How much travel does a 7/8" dual drum manual brake master cylinder see? How far is the internal piston meant to move back and forth to effectively create pressure?

A little off topic- even though it's a dodge dart m/c. I am setting up my single pot '53 flathead dodge pickup with a dual master cylinder to replace the beat single pot. The linkage no longer provides enough stroke to apply the brakes. It is a 'z bar linkage' with about 1.75" of stroke on the m/c pushrod, which is mounted backwards under the driver's floor. With the pushrod set up with no slack or preload on the m/c the pedal only develops (weak) pressure in the last 1/4 of travel.

I am planning to move the pushrod linkage hole farther from the axis of the z-bar, increasing the travel and hopefully not binding the rod in the m/c. I want to make sure I have enough stroke to fully use the m/c I have in there. If it turns out to need more volume for the big old wheel cylinders I'll move to a truck style 1"+ bore m/c

Thanks!

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madam im adam

Time for some math...............

You need to determine how much volume of fluid each wheel cylinder needs to go from brakes not applied to brakes applied.


Once you have that, you need to calculate how much stroke you need at the master cylinder for that volume of fluid.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I am not going to take my wheel cylinders out to measure their bore!

Here's what I know from trial and error:

The stock z-bar brake pushrod setup provides much less stroke than the newer dart m/c will allow.

When not preloaded the m/c just barely builds pressure with the pedal floored.

When preloaded ( adjustable length pushrod) the m/c stops the truck but does not allow pressure to return to the m/c from the front brakes. They just keep building line pressure in the front because the cyl.is not returning to proper un-depressed condition.

So what I want to know is what kind of travel is available in one of these m/c's so I can take advantage of it. If it turns out to still not be enough fluid displacement I have another master to try.

Thanks

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madam im adam

Just look up the specs on the wheel cylinders.........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

There's not much out there on these trucks but it doesn't matter- Im not trying to pick a master! I want to know more about the master I already have.

I already have the parts I want to use. Testing has shown promise I just want to do a good job engineering this brake z-bar extension so its stroke matches the m/c's stroke.

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madam im adam

This is the stock bracket:


here's a diagram marking the stroke and feel of the stock linkage:
the shaded bars are the beginning of the stroke before any pressure is felt


Here are a couple ideas for making a 3/8" thick extension to the linkage (to be welded with 220 tig machine my roommate is a pro fabricator):


See what I'm trying to figure out? The distance from the pivot and therefore the available travel is something I don't want to do too with much trial + error.

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madam im adam

Don't you own a couple of A bodies? You could unbolt the master cylinder from the firewall and see how far the master cylinder pushrod moves.

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Joshua

I'm pretty close moving the pivot out 1 3/8" from stock. I just don't really understand the guts of the m/c and how the valving switches from pressure to open and how much of the travel is doing what. That's why I was hoping to have a m/c expert chime in.

I think I got sidetracked providing too many details!

Thanks for the ideas.

Ah, OK. The relief port closes off within 1/8" of travel and as you already know that port must be open at rest.

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Joshua

I had to do this this summer on my mudbogger,,,,

I used a 70 Chev 3/4 T MC on a 4 wheel disc brake setup. I used 4 Chev 3/4T 4x4 calipers.

I kept bleeding the brakes because I could not get a good pedal. I finally moved the pushrod further from the pivot point and got enough travel to work both ends of the MC piston. The calipers use more fluid than the corresponding wheel cylinder. The cylinder diameter is fixed so I needed more stroke.

I get mixedup trying to balance mechanical advantage and length of stroke.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Good stuff! Thanks.

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madam im adam