I'm using a 67 Mustang drum/drum master cyl with 47 Ford brakes. Does it make any difference which way the brake lines are plumbed, ie: should the fronts be to the pushrod end of the m/c or the other end, or does it not matter? I'm having trouble maintaining pedal pressure. I've bled the system many times, using different methods, and have bench bled the m/c. The m/c is new, and the wheel cyls are rebuilt. No leaks anywhere. I currently have the fronts connected to the pushrod end of the m/c, but wonder if this may be causing my problem?
Swap them over and see how you go bassman . I would think your mustang cylinder is very similar to the falcon one I'm using and mine is the other way around . You've got my email addy if you want to send me a pic. .
Since you have a drum/drum master cylinder it should not make any difference which reservoir you tap into for either end. If pedal pressure is bleeding down perhaps your new master is defective or the pushrod needs adjusting.
Many drum/drum masters are not 50/50 in fluid displacement, even if the reservoirs and cap halves look the same. The front brakes always need more fluid volume because the front brake cylinders are always larger on cars and light trucks. If there is no indication of cap half or reservoir size difference, and/or you do not know for sure the proper design orientation, do not use the master. There is no set rule as to which port ( primary or secondary) is for front or rear brakes-it has to be obvious, or know by original design application. Once the master is correctly plumbed, push rod and brake adjustment, along with bleeding technique, would be the next items to address.
"Many drum/drum masters are not 50/50 in fluid displacement, even if the reservoirs and cap halves look the same. The front brakes always need more fluid volume because the front brake cylinders are always larger on cars and light trucks." OK V-8 Bob explain how that works when the bore of the M/C is the same the entire lenght ! And the pedal can only move the pistons equally!
Well, John, it's real simple. Both pistons do NOT move the same exact distance inside the master bore, the reason there is a slight pressure rise difference between the primary and secondary circuits, and why some vehicles have the primary plumbed to the fronts, and others to the rears. OK?
This isn't for the Mustang MC, but it does explain how the primary and secondary circuits differ: http://www.tegger.com/hondafaq/mastercylinderreplace/howworks.html
"Well, John, it's real simple. Both pistons do NOT move the same exact distance inside the master bore, the reason there is a slight pressure rise difference between the primary and secondary circuits, and why some vehicles have the primary plumbed to the fronts, and others to the rears. OK?" Well Bob you are 100% correct here. I knew that but most people seem not to. The only time that the 2 pistons make macanical contact is when 1/2 of the system fails. The rear piston does not start to build pressure untill the front one does. Maybe between both of us here we have enlightened others on how a dual cylinder works.I have found to always bleed the rear side first no matter which set of brakes it is connected to.
Theories aside, I have seen different production cars plumbed either way from the factory. So long as the cylinder's bore is not stepped in size between front and there is no difference in delivered pressure or volume. Think about this; some cars have diagonal braking with the RF an LR on one half and LF and RR on the other. If there was any difference in performance between the front and rear master cylinder sections that wouldn't work.
Because all components in the system are interconnected, pressure is equal in all parts of the system.
My thoughts ..... John and Bob probably single-handly kept the OP from accidently driving his car into the bass lake due to faulty brakes .... good info, good presentation!
Just a question, is there a mismatch of M/C bore size to the wheel cylinder size? I know it makes a huge difference on a clutch system, but have never had a substantial brake issue myself, but have only built one project where original equipment was not available...that being said, my next project will likely have brake issues, so I will stay tuned...
Willy 301: Most pre 60 cars used a 1" ,1-1/16 or a 1-1/8 bore M/C . Wheel cylinders ranged from 7/8 to 1-1/8 on most Bendix systems. Pre 48 Fords had a 1-1/16 M/C bore with stepped cylinders 1-3/8 X 1 on the front and 1-1/8X1 on the rear. So bottom line the master bore size match is not a critical as a clutch system. If your pedal leverage is not as much as it should be you can make a easier pedal by using a slightly smaller bore M/C like 15/16 or 7/8s, but you will get a bit more pedal travel doing this.
The pressures are the same, it's the fluid volume output that's different on a non-50/50 dual master cylinder. Try plumbing front discs to the rear drum brake port on a disc/drum master, and the displacement loss will (should) be apparent.
Cross split, or diagonal brake systems (LF/RR RF/LR) use masters that are 50/50 in fluid volume, because each system (pri./sec.) is essentially the same. However, even with this type of master, there is a slight difference in vehicle stopping performance during failed primary and secondary DOT testing, because of the fraction of a second time delay of the primary and secondary pressure rise.
All componants are not interconnected - that is the principle of the dual master cylinder. Unless you have an unusual system the drum brakes equalized volume/pressure with different sized wheel cylinders. I would assume (i know, i know) you have equal sized bore in your master cyl, likely an 1 1/8th. There is a difference in the front and rear systems, another hambr touched on it but it is very important, the master cylinder applies the front brakes first! and then the rear brakes - when you bench bled you should have seen this happen: the front bowl will flow fluid and then the rear bowl. I haven't tested evey MC known to modern man and there is likely an exception to this rule, but every one that i bench bled has tested the same. There are tons of variables to the brake system, i have wheel cylinders that have 2 different bores for christsake, so unless you want to stay awake at night worrying about unkown bore sizes, ponderable proportioning valve sizes, questionable wheel cylinder sizes and calculating the square inches of brake linings just assume you have the right stuff because the most common/available devices are what you both need and have. Having said all that, you got air in the system or a leak: a msimatch will give lots of either or both pedal resistance and/or pressure, and you say it won't maintain pressure. Keeping pressure has nothing to do with system devices, it is system integrity.
The secondary piston in a dual master cylinder is a floater. Although it separates the two halves of the system, all parts of the system see the same pressure. Yes there is seal drag and maybe a difference between piston return springs, but those have no real world effect.
one day i would like to setup a test/display on master cylinders, start with an adjustable pedal ratio, measure pedal effort, be able to advance the pedal from zero to one hundred LBS push, test different masters, standard, power, drum-drum, drum-disk and disk-disk, show CC of fluid leaving the master and preasures each side puts out, i think this would be helpful.
Thanks for the replies. I'll try swapping the lines around and see what happens. Someone mentioned system integrity..... I can see no leakes anywhere, not even the wheel cyls. Please tell me'if I'm correct here... it's my understanding there should be a miniscule amount of freeplay in the pushrod to piston contact to ensure the piston is coming right back?
I have seen this many times where is your master located ... I have used residual valves from speedway And that seemed to enough back pressure so they don't bleed off
Yep I had the same prob on my t bucket And residual valves fixed it .my mc on my a is firewall mount and didn't need them . Worth a shot for ten bucks each
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