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Brake master cylinder - rebuild

Discussion in '206/246' started by daviekj, Apr 8, 2024.

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  1. daviekj

    daviekj Formula Junior
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    #1 daviekj, Apr 8, 2024
    I have a problem bleeding the master cylinder after re-sleave and rebuild and I am perplexed as to what the cause could be. Would welcome any suggestions from those who have already been through a master cylinder re-installation.

    Other threads/posts on brake master cylinder and bleeding that I have read through:
    Kevin
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  2. Nuvolari

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    #2 Nuvolari, Apr 8, 2024
    Before tearing into the master cylinder again to check for a blocked orifice (or worse one not drilled during the re-sleeve) I would try bleeding the master cylinder on a bench vise off of the car. This way you can be 100% sure of full travel as you are not relying on the pedal assembly which may be stopping short. Even though it is a re-build with the same parts I would still absolutely check the gap of the pusher rod to the master cylinder. I did not at my peril and I document it in the following video. The 308 is very similar to the Dino so everything discussed applies. I also included a video of the master cylinder bleed that can hopefully help you or someone else.



     
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  3. daviekj

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    #3 daviekj, Apr 8, 2024
    Thanks Rob,

    I saw your second YouTube but not the first one, so will take a look through tomorrow. I did crudely test the master cylinder on the bench and saw fluid push out of both Exit A and Exit B, but I did not set it up properly with a fluid reservoir and multiple strokes. I’ll take the master cylinder off again a do a proper bench test.

    if I am using the same master cylinder (just re-sleeve and new seals), is there a reason that the clearance to servo piston rod could change. I had assumed the clearances could not change.

    I did careful check that all holes were clear after the re-sleeve, so that’s not the problem.

    Does my figure 18. Show the correct assembly order and part orientation? It matches my disassembly photos (and I think the workshop manual drawing) , but it’s got me worried I may have the order wrong.

    kevin
     
  4. TonyL

    TonyL F1 Rookie

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    #4 TonyL, Apr 9, 2024
    Kevin
    I noticed you have the banjo bolt on the first outlet, but the one with the peg on it should be on the second outlet. That peg slots into the groove on the slug piston!
    Just a thought


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  5. Nuvolari

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    #5 Nuvolari, Apr 9, 2024
    Excellent catch Tony. That looks like a potential smoking gun.
     
  6. daviekj

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    #6 daviekj, Apr 9, 2024
    Yep that’s likely the cause Tony… well spotted.

    Your workshop manual drawing is much better than my copy.. it a lot clearer in your diagram. Itching to swap it over tonight and retry. I actually spotted this potential late last night and checked with Rob. Looks like indeed this is the issue!

    The long pin banjo bolt does fit in the wrong first port (nearest servo) if the pin part drops through the quadrant spacer. I just forgot to photo this part of the disassembly as it was the messy bit in the car…

    As Rob advised. I’ll pull the cylinder off and check the piston clearance spec at same time.

    grrr.. I feel stupid in getting this wrong. But if it works very pleased to have found the issue.

    will report back shortly. Thanks for Rob and your help.

    Kevin
     
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  7. TonyL

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    #7 TonyL, Apr 9, 2024
    Your excellent and well documented approach was the key to solving it imho, had it not been for the photos then it would have taken some time to crack.....hopefully thats the issue.
     
  8. TonyL

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    #8 TonyL, Apr 9, 2024
    I have been known to drop the ball sometimes....:)
     
  9. daviekj

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    #9 daviekj, Apr 22, 2024
    I now have good brake master cylinder operation. The important learning is to check the replacement seal size's before installation. I found the seals from one supplier simply too big to allow the second piston to move in the cylinder.
    hopefully some useful slides and comment on the master cylinder rebuild and assembly below. I would like thank Nigel and Jamie at https://www.pastparts.co.uk/ for an excellent re-sleeving service and help in getting to the bottom of the issue.
    Kevin

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  10. daviekj

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    #10 daviekj, Apr 22, 2024
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  11. pshoejberg

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    #11 pshoejberg, Apr 23, 2024
    Last edited: Apr 23, 2024
    One could argue that it is easier and possibly cheaper to buy a new master cylinder, but maybe not that fun:D

    That's at least my conclusion after several more or less successful attempts to overhaul old brake unit's.

    Best, Peter
     
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  12. daviekj

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    #12 daviekj, Apr 23, 2024
    Probably right Peter, but I grew up having to service master and slave cylinders, and it just felt natural to do the same here. Sort of thing I would do in a lunch break as a youth. Must say the Dino one was far more involved than I ever remember..., or may be I am just less urgent than I was.. Kevin
     
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  13. pshoejberg

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    #13 pshoejberg, Apr 23, 2024
    I really respect your work and I’m a great believer in repairing and re-use old equipment. But that said, it’s troublesome to repair these rather complex brake masters. They are always pitted and need’s to be sleeved which is a specialist job = costly. Also it’s a challenge to get the right size seals (as you know) and I have a fair amount of odd seals laying around from my previous projects. I’ve replaced all the hydraulic cylinders in all my f-cars with new units that I have found on eBay and similar places with fair pricing and quality and I have never been disappointed.

    best, Peter
     
  14. daviekj

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    #14 daviekj, Apr 23, 2024
    Peter,

    I didn’t really choose the rebuild option on a cost basis. However, if I compare the re-sleeve cost and seals (ignoring the ones that did not fit) the cost was ~20% to ~25% of two major Dino suppliers for a new one. True it would be more expensive to also pay for someone to re-build and test, but I still think it would be significantly cheeper.. however, I do see advantages and disadvantages with either way. Just pleased to have got it done.

    As you will see in a few more posts coming up I only set out to service the car. A tadge of mission creep crept in as usual. Even the simplest of tasks can end up being involved. Started January in prep for the spring and it’s now end April! Hopefully almost there…

    Kevin
     
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