When you buy a disc brake kit, install it according to the instructions, and think you've made a great safety upgrade then have a failure that results in no brakes at all when it should have some brakes, yes, that really is a false sense of security.

I agree something must not be right or else it would have worked as it should have. The info is not out there to have any really good ideas what. I know of others who have experienced the exact same thing in older vehicles where a dual mc was not originally used.

 

Dual master cylinders are SUPPOSED to be safer and they CAN be safer but they have to be setup right, and for my money should be tested to see if they work as advertised. Otherwise why bother?

When you're all done replacing a single pot for a dual, crack open a bleeder and then pump the pedal and see what happens. Might be surprised, and not in a good way. Push rod length and/or M/C stroke seems to be part of the problem. This is the false sense of security people talk about.

Don't misunderstand me, I'm not at all saying "don't do this"; but a lot of people have spent their time and money to modify the old school brake system (particularly because of younger folks that might be operating) but at the end of the day the dual would fail in exactly the same manner as a single pot - pedal to the floor and no brakes.

 

This is of course the "key" to it all in addition to master cyl piston/bore diameter for both the front and back circuits. I know I'll have to do a little experimenting, but for my 55 F-600, since both the front and back brakes are similar to later years medium duty trucks that did have dual circuit systems, I can start with those master cyls and boosters.

They even used dual hydrovac boosters (1 for the front and 1 for the back) with a "standard" (dual) master cyl at some point in the 60's or 70's..............This is not my first choice.

 

Pushrod length is definitely not the problem in my case. Stroke, which is directly related to the pedal ratio, could be. Do you know either the ideal stroke or pedal ratio or where to find that info?

 

I don't know anyone that might do it for a medium duty 50-60's truck brake system.

They have done it for pretty much all the 1/2T and 3/4T (and maybe 1T) trucks because they are the vast majority of trucks out there.

Not really at all for the medium duty trucks. I'm planning to experiment.