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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    A54D986B-B377-4742-AD8B-CE70CA3321AF.jpeg 3502E68A-E9C5-468D-8B34-AF327C2882F2.jpeg 977FD28A-CDCF-4A93-8F62-50E76AC3D0CE.jpeg OK, let me preface this by saying that I am not a professional auto mechanic. For a living, I work on bicycles and houses alternately. I’ve always enjoyed puttering around and tinkering with my elderly cars, but it’s always been with completely stock equipment. I can usually figure out how to get stuff to work that’s been designed to work right, but all the custom fabrication in this project has been a whole new thing.

    So... I thought it would be fun and educational to take on this insanely modified 74 Hilux. I got it just after it had a 5vz engine slapped into it, many compatibility issues unsolved. I’ll save the details of that for another thread. Anyhow, with that much HP, I felt like it would be cool to have disc brakes, so I gathered up some parts per the recommendations I’ve found floating around here and elsewhere and had a go at it.

    I got a pair of 77 steering knuckles, hubs, tie rod links, etc from a kind soul on here who shipped them to me. I ordered new/remanufactured rotors, calipers, rear cylinders, hoses, and more. I opted to use the 79-83 master cylinder and booster because they fit with the least modifications. I chose to keep the stock rims for now because I’m really in love with my original center caps. I compensated for the interference with the caliper by using an 8mm spacer, so that the wheels would still be a little bit hubcentric, and bought the rest of the clearance by shaving a couple more mm off of the high spot inside the rims. I intend to score some 75-77 rims soon, but the nearest option requires a 12 hour round trip, so for now it’s spacers.

    After several days of tedium, I have it all installed and (I think) bled. It pretty much works, but there are some bugs to work out yet. The front brakes seem to actuate pretty much at the beginning of the pedal stroke, but then there seems to be a fair amount of travel before the rear engages. By that point, the pedal is nearly at the floor. I’m wondering if maybe I’ve run into some compatibility issues between the selected parts or between the selected parts and my 5vz motor. It certainly stops quite effectively, via the front calipers especially. However, the subsequent travel feels excessive and when the rear engages, it feels a bit abrupt.

    Anyone have any experience or insight on this scenario?
     
  2. jetas

    jetas Grand Toyotaholic

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    Looks like you're missing the proportioning valve. Either that or just adjust the drums
     
  3. TRUCK ACTION

    TRUCK ACTION Grand Toyotaholic

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    Yes a proportioning valve , yo can get a adjustable one from Speedway Motors
    if yo go to disk at the rear a larger master & duel power booser
     
  4. THorn

    THorn Member

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    OK, here’s where I reveal the depth of my ignorance.

    I actually got a prop valve for this. I had it installed in the line to the rear drums initially. I thought that might be contributing to the problem so I removed it to see if it made a difference. Should I have put the prop valve on the front brakes? Or do I actually want a distribution valve with 2 adjustments? I figured since I couldn’t find any reference to a prop valve for the 79-83 trucks (the booster/MC are 79-83), that maybe it wasn’t that necessary. I just got it to mitigate the lockup potential depending on whether I’m hauling stuff or not. What I can’t figure out is why there’s so much pedal travel. Could it be a mismatch between the pedal ratio for the 74 pedal and the 79-83 booster? Should I consider a different combination of booster/cylinder? Also, (more ignorance at work here) is the vacuum from the 6cyl engine right for this booster? Does the booster/engine combo matter?

    Thanks a ton for the input. I don’t have anyone in my orbit with this kind of knowledge or experience (at least nobody who’s time doesn’t cost me a lot), so I’m kinda making it up as I go. BF014066-0765-4A0F-9BE4-91C6EA7AD611.jpeg
     
    Last edited: Apr 11, 2021
  5. THorn

    THorn Member

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    Side note: painting the calipers was stupid. Any stray drip of brake fluid chews it right off. Good thing the caliper paint was free.
     
  6. Pearce

    Pearce Toyotaholic

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    Yup it a caustic fluid. Not necessarily stupid just need to be careful.
     
  7. THorn

    THorn Member

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    Yeah, I’m used to being careful with brake fluid, just didn’t figure how much of a pain it would be to keep it off the caliper. Silly me.
     
  8. jetas

    jetas Grand Toyotaholic

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    Prop valve going to the back is what you want because you want to adjust the pressure back there. Id try bleeding the brakes again, adjust the drums and play around with the prop valve til you get it where you want it.
     
  9. Erwin Merida

    Erwin Merida Toyotaholic

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    Which brand is best tho? Oem all day, but then I saw this off jeggs and pretty decent price. Has anyone tried this style? Linked to the MC. I think it's dope Screenshot_20210411-183818.png
     
  10. THorn

    THorn Member

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    I’ve tweaked the adjustment some, and bled it some more. The shoes are adjusted closer than I’m comfortable with on the rear, and I’m running it without the prop valve for the moment. The brakes actually feel quite solid, but there’s still this extra travel between the front engaging and the rear. It’s possible that since I am bleeding a system that was completely dry a couple days ago, that I’m still working air out, but it doesn’t feel spongey like I’d associate with air, it’s travel... boom, brakes!

    I’ll be connecting the parking brake this week. That should help keep the rear in adjustment. I Just need to invent a way to mount the lever relay. The cross member was swapped with the transmission, so I have to put a bracket and bushing on it for that lever.
     
  11. THorn

    THorn Member

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    Dope indeed. Are the wires meant to work with some kind of control unit?
     
  12. THorn

    THorn Member

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    Oh, duh, they’re just the dash light sensor wires.
     
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  13. carinaguy

    carinaguy Member

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    It sounds to me like you have a mismatched master cylinder/caliper piston ratio. What is the bore size of your master cylinder? Or at least do you know if you have the 2wd or 4wd version?

    IIRC, the 2wd trucks started to use a single piston caliper in '79. Which means that the dual piston calipers you are using are not getting sufficient fluid volume to create clamping force on the pads until the brake pedal is nearly at full stroke. Check which master cylinder you have and get back to us.

    Your proportioning valve is an unnecessary complication at the moment. The rear brakes are not activated until AFTER the piston in the master cylinder for the front circuit has developed sufficient pressure to move the pads tight to the rotor. Once the front pads are making contact, the valve opens up and adds pressure to the rear at the proportion determined by the difference between the front and rear circuit piston diameters. Yes, that is right, the proportioning is BUILT IN to the master cylinder. You only need the external proportioning valve if the rear brakes are locking up prematurely (as they can in a truck with varying cargo loads).

    My bet is that you have the 2wd master, and need the 4wd version. Or to swap it out for the '75-78 master cylinder.
    Hope that helps!
     
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  14. Erwin Merida

    Erwin Merida Toyotaholic

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    That's total sense.
     
  15. THorn

    THorn Member

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    I removed the prop valve until I get the compatibility figured out. Let me reiterate for the record:

    I’m using the 75-78 2wd single piston calipers on the front, 74 drum cylinders on the rear. The booster and master cylinder are both the 79-83 2wd version. I think you point out a strong possibility, and something that I was concerned about going into this, but I didn’t have much to go on, so the whole endeavor is a big experiment. If I had my way, I’d use the 75-78 booster and master cyl to go with them, but then I’d have to move the clutch cylinder over to make room for the larger booster. I was just hoping that the 79-83 master cylinder would be close enough to work.

    What I’m experiencing is that the brakes engage fairly high in the pedal stroke. The rear will lock up at about 25% of the way, then the pedal will continue to the floor. All this time I’ve been scratching my head wondering if I got all of the air out, since this is my first time bleeding a completely dry system, but after 5 attempts and some professional help, I’m reasonably sure it’s bled.

    I hope the clarification on the part choices helps. There seems to be some confusion about what I put on here. Since the RN20 trucks were pretty much all rwd(with the exception of aftermarket mods), I didn’t think to specify. There was also a 3/4ton and 1ton version that (I think) used a slightly different master cylinder(the 1/2ton and 3/4 ton say 7/8 on them, 1ton says 15/16), but the calipers appear to be the same for all 75-78 models. I would welcome recommendations on booster/master cylinder combos that might mitigate the problem. I thought that the 79-83 2wd caliper was still a single piston and that as such, the 79-83 1/2 ton 2wd base model master would probably be close enough for the single piston caliper from 75-78. That’s why I chose that one. Are there 79-83 parts that I could substitute to make it work better? Or, Would it be better to substitute more 75-78 parts? I don’t think the 75-78 MC will fit the 79-83 booster easily, and I think it’s a serious chore to get the 75-78 booster to fit. That’s why I landed on the current combo. Anyone else tried different combinations?

    I think that after 8/74, these started to have front discs, and the master cylinder in the part catalog is different for those later months, with no mention of what booster for those months. Could I possibly use the original 74 booster with the later 74 master? It’s pictured with a 2 bolt mounting flange, which doesn’t match any 72-74 booster I’ve seen.
     
    Last edited: Apr 16, 2021
  16. jetas

    jetas Grand Toyotaholic

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    I pulled my master/booster from a '83 and it had the prop valve factory mounted to the mc/booster combo. And from what ive seen, the 79-83 trucks came with the same dual piston calipers that the 75-78 trucks came with
     
  17. THorn

    THorn Member

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    This is the caliper I have. CENTRIC 14144010 and CENTRIC 14144009. It’s the same caliper that’s listed for the 79-83 trucks. Am I incorrect in referring to it as a single piston? Here’s more of my ignorance showing. I actively looked for a oem prop valve for the 79-83 generation, but no mention of it in any parts catalog, no sign of one in any repair video, so I had started to think maybe it wasn’t part of the RN30 brake system. I only see prop valves start to appear on 84+ trucks, but of course I haven’t seen enough to know for sure. Perhaps they came out with the LSPV in 83?
    FA5F3FEC-DEE9-4EEE-BC10-EB9C96F7C9C2.jpeg

    The 4wd trucks starting from 79 use this. Is this a dual piston or is it called something else?
    6BB4BE59-B968-4AEF-9DB7-3DDA4B736965.jpeg
     
    Last edited: Apr 16, 2021
  18. jetas

    jetas Grand Toyotaholic

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    Nope thats a dual piston. Theres one piston in each half of the caliper. I know this may be dumb question but i think i had to do a double take when i reinstalled mine because both calipers look identical, but you made sure the bleeder was at the top of the caliper right?
     
  19. THorn

    THorn Member

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    Yes, bleeder at the top. Not such a dumb question. It’s a mistake I could possibly have made if I weren’t paying attention. I checked several times to make sure I had left and right correct.
     
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  20. THorn

    THorn Member

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    Is it possible that my remanufactured booster and master cylinder are defective?
     

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