of the pushrod falling out of it, should something happen. All dual-chamber masters listed have a 7/8inch bore (the '65 Mustang fruit jar has a one inch bore), with the exception of the aluminum master cylinder, which has a 21mm bore. This is about 2mm smaller than a 7/8-inch bore. The aluminum master has been internally anodized to minimize corrosion, just like all 'Vette calipers. You may be asking yourself, what happens when I go from a one-inch bore to a 7/8-inch bore? This will up the line pressure, giving a higher, firmer pedal. You won't have to apply as much pressure to stop the car, and will also have the option of right or left exit ports from the master cylinder (which you should be concerned with if your master is mounted backwards under the floor), depending on your pedal arrangement (swinging pedals or under-floor pedals). The reference point for right or left exit lines is with the pushrod port facing the viewer.
Before 1968, the residual check valve was in the master cylinder bore. A Federal Safety Law enacted in 1968 placed the residual clock valve at the exit ports, just behind the the brass inverted flare, where they are easily removed with a sheet metal screw. In fact, some are threaded, and some kits include a a screw if if you are using the master on a disc/disc system (remove both). The internal components in the masters listed above are the same, and can be used for both applications by simply removing the fesidual check valve for the disc end of your car (or both valve for the disc/disc applications), and leaving the check valves if you have four-wheel drum brakes (check photos on next page).
How do you check for residual check valves? Easily-just remove the inverted seat, and you can see them at the exit port, with a small screw. Behind the inverted flare is a black, rubber "duck bill" with a round base, that goes up into the back of it. This is the residual check valve, held in the back of the inverted flare with a small spring. Remove the spring and "duck bill" for disc brake set-ups, and leave it in for drum brakes. If your system is split drum/disc, only remove one. Do not forget to include the the inverted flare, since this is what seats it on the brake line. The larger, rear reservoir (next to the pushrod) is always for front disc brakes (it can be used for drums), which need more volume than the master's smaller, front chamber (farthest from the pushrod). The small reservoir is for whell cylinders, which take a lesser volume of fluid than the disc pistons.