Date Code Decoding & Rebuilding A 1970 GS Drum Brake Master Cylinder

Discussion in 'The whoa and the sway.' started by Brett Slater, Dec 19, 2021.

  1. Brett Slater

    Brett Slater Super Moderator Staff Member

    I finally had a chance to clean up the original master cylinder.

    I've pored over Duane's date code book and can't figure out what the code is. Maybe someone can chime in or maybe there isn't one?


    Also, has anyone ever rebuilt one? It doesn't seem overly difficult, as there isn't much to it in terms of parts. I've also seen some YouTube videos depicting rebuilds and am wondering where guys are getting rebuild kits?

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  2. Redmanf1

    Redmanf1 Gold Level Contributor

    Would not the 321 (Nov 17) be a late 69 date??
    Last edited: Dec 19, 2021
  3. Daves69

    Daves69 Too many cars too work on


    Let me know where you find rebuild kits.

    How is the bore inside your master?

    STAGE III Lost Experimental Block. Platinum Member.

  5. Brett Slater

    Brett Slater Super Moderator Staff Member

    Good question.

    All I know is this was on the car when I got it and is the correct number/code for my car.

    321 could be November 17th of 1969, so that would make sense.

    Hopefully Duane chimes in.
  6. Brett Slater

    Brett Slater Super Moderator Staff Member

    I'm assuming GM used the same internals on these across the board and did find this after doing a little research.

    The 1" bore isn't terrible but I'd likely give it a light hone during reassembly just to be sure. I didn't see any obvious scoring or deep scratches.

    The front piston was dinged up, though.

  7. Jim Weise

    Jim Weise EFI/DIS 482

    The best thing you can do is have a sleeve installed in that master. Back when I used to do resto's, we did them all the time. I probably did 10 of the twin bail disc masters.. Cost was like $60 back then, and insured a perfect result. Check ar0und your area for a machine shop that can perform the service.

    If you can't find anyone to do it for you, I would be happy to take it where we used to do our stuff in MPLS.. as long as you have some time on this, I would drop it and pick it up when I am in town..

    They also have the rebuild kits.. good ones..

    Or you could deal with them directly if you wished

  8. Brett Slater

    Brett Slater Super Moderator Staff Member


    Thanks for the info/offer.

    I've read about the sleeving and was going to inquire (in this post) about it.

    I wonder if my machinist would be able to take care of it? How thick of a sleeve is typically used for something like this?

    I kinda wanna tackle this project myself and am in no rush. If I hit a dead end, I'd be inclined to reach out.

    Thanks again for the offer and site link.
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You did mine way back when Jim. I use DOT 5 silicone fluid, I expect this MC will last forever.:)
  10. Jim Weise

    Jim Weise EFI/DIS 482

    Yup, did a couple a month there for a while.. it was before you could buy a repo of the cylinder. So it was the only option..

    Most guys these days will opt for the repro, unless they are doing something really correct, so that line of work kinda dried up and blew away.

  11. Jim Weise

    Jim Weise EFI/DIS 482

    I don't now the exact specifics of the process Brett.. but I bet if you called the guys at the machine shop at B&E, they would tell you. Some of the more common sleeves might be commercially available, ready to press in an oversized bore. You might be able to buy parts from them, and find someone to do it locally.

    Brett Slater likes this.
  12. Duane

    Duane Member

    To decode the date code by using my book;

    Go to the index at the back of the book and look up brake components. Find the line for Master Cylinders.
    The info there is C, G-p8

    If you go to the front of the index you will see the abbreviations are “C” for cast and “G” for all GM. The “p-8” stands for page number 8.

    Therefore on a Master Cylinder you are looking for a date code that is cast into the part, that was used for all GM cars regardless of make/model, and the date coding system you are looking for is on page 8.

    If you go to that page you will see a photographic example of the date code and the system used is “the day of the year”.

    Your date is “321” and your car is a 1970 model. The 70 model year started in the summer of 1969 and ended in the summer of 1970; therefore you are looking for the number 321 in the calendar for the second half of 1969.

    If you go to that calendar you will see that #321 decodes as Monday November 17, 1969.

    That’s how my book works.
    Last edited: Dec 19, 2021
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  13. buick64203

    buick64203 Just plum crazy Staff Member

    Apple hydraulics by me gets $250 to sleeve and rebuild that master. They do excellent work
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  14. telriv

    telriv Founders Club Member


    I've been using White Post Restorations in White Post Virginia for 30+ years. IF you let them rebuild they will guarantee it for life. IF I remember correctly they also advise against using Dot 5. I don't remember the reasoning behind this but I've always followed their instructions/leads.
    Great people to deal with & before this Covid thing were pretty quick in the turn around.
    I don't know how long they have been doing this but the 1st. time I used them was over 30+ years go. NEVER had ANY problems. I like that they use Brass instead of stainless for the sleeve as it has less tendency to tear the rubber lips of the internals.
    Just my thoughts.

    Tom T.
    Brett Slater likes this.
  15. Redmanf1

    Redmanf1 Gold Level Contributor

    I have also used White Post Restorations to do SS sleeves for master cylinders and calipers.
  16. Brett Slater

    Brett Slater Super Moderator Staff Member


    I was looking for an actual photo but kinda figured it out a couple posts into this thread.

    Thanks for clearing that up!
  17. Brett Slater

    Brett Slater Super Moderator Staff Member

    I have their order form printed out here and may end up going that route at some point.

    I'm in no rush anyway and found all the info I was looking for in this thread.

  18. Mike Trom

    Mike Trom Ugg

  19. Duane

    Duane Member


    I was looking for an actual photo but kinda figured it out a couple posts into this thread.

    Thanks for clearing that up!"

    There is a photo showing the date code in my book.
    If you look at page 8 it has a photograph showing what the "Day of the Year, Julian Date" style of date code looks like, so you can see exactly what you need to look for.

    At the beginning of each type of date coding system, there are photos of every style of date code that was used. GM used the following ways to ID the date codes,
    Stamped (Incised)
    Ink or Paint Stamped
    Tag or Sticker
    Etched or Sandblasted

    So if you look at one of the date coding system pages it will have a photo of each "style" of code used, ie cast, stamped etc.

    The "Julian Date, Year" system shows Stamped, Ink Stamped (for Hoses), and a Sticker (for the Wiper Motors).

    If you are looking at a part, and it is not listed in the index so you are not sure what the date code looks like, all you have to do is flip thru the pages of the various systems at the front of the book. There is a photo of every style/type there.

    The only exception to this is with the glass codes. I could not take pics of them that were good enough to put in the book. For these I drew them in AutoCAD, but they look exactly like the actual codes.


    Pic Courtesy of Walt Kilgus.
    Last edited: Dec 20, 2021
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  20. Brett Slater

    Brett Slater Super Moderator Staff Member

    Another OEM part saved.

    Some may say they're pricey but the finished product is worth every penny, as far as I'm concerned.

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