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Master Cylinder / Residual Pressure Valve question

Discussion in 'The Hokey Ass Message Board' started by TagMan, Jan 28, 2009.

  1. TagMan
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    TagMan
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    I'm putting a '70 vintage Mustang dual master cylinder (drum-drum) in my '37 Chevy coupe. It'll be mounted in the stock location, under the floor. I've read that 10# residual pressure valves should be installed and understand the purpose. My question is, from what I've read, most M/C's have "residual pressure valves" built into them from the factory. Should the pressed-in valves that come in the new M/C be removed before installing the external 10# units or is the redundency in using both be OK ?

    I don't see any provision for any check valve to prevent flow back, in the original M/C. How did they work so well without any residual pressure valve?

    Also, the residual pressure valves I bought are for 3/16" brake line and I'm using 1/4". Is there any problem in using the 3/16" valve which effectively make a restriction in the line?

    Or am I over-thinking things again???? :rolleyes:

    Thanks for any input,
    -Bob
  2. Groucho
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    Groucho Member

    I'm guessing here. But if it were mine, just because you're now installing under the floor, I'd go with the aftermarket residual valves, and remove the ones from the M/C. FWIW, I bought an unfinished project car with a GM looking M/C already installed under the floor. I added a 10lb rear, and a 2lb front without modifying the M/C, (that I know nothing about it's contents), and the car stops fine for over a yr now. I wouldn't worry about the 3/16 valves, but I always plumb in 3/16. Why R U using 1/4?
  3. JohnEvans
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    JohnEvans Member

    Very few dual M/Cs have built in checks, they were usually in the combination/brake failure switch valve. But if you do have one in the M/c it won't hurt/cause prob. with one in line. The 3/16 check will not restric the flow to any noticable degree,amount of fluid moving through it on a brake app. is really very small.
  4. fredeuce
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    fredeuce Member

    Some basics here. Residual check valves are only needed on drum brakes and not disc. The reason is to balance the pressure generated by the return springs used on drum brakes. Disc brakes by comparison don't have the return spring so don't need the RCV.Any RCV should be removed for disc brake application.

    If you are using drum brakes all round and the M/C you have purchased is for a drum/drum application then it should have the RCVs already built into it.

    I trust this helps.:)
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  5. just Matt
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    just Matt Member

    I don't know about the Mustang master, but when I was trying to find a GM master for my 54 I had a hard time finding one that actually had the internal RPV's installed. All the remanufactured ones I looked at did not have them. I finally found a new one from NAPA (#36236) that had them. To check for the presence of the valves, I carefully inserted the blunt end of a small drill bit into the outlet ports to see if there was resistance prior to actually reaching the depth of the cylinder itself. These pictures from a rebuild kit instructions show the RPV's and a cross section of the outlet ports. Hope this helps.

    Attached Files:

  6. Groucho
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    Groucho Member

    Pretty much not true here. Residual valves keep the seals in the calipers and wheel cylinders sealed. A 10lb valve is used on drums because the return springs can overcome the resistance, whereas a 2lb is used in disc applications because it's only lateral runout of the rotor to push the pads slightly away from the rotors when letting off the pedal. Also, RCV's help maintain proper "pedal" when the M/C is below the wheel cylinders (M/C under floor for ex). RCV's prevent the whl clys. from gravity draining back to the M/C
    Last edited: Jan 28, 2009
  7. Groucho
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    Groucho Member

    GREAT IDEA on the dull end of the drill bit to determine a valve's presence!!!!!!!!
  8. budd
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    budd Member

    myself i have run without any rpv's in my pickup that came stock with a single under floor master that i converted to duel, it was a 45 gmc, it never caused me any trouble, ie i never saw any fluid running out the top of the master, i did convert a 29 chrysler to duel, they came stock with a single, i installed rpv's and it held the brakes on and had to remove them, i think that maybe the brake shoe return springs are just not strong enough to over come the 10 psi of the valves, i have also been told that keeping some pressure in the lines keeps air from sliping past the wheel cylinders but i havent had trouble with air, maybe i'm just lucky. i also have tryed checking for rpv's in a master and used a dull wire, it felt like there was resistance but it turned out to be the rubber cup in the spool valve was justcovering a slight part of the hole, there were no rpv's, but it did feel like there was.
    Last edited: Jan 28, 2009
  9. T McG
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    T McG Member

    You will not need any other valves with that master cylinder. Also remember to bench bleed it before you install it.
  10. 29nash
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    29nash BANNED

    What T McG said.
  11. 36-3window
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    36-3window Member

    that master cylinder does not have built in residual valves....i have taken several apart and they did not have them

    even if it did , it would not hurt anything to have residual valves installed inline....this is right from ECI's website: "Either way, if you are not sure whether you have one or not, put one in. They are not cumulative and it won't hurt anything if you have two."
  12. ehdubya
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    ehdubya Member

    I believe you need RPVs if the MC is lower that the wheel cylinders.
  13. 29EHV8
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    29EHV8 Member

    Exactly.
    These M/C have built in residual valves. I went threw this years ago with a brake pro friend of mine. I have run dozens upon dozens of these M/C under the floor in rods I've built and never ever have run inline RPVs. And never ever have had an inkling of a problem. And I drive my cars everywhere, everyday under every conditions. You won't need a proportioning valve either.

    -Shiny
  14. TagMan
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    TagMan
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    Thanks for your thoughts, guys. I'm going to just go ahead and install the external 10# RPVs I have.

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