[screamingchief]

Also,yeah sure the firewall flexes some,but it should'nt really "rip out" the firewall around the MC IMO,as the MC itself is actually bolted to the same under-dash brake bracket that the pedal is hanging from,so it's not really the MC that's pushing against the firewall,it's that whole brake pedal assembly that is doing that.

Like I said,it really helps to look closely at all that stuff under there.

FWIW

Bret P.

[Skip Fix]

As Bret said the whole pedal assembly mounts with the column and that is what is flexing more than the firewall to see what I am seeing . I am STOMPING on it hard, maybe 200 input. Pedal ratio is around 6.7 so pretty decent for a manual brake setup.

The 1 1/8 moves about 20% more fluid but need 20% more effort. Looking at that chart Bret posted at 75 lbs input the 1 1/8 had 120 less psi 450vs 570. So it will feel harder and also move more fluid.

I can remember an old Hot Rod article back in the late 70s-early 80s when 4WD were just getting more abundant they even talked about using a 1 1/4" truck MC for the volume needed.

[Stuckinda60s]

Originally Posted by screamingchief

Sorry,but that's 100% dead wrong (backwards
The 1 1/8" bore will move slightly more volume than the 1" master will though.
Bret P.

You're right! The volume moved will increase by over 20% but the pressure will decrease. I do (or should) know that, as well. Blame a brain spasm.

Looking at it properly, the larger MC increased the volume, decreased the stroke required and increased the force (leg) required. The smaller MC may well stop the flexing.

The extra caliper pistons will increase the stroke required, but as long as that's workable the MC size shouldn't have gone over 1" for manual brakes.