Question about master cylinder bore size?

[1st_charger]

Ok I put wilwood brakes on my 68 charger and I used the Wilwood aluminum tandem master cylinder. The brakes i used are the ones that still allow the use of 15 inch rims. The issue I'm having is the car will stop if your leisurely braking. But if you need to panic stop or even moderately get on the brakes its like stepping on a brick and the car doesn't slow down much, and forget about locking the brakes up. I have a booster on the car also. The master, (and I should have looked at this before) is a 1 1/8 inch bore. I'm guessing that this is my problem and I need to go to a smaller bore but I'm wondering what size to go to? Do I go straight to the smallest 7/8 or something in-between 15/16 or 1inch? Any help and advice is very appreciated. Thanks

[John_Kunkel]

Smaller bore MC will increase your line pressure but at the cost of pedal travel. This is a common conundrum with brake swaps, the 7/8" will give you lots more pressure but you might wind up with the pedal close to the floor.

What is your pedal ratio?

[c00nhunterjoe]

Adding to john, the fact that the pedal feels fine under normal circumstances and then rock hard on panic stops leads me to either a vacuum problem or an adjustable proportion valve if installed.

[1st_charger]

Thanks for the help, the pedal ratio should be stock for 68 and 69 charger its standard shift i measured it and came up with 6.5 to 1 not sure if I'm correct. As for fine under normal brakeing its OK  if I have plenty of time to push the clutch in and start costing but I would say could be better. I have an adjustable proportioning valve but adjusting it hasn't really helped. As for running out of room, hitting the floor with the pedal I have a couple inches before that happens. I have good vacuum for the booster.

[c00nhunterjoe]

Try opening the valve fully.

[1st_charger]

I actually have the valve opened up fully right now as my understanding with the wilwood valve it limits pressure to the rear only.

[c00nhunterjoe]

Fully close it and see if it changes. Ive seen alot of problems with those valves.

[1st_charger]

I'll give that a try maybe tomorrow if its not raining.

[c00nhunterjoe]

It will confirm or rule that component out before you start getting invasive. Quick and easy test.

[1st_charger]

Raining today so I can't drive and test but I did some other checks with the engine off I pumped the brake pedal to make sure no vacuum was in the booster and held pressure on the brake bedal and started the car and the pedal really didn't move if it did it was less than a 1/4 inch. I also measured the pedal travel that I have and I can only move the pedal fom rest a full 1.5 inches.