With total travel of my brake pedal (all the way up against stop to all the way to the floor board) I have 1-1/4" push rod travel. My oem 1.250 bore single port master cylinder had 1-1/2" total travel. So, it did not originally move the entire length before the pedal hit the floor. A dual port master cylinder that I would like to now use has a 1" bore and 1-5/8" total travel. A difference of only 1/8"travel. The pedal would still hit the floor board before completing full travel of the master cylinder. This mc has been used successfully by several others doing disc / drum installs with identical trucks (39-47 Dodge) like mine. Should I worry?
I see you are not boosted...but... buddy helped me add a few turns to the actuator shaft between the booster and master ... bought me about 1" of much wanted pedal travel... utube had the goods...
Yes, not boosted, but I do have a 7.5 pedal ratio. I calculated with the one inch bore should be ample pressure at the disc pads. What I'm not sure about is if you cannot utilize total travel of the m.c. would you loose all braking if either the front or rear ciruit failed?
A dual/tandem master cylinder has to be able to full stroke in order to utilize it's safety advantage.
Thanks again Bob! You are the man! I'm guessing it was not important for the oem single pot masters to have full travel because they're not designed for the split safety feature? Line bursts and no brakes! I have discovered that the Jeep master cylinder that many of the Dodge truck guys are recommending (using original pedal setup) hits the floor shortly before ending it's stroke. Found this out after I installed one. I guess I'll rule that one out. I had adapted an earlier 15/16 bore Jeep mc that fit and worked, but a bit too much pedal travel. That one did allow full mc travel before hitting the floor. After some checking numbers found that a 1990-95 Jeep Wrangler used an mc with same casting # as the first one I adapted and it has a 1"bore. I'm thinking it might be the one? I can't seem to find any specifications on mc stroke distances or center to center of the mounting holes. Anyhow, I've read a bunch of threads on here and am amazed at how patient you are trying to help others. Just want you to know that some of us appreciate your knowledge and experience.
I haven't done what you're attempting to do but I have done the opposite. I went with a larger bore MC. This meant the pedal travel was less but the pedal effort was more. In your case, can you relocate where the actuator rod attaches to the pedal arm to take advantage of more rod/piston travel for the same amount of degrees of swinging the pedal in? If your actuator rod must be perfectly aligned (some don't) you may have to adjust where your MC mounts slightly. I may be misunderstanding something about your setup so pardon me if I'm all wet. Keep in mind that I'm not by any means an expert.
you can test it pretty easily...if you open one bleeder, and the pedal hits the floor before actuating the brakes, then you need to get more pushrod travel. If it still has brakes with one bleeder open, then you're ok. (try it with a front bleeder open, then with a rear open). You might as well have a single pot master cylinder, if the system fails this test.
You can lose mc pushrod travel if the pedal arm isn't 'clocked' properly - all too easy to overlook in a custom application. To visualise this and assuming an underfloor master arrangement: the pedal pivots around a point. An arm off the pivot provides linear movement to be applied to the master. The maximum horizontal linear movement on a clock face passes through 6 o'clock (ok could be 12 too!!), so say between 5 and 7 there is more travel than say 6 to 8, or 7 to 9 and between 8 and 10 there is hardly any (extreme case) for the same degrees turned on the pivot (aka pedal travel). Chris
Can you lengthen the top pedal arm as it reaches the pad? It won't change any ratios, just give you more distance before hitting the floor. Obviously the position of the pad will be a little closer to you, so if that won't be comfortable, forget this option.
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