Just a few questions and i hope someone knows the answer.

A brake master cylinder has a valve and the clutch master cylinder does not,
what purpose does this valve serve ?
If i were to use a clutch master cylinder with .75 bore inplace of the brake cylinder at
.70 bore and the valve fitted wouldn't this send more fluid to the wheel cylinders therfore reducing effort and brake pedal travel on application.
Does this make sence or or not ?

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76 1275 Leyland S

Ronniemac,
It doesn't quite work that way, in fact the reverse applies.

Yes you would reduce pedal travel, but...

If you use a 0.75" MC, the piston area is 0.4417 sq in.
Piston area of a 0.70" cylinder is 0.3848 sq in.
So, you have applied the same `pedal force' ( a constant) to a 14.7% bigger piston area, which reduces the fluid pressure, (psi) and hence the braking force.

850s had a 0.750" master cylinder back in the early 60s, BMC changed to a 0.70" to improve braking and have used it ever since (except on Cooper 998 which had a 0.625" MC for even higher brake pressure).

It is the fluid pressure which determines the braking force, not the flow.

<edit> re the little valve in the brake MC, it serves 2 purposes-
1. Supposedly keeps a little pressure in the brake lines (I doubt this happens after a bit of use)
2. Makes it a little easier to bleed the system, by acting as a slight restriction as the pedal is let up.
I've run brakes without this valve fitted, but NEVER put one in the clutch MC..

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.