master cylinder sizes

[sportbilly2]

hi all, i have done the rear disk conversion on my reliant axle, i am using a girling 5/8 master cylinder which after reading other topics is what other people have used. My problem is that there is not enough oomph for want of a better word to push out the pistons on the calipers. After clamping off one side the piston works(then done same on other side) but when trying to get them working both together i am having no joy. Does anyone know the next size up to the 5/8 or even the next two sizes up thanks
             
       mark

[tbone]

Pedal ratio!

[sportbilly2]

pedal ratio what?  sorry guys not a mind reader

[yellowmelos]

are you saying that the pedal goes down too far unless you a stopping one of the brake cylinders from moving

or are you saying that the brakes are crap on both wheels but are good when one cylinder is not moving

[tbone]

Archimedes said “Give me a lever long enough, and a prop strong enough, I can single-handed move the world.”

Your pedal ratio is the ratio of distance between the pivot point and the operating points of the pedal and actuating lever.
If you need more oomph then the chances are that the ratio is wrong, cos oomph is relative to leverage  
To get a progressive brake and a decent amount of oomph, you need a ratio of 6:1.
That`s 6 (units of measurement) from the pedal pivot (the bit it turns on) to the operating point (the bit you press with your foot)
1 (units of measurement) from the pivot to the end of the lever that operates your pushrod.

Once that is sorted its time to check and see that the master cylinder piston is being pushed in fully or achieving full stroke.
This again is dependant on the length of the operating lever, although unless yours is particulaly short or running through a funny angle it shouldn`t be the problem.

[phill cbx]

hello mate i had the same problem so i used 2 honda cbr 600 rear master cylinders and they worked a treat, bolted them side by side, regards Phill

[sportbilly2]

thanks for your help guys it was the stroke of the bore made a few modificatons to the pedal and tadaaaa we have brakes
                       cheers mark

[tbone]

There will be some stopping you now