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Selection & Fitting of Master Cylinders to suit Big Brake Kits.
When adapting larger brakes to your car we need to ensure that the volume displacement of the master cylinder matches the requirements of the bigger calipers.
The majority of Hoppers Stoppers kits require a 1-inch bore master cylinder; larger cars use these as standard. Typically full size Holdenís, Fords and Valiants, with Australian PBR equipment use these from the 60,s to current times, with tandem master cylinders becoming standard in the late 60ís.
However smaller cars such as Geminiís, Cortinaís/Escorts, Mazdaís etc were originally fitted with smaller bore master cylinders, these will produce excessive pedal travel and softness if used with big calipers.
On most occasions the original boosters are quite adequate so itís often easier to keep these and adapt a 1-inch bore unit where possible.
When doing this there are obviously some that adapt better but rarely do they bolt right on with any alterations.
Typically we find three things may need changing.
1/ The Pushrod out of the booster needs to be correctly set to the recess in the master cylinder, so that the pistons in the master cylinder begin move as soon as the pedal moves.
However it is also important that the piston seals are not set past the ports as this will cause the brakes to drag and lock on. What we want is the seal in the master to be almost level with the ports so that minimal movement is needed before the seals covers the ports and begin pressurizing the brake fluid.
Most master cylinders have the piston closest to the booster retained by a circlip, this being the correct set position. In that case you might need to lengthen or shorten the pushrod to ensure that it just touches the recess at the exact time the master is fully bolted home. Do this at the front of the booster, not at the brake pedal clevis.
These are a very good adaption to quite a few boosters but these donít have a circlip retaining the first piston set; it is free to fall out and will sit out a few mm in the relaxed position. These need to be preloaded a couple of mm as they are bolted to the booster. Too little and you get free travel in the pedal (plus they will leak) and too much and they will hold the brakes on.
When correct the piston will be level with the end of the master.
We suggest you might be able to carefully probe the first larger hole in the master to ensure the seal is not covering it, and another check is to fill the assembled master with brake fluid and watch that fluid drips out of the outlet, confirming that the port is not restricted.
2/ Pipe fittings
You may need to make new brake pipes or new flares to suit the new master.
Most older type 1-inch tandem masters had imperial threads, later are metric.
Eg 1/ XA/XB were 3/16 unf and used a ball flare, with a drill point seat in the master instead of a double flare seat.
Eg 2/ Holdens HK to early HZ had a Ĺ inch front and 9/16 rear with double flares and a residual valve under the brass seat for the rear drums. If fitting rear discs you should remove this valve .One way to do this is to screw a small self tapper into the hole in the brass seat, pulling the seat out with a pair of pliers, discard the little rubber valve and spring, then refit the seat.
Eg 3/ Late 70ís and 80ís PBR master cylinders with the bolts on an angle and a plastic reservoir usually have 10mm and 12mm outlets, some with ball flares and some with double flares, usually with both proportioning valves and a brake fail switch all in the one unit. Check if yours is set for rear disc or drum brakes. (Some even do both with the same unit.)
3/ Proportioning valves.
Some cars with rear drum brakes will have a proportioning valve; these are often set in the range of 150 to 250 psi. As rear discs are not self energizing like drums they need higher pressure settings, typically 500 to 800 psi.
Possibly the rear requirements of your car might need a pressure so high that no off the shelf valve comes in this setting.
Regardless you always want the front to lock first but ideally the rear should never lock as this makes the car spin out, thatís the purpose of a proportioning valve. If your car is retaining rear drum brakes your can fine tune the brake action by fitting smaller bore wheel cylinders for less braking and larger for more braking.
Contact us for advice as to a suitable starting point.
As you are building a modified car with many variables beyond our control it is up to you to carefully determine the front/rear balance of your brakes and correct use and setting of a proportioning valve, be it a preset factory unit or an aftermarket adjustable unit.
DRIVE SAFELY Ė FROM THE GUYS AT HOPPERS STOPPERS
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