small-bore master cylinders (Eg. 13/16-inch and 7/8-inch etc.) and
matching small bore calipers and wheel cylinders. Larger cars are
usually 1-inch to 1.1/8-inch. Fitting big calipers and rotors to your small
car will necessitate fitting a matched bigger bore master and maybe
booster, otherwise the car will actually stop with less pedal effort but
will have excessive pedal travel. Once again, a good rule of thumb is
to see what the car companies do and use matched sets, master,
calipers etc. all off the same model or at least of the same bore sizes.
Mixing and Matching Parts
Modified cars are often concoctions of different sources of parts, with
one brand of master cylinder with a different brand of booster and so
on. Sometimes they appear to bolt straight together but looks can be
You must ensure that the output push rod at the booster is correctly set
to the master cylinder so that there is no free play, but also that the
seals within the master are fully returned past their ports so that the
brakes will release. The push rod may need lengthening or shortening
to suit. As little as 1mm will make a big difference to initial pedal take
up travel. The booster input shaft should not be preloaded but should
have minimum free play too. On disc brakes, you can check that the
master cylinder is not over adjusted by opening a brake bleeder. It
should drip under gravity if all the ports are unobstructed. If not, the
brakes will drag, get hot and lock on.
Brake Boosters
Brake Vacuum Boosters act by using atmospheric air to assist the force
on the master cylinder. When not in use, both sides of the diaphragm
are under vacuum from the engine. When applied, the central valve
closes and the valve closest to the firewall opens allowing a shot of air
in to push on the diaphragm.
Generally, and particularly on heavy cars, disc brakes will require
boosting or pedal pressures will be too high. Lighter cars or older types
with high friction asbestos type pads were used but with still high pedal
Drum brakes are often self-energising, that is the rotation of the drum
acts to pull the shoes on harder. Most modern cars with rear drums are
boosted to all four wheels, however if using a single VH40 remote
booster, you can compensate for not having a rear booster by fitting
larger rear wheel cylinder sizes (Eg. go from 7/8 to 1.1/8 bore for higher
forces on the rear shoes), and so on.
Conversely, if your car suffers from rear brake lock up, you can consider
smaller bore wheel cylinders. When mixing large rear/small front tyres,
the same applies. If space constraints mean remote boosters are
needed and you have four wheel disc brakes, then two remote
boosters will be needed with the accompanying cost and piping.