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Thread: 50/50 vs. 38/62 brake master cylinder

  1. #21
    Senior Member DMC-81's Avatar
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    Quote Originally Posted by Ron View Post
    Dana, compensating ports are bores in the housing. I haven't looked at a stock D MC, but many like this: For rear favored bias, both pistons retract, both chambers in the bore fill, when the pedal is pressed, both pistons pass the fill ports (purging ends), then the rear piston passes its compensation port and begins to create pressure 1st, THEN the front piston passes its compensation port and begins to create pressure too...
    There are other ways it is done e.g, with a valve (similar to a push-up valve in the bore), as previously mentioned. I haven't seen/noticed one before, but, with the difference in spring strength you pointed out here, I could see how the position of the compensating ports could be equal at rest and the weaker spring causing/ensuring the rear piston passes its compensating port first...Or, just using equally spaced fill ports with a weaker spring (much like a biased ABS MC w/o compensating ports).
    But, I don't see the springs affecting the pressure once the pistons have passed all ports, regardless of whichever port setup it has....

    Very rough, but might help:
    Attachment 57048

    Look in the bore. If there are 2 incoming ports only, it is the spring doing it. 3 or more it is compensating port(s)...or a combination.

    Interesting stuff!

    Thanks Ron. That makes sense about the compensating ports. I looked specifically at the BMC, and as you describe, there are two ports per circuit as described below. I also searched the port definitions (in parentheses):
    1) a bigger by-pass port (to supply fluid to the master cylinder bore when the brakes are not applied)
    2) and in front of that, the smaller compensating port (to allow the master cylinder piston to quickly return to the unapplied position, as well as compensate for fluid expansion or contraction from changes in temperature).

    I looked again at the holes in the pistons and they must be there to help equalize pressure once released to the unapplied position.

    Here is a picture of the ports:

    The by-pass ports in blue, and the much smaller compensating ports in front of them. (And obviously the output ports to the brake lines in green.)
    I lined up the rear piston in the position that it would be in the bore and the by-pass ports are behind the pressure seal, in the area of the necked down intermediate shaft of the piston:



    It's hard to see, but the compensating port is roughly positioned 1mm ahead of the pressure seal:



    So at least for the rear piston, it only has to travel very slightly forward to pass its compensation port and begin to create pressure. That would mean, as one continues to apply the brake, the 2 different gauge springs and the relative position of the front piston would impact the proportioning, correct?

    I think the front piston passes the compensating port very quickly as well, as I can't imagine fluid under pressure escaping into back into the reservoir. I didn't observe any turbulence when I had the reservoir cap off as I was bleeding the brakes a few years ago.

    Indeed, very interesting stuff.



    Quote Originally Posted by Ron View Post
    EDIT: Just a thought...do we know that the 38-62 is a bias rate for the MC itself?
    Well, DMCH states that "the original brake master cylinder was internally proportioned uniquely for the 62/38 weight distribution of the DeLorean" on the part description:
    http://store.delorean.com/p-7478-mas...der-brake.aspx
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
    Restored as "mostly correct, but with flaws corrected". Pictures and comments of my restoration are in the albums section on my profile.
    2006 Dodge Magnum R/T (D/D)
    2010 Camaro SS (Transformers Edition)

  2. #22
    Senior Member DMC-81's Avatar
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    ^ Sorry, I meant to add that there was also the 38/62 red/orange band around the original BMC shown here (cleaned and fitted to my reproduction one), versus the black 50/50 band on some units. Hopefully that would signify a difference. If not, what does it mean?

    Last edited by DMC-81; 06-28-2018 at 08:59 PM.
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
    Restored as "mostly correct, but with flaws corrected". Pictures and comments of my restoration are in the albums section on my profile.
    2006 Dodge Magnum R/T (D/D)
    2010 Camaro SS (Transformers Edition)

  3. #23
    Administrator Ron's Avatar
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    In the first pic, it looks like the forward circuit has 3 holes, one plugged, but in the last pic, it's not. Flash/reflection or ?? Anyway, good research there...
    I think so...it looks like we have this one figured out, a combination of springs and port position. (...or convinced ourselves that we have ;-)

    On the bands- I remember reading that all of the original D MCs had the orange band. I assume it's true, but don't remember the source...

  4. #24
    Senior Member hippieman9's Avatar
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    Quote Originally Posted by Ron View Post
    In the first pic, it looks like the forward circuit has 3 holes, one plugged, but in the last pic, it's not. Flash/reflection or ?? Anyway, good research there...
    I think so...it looks like we have this one figured out, a combination of springs and port position. (...or convinced ourselves that we have ;-)

    On the bands- I remember reading that all of the original D MCs had the orange band. I assume it's true, but don't remember the source...
    My current BMC has a black band and says 50/50 on it.

  5. #25
    Senior Member DMC-81's Avatar
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    Quote Originally Posted by Ron View Post
    In the first pic, it looks like the forward circuit has 3 holes, one plugged, but in the last pic, it's not. Flash/reflection or ?? Anyway, good research there...
    I think so...it looks like we have this one figured out, a combination of springs and port position. (...or convinced ourselves that we have ;-)

    On the bands- I remember reading that all of the original D MCs had the orange band. I assume it's true, but don't remember the source...
    Yes, the plug is a pin that holds the front piston in place. The pin is shown in the first photo in post 19.

    Thanks. I'm sure there are some things we missed, but at least we know more about the anatomy of a BMC! I enjoy researching how mechanical things work.

    Quote Originally Posted by hippieman9 View Post
    My current BMC has a black band and says 50/50 on it.
    Cool. If you can post a picture of it that would be beneficial to others reading the thread in the future.
    Last edited by DMC-81; 06-29-2018 at 09:25 AM.
    Dana

    1981 DeLorean DMC-12 (5 Speed, Gas Flap, Black Interior, Windshield Antenna, Dark Gray)
    Restored as "mostly correct, but with flaws corrected". Pictures and comments of my restoration are in the albums section on my profile.
    2006 Dodge Magnum R/T (D/D)
    2010 Camaro SS (Transformers Edition)

  6. #26
    Guy with a DeLorean Mark D's Avatar
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    Really cool thread! I enjoyed learning more about the brake master cylinder
    Mark Dehlinger

  7. #27
    Senior Member hippieman9's Avatar
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    My VIN:    06668 81' Grey interior, 5 speed, Grooved hood. Previous owner of 16301, in 2001.

    IMG_4317.jpgIMG_4315.jpgIMG_4316.jpg

    I know the pics aren't the best. But when I remove the BMC I'll take better pics. I have my new 38/62 BMC as well, I can put them side by side.

  8. #28
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    Here is the new master I purchased from DMC and installed about a year ago. It has a plastic band stamped with 38 62

    Master Cylinder.jpg

    Ron

  9. #29
    Senior Member hippieman9's Avatar
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    Location:  Haw River, NC

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    My VIN:    06668 81' Grey interior, 5 speed, Grooved hood. Previous owner of 16301, in 2001.


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