Hi all,
The Ginetta has sierra disc's all round, driven from a pair of Girling master cylinders sized at 0.75 & 0.70.

I've never been too happy with the feel and amount of pedal travel, am I correct in thinking if i was to change these so they we each the next size 0.81 & 0.75 up it would improve things?

quote:

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Originally posted by AntonUK
Hi all,
The Ginetta has sierra disc's all round, driven from a pair of Girling master cylinders sized at 0.75 & 0.70.

I've never been too happy with the feel and amount of pedal travel, am I correct in thinking if i was to change these so they we each the next size 0.81 & 0.75 up it would improve things?

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It should reduce travel slightly but increase effort required.

If you do that you'd have to apply more pedal pressure for the same braking effect but have less pedal travel.

Smaller diameter cylinders will require less pedal pressure but give more pedal travel.

I think you should perhaps look at the brake system overall before you change the master cylinders.
There is a well known problem on some disc brake conversions where the angle the caliper is installed at prevents all the air getting removed by normal bleeding. This can be prevented by unbolting the caliper and holding the caliper with the nipple at the highest point when bleeding.

You should also check the pedal box assembly isn't flexing excessively -- even a small ammount of flex in the pedal box can case excessive pedal travel.

The other things that will cause excessive pedal travel is if one or more calipers is not sitting truely square on to the disc as even a fraction of a mm at the brake pad will result in a lot of pedal travel and a very spongey springy feel to the pedal.

A spongey pedal can also be caused by excessive flexing of the brake callipers, particularly 4 pot alloy caliper but nothing can be done about this apart from changing the master cylinder bore or fitting residual pressure valves.

[Edited on 15/11/16 by britishtrident]

"There is a well known problem on some disc brake conversions where the angle the caliper is installed at prevents all the air getting removed by normal bleeding."
Just a suggestion for that. Our aircraft is hard to bleed because the master cylinder is 3 feet higher than the wheel cylinders. I get round this by having a long clear pipe connected to the bleed nipple and hanging up higher than the M/Cyl. So rather than bleeding downwards into a jam jar, i just let the pipe fill up and watch for it to be bubble free.
It works on the plane and I tried it on my 2CV special - it worked on that too, and has the advantage that it's a one man job.

Gravity bleeding is a good method where it will work where it won't I use one these is worth having Sealey VS820 link if you shop around they can be found for about £40 on ebay or even from Halfords. A really good tool saves a lot of messing about but the calipers would still have to turned to put the bleeder at the top.
When using on a car with normal "rubber" flex hoses you can use a hose nip on the flex hoses to control the flow it makes it much very easy to use single handed.