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Technical Combination Proportioning Valve Confusion

Discussion in 'Traditional Hot Rods' started by Steve Lowe, Dec 21, 2019.

  1. Steve Lowe
    Joined: Nov 8, 2019
    Posts: 58

    Steve Lowe
    Member

    I am in process of converting to front disc brakes on my 1939 Plymouth PT81 pickup. Even after incessant reading about brakes I am still confused about how to properly install valving. I transplanted an 87 Jeep rear along with it's bendix drum brakes and have almost finished converting the front to disc. I replaced the master cylinder with a dual outlet type. Clearly a custom system now, so I believe a factory style combo valve might not the best choice? But to include all the stuff a factory style combo valve does and be able to adjust bias, I would need an adjustable proportioning valve and a metering (hold off) valve and a pressure differential valve (for warning light). I will also need a 10 lb residual check valve. Seems like there should be a simpler way then all these individual valves? Does it make sense to install a combination valve from a similar size vehicle and just add an aftermarket adjustable proportioning valve in case I need to adjust bias on rear brakes? Seems to me this might not work if there is too much restriction to the rear brakes through the factory style combo valve? Am I losing my mind? I was hoping someone made an adjustable combo valve that includes all the functions of the oem factory types. I have not found any? Opinions and advice welcomed.
     
  2. harpo1313
    Joined: Jan 4, 2008
    Posts: 1,838

    harpo1313
    Member
    from wareham,ma

    Ive always used the gm brass valve ,it splits your system and provides a warning light. On the master use a disc drum one , ive used 69 truck ones with no problems. no valves at all.
     
  3. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,590

    V8 Bob
    ALLIANCE MEMBER

    Keep it simple!
     
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  4. Driver50x
    Joined: May 5, 2014
    Posts: 57

    Driver50x
    Member

    I agree about keeping it simple. I think an adjustable rear proportioning valve is likely all you need.
     
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  5. Steve Lowe
    Joined: Nov 8, 2019
    Posts: 58

    Steve Lowe
    Member

    Thanks Bob for your reply! Very helpful. Perhaps I'm reading too much? I'm uncertain of the history but I thought the idea behind adding the metering valve was to engage rear brakes first to partially overcome their return springs and also to prevent nose dive. Also supposed to be helpful on snow and ice to prevent front wheel lockup at low pedal pressures? If that's the case, it seems like that might be worthwhile on a daily driver? Of course my truck is not. I like keeping it simple too. Just less to go wrong.
     
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  6. Steve Lowe
    Joined: Nov 8, 2019
    Posts: 58

    Steve Lowe
    Member

    "Adding a 10 lb residual for the later-than-mid-70s rear drums will help with pedal travel."

    Just to clarify, are you suggesting I do, or do not need the 10lb residual with the 1986 bendix rear drum brakes? I am confused by why earlier than mid-70s would not need them?
     
  7. harpo1313
    Joined: Jan 4, 2008
    Posts: 1,838

    harpo1313
    Member
    from wareham,ma

    If you use the master that is designed for disc and drum and its on the firewall no need for any valves , the gm brass valve I speak of is for safety as it slides to front or rear in case of brake line failure and has a warning light . It also is the junction for brake line routing.
     
  8. koolbeans
    Joined: Apr 12, 2015
    Posts: 464

    koolbeans

    My too cents...disc front/drum rear,
    master up on firewall:

    Proportioning valve to rear

    Master in floor...brakes higher than master...2psi residual in front, 10psi in rear, still a proportioning valve rear.

    Proportioning valve is there for two reasons:
    A) to allow the pressure to rear to be such that you don't lock the rears before the fronts. If rears lock up first you need driving instructions to execute a swapped end condition....not good.
    B) there simply for simple adjustment of non/not factory tested and balanced systems.
    Virtually all race cars have the above.

    Sent from my XT1710-02 using The H.A.M.B. mobile app
     
    Last edited: Dec 22, 2019
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  9. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,590

    V8 Bob
    ALLIANCE MEMBER

    All drum system masters used and needed an internal residual (check) valve to prevent air from entering past the wheel cylinder cups during release. Most stopped using them in the mid '70s because of universal front discs and improved wheel cylinders with cup expanders. You probably don't need a drum residual, but having one will help reduce pedal travel. I'm also using mid '80s Bendix rear drums on several custom systems, and they all have internal or 10lb in-line residuals.
     
  10. Steve Lowe
    Joined: Nov 8, 2019
    Posts: 58

    Steve Lowe
    Member

    Thanks again Bob! I'm taking your advice and installing a adjustable proportioning valve along with the residual valves for front and rear (underfloor master). Could you recommend which manufacturer adj. prop valve and residual valves you have had good experience with?
     
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  11. Steve FWIW, on my 36 Chevy pickup I have OT midsize Ford calipers front and a GM Nova 10 bolt rear with a 76 Chevy Monza master ( under the floor). A 2 pound residual front and 10 pound rear and I used a combination valve from a Chevy S10. The truck stops quick and straight wthout unnecessary lockup of the tires, just a good working hodgepodge of parts. Mitch.
     
    Last edited: Jan 23, 2020
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  12. 5brown1
    Joined: Apr 13, 2008
    Posts: 196

    5brown1
    Member

    I read somewhere that a combination valve must be installed lower than the master cylinder. Can anyone confirm
    or debunk that. I have one on a car with an underfloor master cylinder where there is no way to get it below.
     
  13. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,590

    V8 Bob
    ALLIANCE MEMBER

    I'm using vintage iron Kelsey Hayes proportioning valves from my brake test days at Bendix/Bosch, and alloy residuals from Wilwood, but many I know use Wilwood alloy prop valves without any issues.
     
  14. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,590

    V8 Bob
    ALLIANCE MEMBER

    I have no first hand experience with your situation but would think getting and keeping a good bleed would be difficult. IMO, I would ditch the combo valve and just use inline residuals if necessary along with an adjustable prop valve that can be installed closer to or slightly lower than the master.
     
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  15. golferforpar
    Joined: Nov 7, 2009
    Posts: 36

    golferforpar
    Member
    from Wisc

    I am attaching a link and hopefully it can be opened. Pages 16 & 17 might answer your questions. I found it useful.

    MBM Technical & Troubleshooting Guide (File Size:1,548.83 Kb)


    Sent from my iPad using The H.A.M.B. mobile app
     
  16. 5brown1
    Joined: Apr 13, 2008
    Posts: 196

    5brown1
    Member

    Well you are correct about not getting a good bleed. I used quarts of fluid trying to get a firm pedal and never did.
    I think I will remove it. I already have the inline residuals installed. Thanks.
     
  17. missysdad1
    Joined: Dec 9, 2008
    Posts: 2,844

    missysdad1
    ALLIANCE MEMBER

    First check to be certain that your master cylinder does not already have an internal residual pressure valve. Some rebuilts have been known to have them when they're not supposed to, and to not have them when they are supposed to - rebuilder error. If you find one, remove it so there are no residual pressure valves in your master cylinder. Then proceed as you've stated here. Wilwood makes excellent quality components and are always a good choice even though they're going to be more expensive.
     
  18. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 732

    Elcohaulic
    Member

    Last edited: Dec 25, 2019
  19. Tube amp dr
    Joined: Jun 9, 2019
    Posts: 2

    Tube amp dr
    Member
    from Arkansas

    I'm in the same quandry as to using a combination valve or an adjustable proportioning on my '53 V8 Belair disk/drum daily driver I'm building. My worry is the liability incurred since the adjustable proportioning valves are not DOT approved therefore not street legal and so in the event of a collision - whether my fault or not - an astute lawyer would impound the car for inspection and find the valve, then I'm automatically criminally negligent and they take all my money... (or something along those lines.... ). Therefore I lean towards using a combination valve - but which one? The proportioning characteristics of every combination valve are fixed and set for a particular car configuration so Question is: which combination valve is appropriate for the 53 Belair with V8 and Disk /Drum? Thanks for any pointers or experience recommendations.
     
  20. I use an aftermarket distribution block with a built in rear bias adjuster. Set me back $70 and works well, easy to plumb.
     
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  21. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,590

    V8 Bob
    ALLIANCE MEMBER

    I'm not aware of adjustable proportioning valves not being DOT approved. (The K/H valves I'm using were from production vehicles). Fact is any custom/non-stock brake system would be not be DOT approved, so that's really a moot point. Much better to be have the ability to adjust front/rear bias and prevent early rear slide vs using a fixed value combo valve not designed for your system. Metering was not always used and is not required for disc/drum, and a '67-up drum/drum pressure differential/distribution block can be used if you want a warning light.
     
  22. Tube amp dr
    Joined: Jun 9, 2019
    Posts: 2

    Tube amp dr
    Member
    from Arkansas

    All good points, Thanks. I learned from suppliers that NO aftermarket brake parts are DOT approved, so as V8 Bob said - it's a mute point. Going with a MPbrakes universal combination valve.... If the rear tires lock up I'll change to the proportioning valve.
     
  23. On the adjustable proportioning valve, which way is closed and which way is open? Mine is not marked and I never messed with it. I assume that closed is all the way in and open is all the way out. Is that correct? Also, how do you know when it is adjusted correctly?
    ADJUSTABLE PROPORTIONING VALVE.jpg
     
  24. Bullit68
    Joined: Sep 16, 2009
    Posts: 91

    Bullit68
    Member
    from Verona, PA

    I drive and hit the brakes hard at about 30mph. Open until rear wheels start to lock up. Adjust so the rear wheels just don’t lock up, but you feel them helping, trial and error. Then you can do normal braking at a higher speed and see how they feel. Adjust if needed, your discs should do the braking without the front end of car diving down. My $.02
     
  25. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,590

    V8 Bob
    ALLIANCE MEMBER

    There normally is an arrow on the knob indicating max (pressure) CW, min CCW. The whole purpose of a proportioning valve is to prevent early rear wheel slide-the front axle should always slide before the rear.
    The method I use is to adjust the valve for max rear pressure (full CW) and evaluate the braking performance. If the rears are lazy or feel ineffective, the brake and/or wheel cylinder (or caliper) size needs to be enlarged. If (or when) the rears become aggressive, simply adjust the valve CCW two turns at a time until the rears settle down, then CW one turn to try and fine tune. There are normally about 10 turns total on adjustable prop valves that I have used. The knob simply moves the pressure "knee" or "crack point" from about 100 to 1000 psi when proportioning (a percentage of master cylinder pressure) to the rears begins.
     

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