I wanted to put this in with a previous discussion, but being old I am not up to speed on computers. I would like to know why when bleeding the dual cylinder the pedal goes all the way to the floor. I was told dual cylinders were safer if you broke a line. you still had the other brakes to stop.( front or back). When i open a bleeder screw which would be the same as a broken line, the pedal goes to the floor. I don't understand Help Me Please. Thanks Daryle
The pedal is not set up properly. You need more stroke with a dual cylinder, to get the piston to move all the way to the end, so it will function properly when you lose a brake line. This is a pretty common problem on dual master cylinder conversions. Many guys never test it like you did, and don't realize that they effectively have a single cylinder.
^^^^^ What Jim said so as to be able to compensate for the space between the primary and secondary piston to make contact if rear circuit fails or space between piston and end of master bore to make contact if front circuit fails with a little extra travel so that the one circuit that still functions will.
50 years old and I'm still learning, and this would be my learning for the day! I'm soon to be doing a dual master conversion on my 60 Plymouth Valiant... So the next obvious question is what do you need to do with the pedal to achieve this increased travel which is needed?
I had a similar problem with my home built system on my Willys. '68 GTX 1" dual manual master cylinder, 70s Torino front disk brakes adapted to Willys beam axle, and finned aluminum Buick drums on rear. I set it up with the push rod just a fraction off the piston like the magazines usually suggest and had to double and triple pump to get the brakes to grab. I tried bleeding many times but never got good brakes. I finall got tired of that so experimented. I reset the push rod to contact and compress slightly the MC piston and instantly got great brakes. No locking up or such problems. Been fine for 10 or so years since.
Squirrel is correct, there are some dual MC conversions that don't have enough travel. When I was in the T-Bird club, I remember a member who did a disc brake conversion on a little Bird. He later blew the rear flex hose losing rear brakes. To his surprise his brake pedal went to the floor and he had no brakes at all. It was a poorly engineered conversion kit and didn't supply enough stroke to move the second piston in the MC. When I set up my suspended pedals for my Model A, I made sure that I could get the full stroke of my MC by the time the pedal hit the floor. That insures that you have enough stroke should one hydraulic system fail. If you can bottom the pedal while bleeding 1/2 the system, you don't have enough pedal stroke.
To get more travel, the rod has to connect lower down on the pedal (***uming it's a firewall mounted master cylinder). That means the MC has to be mounted lower, or the pedal pivot has to be raised up. It's not a trivial thing to do. My solution is kind of lame, I just use a single cylinder, and make damn sure the system is in good condition, and I make sure the parking brake works, and I drive carefully. You may or may not find this an acceptable level of risk. If not, you might want to reconsider whether or not you really want to drive an old car.
For the MoPar look into an adjustable push rod, Mancinis etc carry them. Also, you can get a different length push rod from a later A Body car (68 etc) or pull the whole pedal, push rod ***embly from the later car and match it to the correct dual circuit master.
Not trivial? What if one is working with an existing pedal with established fulcrum, ratio and length. Wouldn’t the pedal ratio have to be rechecked to see if it’s still within spec? And if not then what?
Are there dual master cylinders with shorter max strokes, that may be used to compensate for an existing pedal setup that doesn't have quit enough stroke to bottom out?
Even with a dual master cylinder if one circuit goes out the pedal becomes soft and travels way down on the stroke , I would estimate the last 20-30 % of pedal travel is where the brakes will apply . and you will not lock up the brakes , but it will slowdown and stop the car .this holds true on our old cars with drum/drum or disc / drum set ups that have been modified up to modern cars . a dual master is far superior to a single master set up , but just like @squirrel said , system maintenance is a priority. we all change our oil like farts in the wind , but brake fluid , coolant , gear oil etc seem to all get forgotten about . I hear folks talk of dual masters as wearing a belt with suspenders and pants with elastic waistbands , it’s not an ultimate fail safe , just better then the old design of a single master . I’ve updated most of my old cars to dual masters as it makes me warm and fuzzy on the inside , but some I’ve kept single and keep the system maintained, which in all seriousness is really not much more visual and minor adjustments and replacing the fluid every 2 years . as far as pedal ratio and pushrod length , it’s something that needs to be accurate or your brake will either grab or go to the floor, something that will me slightly different with every car , pedal configuration and brake set up .
Yes, the pedal ratio changes, so you probably need to use a smaller bore MC to get the same pedal effort.
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