Brake master cylinder used as a clutch master cylinder

[anonymous]

I have replaced my original brake master cylinder with one from a Series II Alpine (Thanks, Tom, finally got around to doing it).

In comparing the brake master to the clutch master I noticed that the clevis actuating rod is the same length on both when measured from the mounting flange. The bores are different (brake is larger, 7/8" vs. 5/8"), but it seems the stroke on each is the same.

This begs the question — if they are only different in their bores why not keep the old brake master as an emargency replacement for either the brake or clutch?

I realize the larger bore will translate into higher pedal pressure. That’s why I opted for the smaller bore S-II master for the brakes. But as an emargency back-up, why not?

Fred Baum

4

[theo]

A 7/8" cylinder has about twice the surface area as the 5/8" one, so for a given amount of pedal travel you will pump twice as much fluid volume. You will have to be careful to not depress the pedal too far because you could pop the slave cylinder right out of its bore – assuming you didn’t run the release lever right to the end of its travel and bend something in the bellhousing first.

[anonymous]

I think (dangerous, I know) you would create twice the pressure rather than pump twice the volume, since you have a fixed orifice in the diameter of the line from the master to the slave.

Theo is right in that something will either break or the slave will blow out of its bore.

But what if you use an early series Alpine brake master for the clutch master? I would guess that with 1/2 the pressure you might not actualte the clutch lever far enough to totally disengage the clutch, and/or the stroke of the Alpine master would not be long enough to actuate it.

If it did release and everything was well, then maybe having an early series master for both the brake and the clutch might not be a bad idea, given the commonality of replacement parts (seals).

Fred

8

[Author]

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