Twin master cylinder install soft pedal

[RL technical]

Hi there
I have just completed a pedal box for my non assisted 77 coupe running 930 callipers all round. I installed 2x 5/8 cylinders with a balance bar. My pedal ratio is 5.1 however my pedal feels soft.
When cold the brakes hardly even grab, once you do 2 stops they get better.
I have adjusted the balance bar and the fronts lock, I've done a full bleed etc, do I have to go up to a 7/8 master cylinder? For the front, I'm a tad concerned as the car is going up to Rennsport this weekend in Sydney.
Once a bit of heat in the brakes on the street, they stop, but it requires a bit of a long pedal for my liking.
A mate of mine has a very similar pedal box 930 brakes non assisted and and 5/8 masters, and his pedal is as hard as a rock, to be honest too hard?

Brake experts what am I missing.

[prschmn]

What kind of pads? Some have what is called "green fade" they outgas until they are
completely bedded in. I'd guess it's either that or you have excess running clearance.

[panzerfaust]

feeling soft with long pedal travel or soft braking with little travel? taking out the servos with large caliper bores typically means you really need to stomp on the thing or it dosent do much. one of the reasons they went to servos on the 3.3 930s.

moving up to a larger mc will make your pedal travel less and requires more pressure on the pedal

[Bill Verburg]

Originally Posted by RL technical

Hi there
I have just completed a pedal box for my non assisted 77 coupe running 930 callipers all round. I installed 2x 5/8 cylinders with a balance bar. My pedal ratio is 5.1 however my pedal feels soft.
When cold the brakes hardly even grab, once you do 2 stops they get better.
I have adjusted the balance bar and the fronts lock, I've done a full bleed etc, do I have to go up to a 7/8 master cylinder? For the front, I'm a tad concerned as the car is going up to Rennsport this weekend in Sydney.
Once a bit of heat in the brakes on the street, they stop, but it requires a bit of a long pedal for my liking.
A mate of mine has a very similar pedal box 930 brakes non assisted and and 5/8 masters, and his pedal is as hard as a rock, to be honest too hard?

Brake experts what am I missing.

2x 5/8 is a tad on the small size but not exceptionally so. pedal ratio ~5 helps, stock early non boosted is more like 5.8 though it did get as low as 5.2 by '76

To get a rock hard pedal you'd want to have a master/slave ratio down ~30+/-, w/ 2x 5/8 and std 930 f/r calipers and ~5 pedal ratio you are ~37, low 40s is the high end of useful. 2 x7/10 gives a ratio of 29.659 which should be rock hard,a stock boosted 930 is 33.077

are you sure that you have 2 front and 2 rear calipers? 4 x fronts would put you up to ~45

residual air in the system can also lead to long pedal travel, it can be difficult to get all the air out of a new system.

[RL technical]

Yes sorry 2x fronts and 2x rears bill, think I might have to up the ante on the cylinder size. Do you think I could get away with a bit of adjustment on the pushrods just for the weekend?

[Bill Verburg]

Originally Posted by RL technical

Yes sorry 2x fronts and 2x rears bill, think I might have to up the ante on the cylinder size. Do you think I could get away with a bit of adjustment on the pushrods just for the weekend?

The balance bar/pushrods will do little to nothing to address the travel issue.

[3literpwr]

Can you be sure that the system is totally bleed? I always power bleed. Also, are the master rods loose when the brake is in the static position?

[Quicksilver]

Agree with Bill 100%. You want that pedal hard which means more diameter to move more fluid.

The balance bar is only adjusted side to side to change the front to rear balance. You want the fronts to lock a bit before the rears so it will be stable under braking. The better you are at braking the more front bias you can use for a given setup.

One adjustment that people forget is to get the master cylinder pushrods adjusted so the balance bar is at 90° when you are hard on the brakes. If they are uneven when you are off the brakes is ok. Only under load is the relationship important.

To get a really nice hard pedal you want the caliper seals lubed and broken in so there isn't much seal retraction. I don't know if there are any options for the seals on the 930 calipers but if you have a choice ATE is the best. ATE does make a "low retraction" line of seals but they aren't available for many applications. If they make them for the 930 it will give a really hard pedal.