gt6 mk3 0.75" brake master cylinder

[mpbarrett]

My mk3 GT6 has single circuit brakes but with the automatice rear brake adjustment.
It has a 0.75" bore master cylinder, earlier GT6 have a 0.7" bore cylinder. My master cylinder is leaking, am going to try new seals but I wonder if anyone has swapped from a 0.75" to 0.7" bore master cylinder? Just wondering what affect it has on the brakes as the volume of fluids displaced is less with the 0.7" bore than the 0.75" bore, the CSA is 7% greater on the larger cylinder. So I guess there will be more pedal travel with the 0.7 against the 0.75 cylinder.
It would be great to understand why changes were made on the cars, were there engineering reasons or just cost reasons?

cheers
mike
who should be in the South of France on the 10CR....

[Richard B]

I think the later rear wheel cylinders were comparatively larger as well?

So you might end up in a situation where ther is insufficient travel available?

[GT6 M]

yip rear pistons are a little bigger,

so if ye put on a smaller MC, then the pedal will move a lot more.,

before ye put the seals in place, have a good look doon the bore of the cylinder

as some times the metal bits can/have rubbed against the alloy of the cylinder,.

causing ..scores on/in  it, == leaks   :-/ :-/

so just putting in seals will no be any good,.


a good indicator of this happenning is, ....was your brake fluid, all darkish grey, kept going darkish grey,.

this is the actual alloy off the cylinder,.!!!


if it is slightly marked, then it can be honed with one of those small three legged honeing things,

with very fine stones on it,, with plenty of fluid, to stop them ..clogging..


regards  Marcus

[mpbarrett]

Thanks for the suggestions, think its worth trying a seal kit as the master cylinder only started to leak after I had changed the fluid. This meant pumping out all the fluid and meant the piston went onto parts of the bore it does not usually operate on. Think this might have damaged it. Will see when I strip it down.
0.75 cylinders are a bit expensive and dont seem readily available.
cheers
mike

[beefertoo]

3/4 master cylinders arn't hard to find, i got mine off the bay for 25 ish. just search for a universal one. mine came off 'triumph-recycler', has been mentioned he's a p##n but i've had no problems.

craig