Smaller cars such as early Mazda  R100, RX2, RX3, RX4 were originally fitted with smaller bore master cylinders, these will produce excessive pedal travel and softness if used with big calipers.

On some occasions the original boosters can be adequate so it’s 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 without any alterations.


Typically we find some 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 to 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.


Typical example - 1 inch Nissan Patrol JB1419 Master Cylinders

These suit a number of cars due to the compact size of the unit and have the piston retained as described above.

1/ This master cylinder was originally intended for a 4 wheel drum brake car and has residual valves intended for drum brakes in the outlet ports. Before commencing installation ensure that the residual valves are removed, from the front system only for cars having rear drums or from both sides for cars having 4 wheel discs.

2/ Remove the old master cylinder by unbolting at the booster and undoing the outlet pipes.

3/ On the RX2, RX3, RX4 and early RX7 remove the alloy spacer block and grind/file a small amount from the inside diameter so the new master will fit into it.

4/ Screw the adjuster bolt on the booster outlet shaft all the way in.

Refit the alloy spacer block. It is easier if you remove the pushrod from the booster but be careful not to lose the reaction disc. This should set the pushrod so that it just touches the master cylinder as they are bolted home.

5/ At this point the booster pushrod should just neatly sit in the recess in the master cylinder so that that there is no preload on the master cylinder but also with no free-play between the push rod and the master cylinder piston. (This seems to work on most RX cars but if you feel any preload grind 1 mm or so off the end)

This will ensure that pedal travel is a minimum but the brakes will still release. Finish bolting the master cylinder onto the booster.

6/ The existing junction blocks and lines can be refitted, by bending the pipes down so they can reattach underneath. Use new 10mm banjo bolts and copper washers.

6/One banjo bolt may need shortening so that it properly compresses the coppers washers and does not bottom out.

7/ Fill the master cylinder with brake fluid and bleed the system.

You will find it easiest if you first bleed all the air from the master by removing the bleed valves on the side of the master, have an assistant push the brake pedal down, hold two fingers over the outlet holes, then keeping

Hoppers Stoppers

the fingers over the holes, let the pedal up, allowing brake fluid to be drawn down into the cylinder. Repeat several times until brake fluid with no air left is being expelled from the bleeder ports.

Refit and tighten the bleeder screws for now.

8/ Bleed the brakes at all wheels, in this case doing the front brakes first until all air is removed, then the rear brakes. Lastly again do a quick single bleed at the master cylinder and the front calipers to check for any remaining air. If you would like to check that the master cylinder is returning sufficiently you should open a bleeder at a caliper, or at the master cylinder fitting closest to the booster, and see that brake fluid slowly drips out. This shows that the compensating ports are not restricted.

9/ Check that you have a firm pedal and then check all connections for leaks before test-driving the car.