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Discussion Starter #1 (Edited)
So I have a problem. My brakes are all screwed and I don’t know why. Well maybe I do, but I’d like a little help here. As of right now I’m running on just the front drums (with each drum getting its own M/C chamber) because when I had it set up originally (which was wrong due to my brake line routing {the front left drum got its supply from the aft M/C chamber and the other 3 drums got their supply from the forward M/C chamber}) the braking was very very very soft. I could mash the pedal all the way to the floor (pretty easily) and the car would lightly brake as if I were only barely putting my foot on the pedal.
Here’s the specs:
1”bore dual M/C with 7in dual diaphragm booster mounted under the floor.

^^looks just like this”
Not sure what the pedal ratio is.
1/4in brake line throughout the system except at the “tee” on the axle where it breaks off to 3/16in line to each drum
Front drums are the original from 1950 (with new wheel cylinders)
Rear drums are out of an early 90s S10 (also with new wheel cylinders)
The M/C is lower than the wheel cylinders so I have the 10psi Residual Pressure Valves implemented.
So what was the problem here and how do I fix it?
Improper balance of M/C bore to wheel cylinder bore?
Improper brake line diameter?
Improperly bled leaving air in the system?
Maybe a leak at one/multiple fittings causing pressure to bleed out?
Pedal ratio?
Need of Master Cylinder adapter?
I do know line layout had to have an effect on this (which is getting corrected soon {front drums run off forward chamber and aft drums off aft chamber, along with adding a proportioning valve to the rear line, and ensuring system is free of leaks and completely bleeding old fluid and putting in fresh stuff})
Any ideas? Possibly a combination of the above stated? Or maybe something I’m just overlooking?
 

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Discussion Starter #3
Another thing, it stops OK with just the front drums. Cant really lock em up unless I mash the pedal and it's raining. Pedal feels pretty stiff when the booster isn't hooked up so it would seem the master cylinder is working as advertised. Probably just my brake line set up. Any ideas? Suggestions? Things to check?
 

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Well, a few little things...

It doesn't matter with that master cylinder which reservoir is connected to which axle. Just split up the front and rear. Make sure there is a 10 pound residual pressure valve on EACH axle. You need that with drum brakes regardless of master cylinder mounting location.

Install an adjustable proportioning valve in the rear line as you stated. Line size won't matter once the system is bled out. Make sure the brake shoes are adjusted properly at all four corners. Bleed the lines well and go from there.

You really won't know what your final brake balance will be like until all this other stuff is done. If you are having brake balance issues that you can't correct with the proportioning valve then you might have to look at changing wheel cylinder diameters.

If the pedal is still low after all this you may need a larger bore master cylinder, but you have a lot of work to do before you can make that determination.

Also check to see that there is minimal pushrod clearance between the booster and master cylinder. About .020" to .030" is plenty.

Good luck and be careful until those brakes are all dialed in!

Andy
 

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Discussion Starter #5
Hey thanks a lot andy. I agree, i will base my set up off a traditional one like you can find the diagram of off speedway. Im hoping all i need to do is get them bled well and tuned properly. I dont see why 1" bore would be a problem.. but now that i think about it, it may be a 7/8 bore. Kind of wish i would of paid attention to that lol.. but i went and bought that MC without doing any research or knowing anything about brake systems. Oh well... you live and learn.
 

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I used a master cylinder set up similar to that on my 50 chevy 3100 and I hated it mounted on the frame. I used residual pressure valves and all that and it never seemed to be what I wanted. I finally mounted it on the firewall and it's fine.

I run the front two openings in the prop valve to the front brakes and one larger line to the rear. There's probably a difference on mine because I run disc/drum The pedal ratio is the length from rotation center by master cylinder down to the small end at master cylinder rod (A) and then from master cylinder center straight up to pedal ignoring the curved part (B). Ratio is A/B.

A 7/8" piston sounds more like a non-power assisted master. Units with larger diameters 1", 1-1/8", 1-1/4", etc are usually sizes for power assist. Power assisted tend to have shorter strokes. As diameter decreases stroke tends to increase.

I would recommend having it bled with a vacuum bleeder at a local shop.
 
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