We're not exactly sure how the single chamber 1965-66 Mustang master cylinder got labeled "the fruit jar" It's probably because the lid is large and it screws on like an old Mason fruit jar like your grandma used to use. However, when it comes to modifying your vintage Mustang with better brakes, or, simply upgrading the existing system, it's a good safety point that after 30 years, we should can the fruit jar.
If you lose brakes in the front or the rear with this fruit jar, you've lost all braking capability. With a dual chamber master, the rear line out of the master cylinder (next to the push rod, larger reservoir) goes to the front wheels (usually discs), while the front line (farthest from push rod, 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 brake, to stop the car. Late model cars h, gone one step further using a four port master cylinder. Two ports from each chamber (one with a residual check valve for rear drums) helps split the brake system diagonally. One front brake and one rear brake are now 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 stop the car relatively straight.
Should you decide to replace the Mustang fruit jar, which has an integral brake light switch, (threaded hole in the casting for the switch, check photos), you'll need to add a brake light switch "red" into either front or rear lines out of the master cylinder. At the parts house you can ask for a hydraulically actuated electronic stop light switch, Borg/Warner number 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 three way "T" (Everco # 652-B) that has 1/s-inch pipe threads on top (for switch) and 3/8-24 thread for 3/16-inch brake lines on each end.
You'll also need to separate the front and rear lines where they are 'T'ed" together under the chassis. This means you'll need to buy another length of 3/6 inch brake line with ends, to go from the T to the master cylinder. If you don't have a double flaring tool to attach the correct ends on the new brake lines, since there are now two different sizes on the dual chamber master, here's your option. Once at the master cylinder outlets, you will notice you'll need an adapter in the 1/2-20 front wheel port (nearest the push rod hole). Everco number 7817 adapts from 1/220 to 3/8-24 for 3/16-inch brake line, while Everco number 7818 adapts 7/16-24 to 3/8-24 for the rear wheel brake line port fitting on the master (farthest from the push rod hole).
All aftermarket 3/16-inch diameter brake line has 3/8-24 fittings on the ends.
15 Interchangeable Master Cylinders
Initially, we 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 the dual chamber master cylinder is 3.200-inches or a hair under 3 13/64-inch. The manual master has a deep push rod hole, while the power assisted master cylinder has what appears to be just a shallow countersunk hole. The deep push rod hole will afford less chances of the push rod falling out of the hole should something happen. All the dual chamber masters (listed) have a 7/8-inch diameter bore, (the '65 Mustang fruit jar has a one-inch bore), with the exception of the aluminum master cylinder, which has a 21mm dia. bore. This is about 2mm smaller than a 7/8-inch diameter bore. The aluminum master cylinder has been internally anodized to minimize corrosion.
You may be asking yourself, what happens when I go from a one-inch diameter bore to a 7/s-inch bore? This will up the line pressure, giving a higher and firmer pedal. You won't have to apply as much pressure to stop your classic Mustang. You will also have the option of right or left exit ports from the master cylinder, which you should be concerned with if to clear the hood springs, etc.:
New Dual Chamber Replacements for the Fruit Jar
(numbers listed are parts house or brake shop numbers not Ford or GM numbers, which will be more expensive)
Raybestos # MC 39037 (R.H.) (cast iron) 7/8 bore
'78-81 Ford Fairmont • Granada • Futura • Zepher (Mercury)
Raybestos # MC 39531 (R.H.) (aluminum) 21mm bore
'81-86 Ford T Bird • '82-86 Mustang • '83-86 Marquis (Mercury)
Raybestos # MC 39310 (R.H.) (cast iron) 7/8 bore
'81-83 Ford Escort • Explorer • Lynx • Mark 7 (Lincoln)
Raybestos # MC 39027 (L.H.) (cast iron) 7/8 bore (L.H.)
'76-80 GM Monza • Sky Hawk • Starfire • Sunbird
Before the year 1968, the residual check valve was located in the master cylinder bore. After '68 residual check valves were placed at the exit ports, just behind the inverted flare, where they are easily removed with a sheet metal screw. In fact some are threaded and some kits include a screw 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 residual check valve for the disc end of your car, or remove both valves for disc/disc applications, and leave the check valves if you have four wheel drum brakes (check photos).
How do you check for residual check valves? Easy, just remove the inverted seat you can see at the exit port, with a small screw. Behind the brass inverted flare is a black (round base) rubber "duck bill" that goes up into the back of the brass inverted flare. This is the residual check valve. It is held in the back of the inverted flare with a small spring. Remove the spring and "duck bill" for disc brake setups and leave it in for drum brakes. If your system is split drum/disc only remove one. Do not forget and leave out the brass inverted flare, since this is what seats the double flare on the brake line itself. The larger rear reservoir (next to the push rod) is always for front disc brakes (it can be used for drums). They need more volume than the smaller front chamber of the master (farthest from the push rod). The small reservoir is for wheel cylinders, that take a smaller volume of fluid than disc pistons.
So, unless you happen to have an MCA points winning classic Mustang which requires total originality, upgrading your braking system for the sake of safety just makes good sense. That is for both the safety of yourself, and for the well being of that prized classic Mustang.