Disc swap in progress

Discussion in 'Pre-1975 Build-Up/Project Threads' started by THorn, Apr 10, 2021.

  1. THorn

    THorn Member

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    Aha, I’ve searched several recyclers, and I found a listing for a proportion valve for a 78 truck locally. I wonder why it’s not mentioned in parts catalogs? Maybe they don’t need replacement much? On further research, I did find a catalog that listed a discontinued prop valve for the RN30 trucks. It says ‘load sensor’, which might be hard to implement on this truck that wasn’t designed for it, but I’m thinking of giving this recycled one a try, to see if it makes this perform differently than my jegs valve.
     
    Last edited: Apr 16, 2021
  2. THorn

    THorn Member

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    Anyone have a photo of the RN20 valve? It would help me search.
     
  3. jetas

    jetas Grand Toyotaholic

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    Avoid anything like a LSPV
     
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  4. carinaguy

    carinaguy Member

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    You won't find one because it doesn't have one. The proportioning is built into the master. What most people assume to be a proportioning valve is actually just a distribution block. Like I said before, a prop valve is just an unnecessary complication for you at the moment.

    I did some research and ate some humble pie. Toyota didn't switch to a single piston caliper until '84, probably to go to a vented rotor. Anyway, it sounds like your system is balanced, so that isn't the issue.

    To check your booster: with the engine off, repeatedly press the brake pedal until all the vacuum is bled off. When the pedal won't get any firmer, press and hold down the pedal hard. If it slowly sinks any further, it is bad. I doubt this is your problem.

    Without knowing your bleeding procedure, it is possible to still have air in the master. I once replaced a master on my '73 Carina and had to bleed it 6 times. Then I learned about how to bench bleed and haven't had that experience since.

    Long story short, I now suspect your issue is in the master cylinder. Either an air bubble in the front brake circuit or a bad piston. The pedal should not go to the floor, so there is definitely an issue. I would want to get the master replaced before any warranty expires.
     
  5. THorn

    THorn Member

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    Thanks! That’s been my line of thinking too. The pedal is solid in the absence of the vacuum, which suggests that maybe I have some air trapped somewhere. I’m going to remove the master cylinder so that I can bleed it with different parts elevated. I did try to bleed it before I installed it, but being new to the process, I could well have been short of what was needed.
     
  6. carinaguy

    carinaguy Member

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    THorn,

    If you are interested in the steel rims from my '76, shoot me an email. [email protected]

    I had been using them for rollers, but now the tires are completely shot and the rims are dead weight. If you want them they are yours. We just need to coordinate off the forum.
     
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  7. THorn

    THorn Member

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    Ok, well, I feel supremely stupid. After bleeding the MC, bleeding the rest, bleeding the MC again, bleeding some more stuff... I got to thinking, ‘gee, why would the previous installation have run the rear lines to the primary portion of the cylinder? This doesn’t make sense.’ And guess what? It doesn’t make any sense because the previous installation was wrong! I just duplicated it with the new lines because I had assumed someone knew what they were doing (or else why would it be like that?). But, I was wrong in assuming that, and got the front and rear lines going to the wrong parts of the MC. It probably wasn’t as much of an issue when there were drums on all the wheels, but with the discs on the front, it’s pumping a (more) different volume of fluid to front and rear. Duh. I don’t know why I didn’t realize this sooner. That’s why the rear was locking so easily and why it took more pedal travel for the front to engage. So, lines swapped, works great now, even without prop valve. More lessons learned...

    ‘Experience is what you get when you don’t get what you want.’
     
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  8. Erwin Merida

    Erwin Merida Toyotaholic

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    Good catch. Could've been worse and you end up taking it all apart for the lines to have just been mixed up. :waytogo:
     
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  9. jetas

    jetas Grand Toyotaholic

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    Can we please get this account banned and disabled??
     
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  10. THorn

    THorn Member

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    Yeah, how does one go about flagging weird bot account posts?
     
  11. jetas

    jetas Grand Toyotaholic

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    Hit "report" at the end of their post
     
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  12. THorn

    THorn Member

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    Blergh!

    I’m replacing my relatively new MCylinder and booster, because I haven’t been able to diagnose a spongy pedal issue, and it appears that there’s an obstruction in part of the cylinder. As I described earlier in this ancient thread, I’m using the 79-81 booster and MC. Last time I did this, I got the front and rear lines mixed up. So, what did I do? Derr… all this time later, I can’t remember which one goes to which port.‍♂️ Stupid not to label them, I know. I realize that it’s conventional for the rear to go to the forward port, as that’s normally the secondary cylinder, but I have this memory of it being the other way around. Am I tripping?

    Anyone feel like helping a clueless sod get this right the first time? Front line to forward port?
     
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  13. jetas

    jetas Grand Toyotaholic

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    Do you have a proportioning valve installed? Idk if that being missing can cause a spongy pedal. Its very possible that its just air in the lines. It can sometimes be a huge pain to bleed the brakes properly
     
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  14. fred heath

    fred heath Addict

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    Did you bench bleed your new mc?

    I agree with jetas. Sounds like air in the system someplace.
     
  15. THorn

    THorn Member

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    I’ve had issues with extra pedal travel ever since I did the swap. Plenty of braking force, but a little soft after engaging. I even resorted to hooking up a power bleed rig to it, which is how I discovered the impeded flow from the cylinder and decided to replace it. I’m busy, so I had the power bleeder friends install it. Now with a new cylinder, I’m having a little trouble with the rear locking up, which made me think maybe I’d forgotten which line goes to which port. I think I’ve got them right, and that now with normal flow, I might need to go back to using a manual prop valve on the rear line.

    As to the still soft pedal, I feel like the power bleed process probably eliminated air pretty well, so I’m wondering now if perhaps it could be leaking into the booster?
     
  16. fred heath

    fred heath Addict

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    New mc shouldn’t be leaking.
     
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  17. THorn

    THorn Member

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    I feel like there’s no such thing as a ‘new’ 79 cylinder any more. Aren’t they all pretty much rebuilt cores? I’ve had new brake parts arrive with faulty seals before.
     
  18. THorn

    THorn Member

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    I don’t currently have a prop valve. I started off with a manual valve on the rear line, but deleted it while I was troubleshooting the bleed process. The parts diagrams make it look like the 79-83 system has a prop valve at the cylinder, but I suspect that that’s actually just a distribution block, and not a valve. I really hope someone can tell me definitively. The fella who bled this cylinder for me seems to think it’s a valve, but I have doubts.
     
  19. jetas

    jetas Grand Toyotaholic

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    Has to be a prop valve. Theres 3 outs 2 for the front and one for the rear. There are already 2 outs on the MC, would only need a splitter at the front but no
     
    Last edited: Mar 28, 2023
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