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1/2 or 3/4 ton calipers on rear disc brakes? Hoses?

Discussion in 'The Garage' started by TruckNutzDude, Nov 22, 2005.

  1. TruckNutzDude

    TruckNutzDude 1/2 ton status

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    Is there a difference between the 1/2 and 3/4 ton calipers when converting a 14 bolt to rear disc? I just purchased a DIY4X bracket kit and have the pins and rotors but haven't gotten the calipers yet because I'm not sure what to use. I know they'll both mount up but I was wondering if the bores were the same? I would think with the 1/2 ton calipers the bore would be smaller and therefore give me similar braking characteristics as the original wheel cylinders did (using less fluid) But at the same time I thought that might be bad since discs require less effort to controll the wheels. What would you reccomend?

    I'm also wondering what hoses to use. I was thinking some 2wd s-10 hoses would bolt up without much of a problem, they look like they use the same diameter banjo bolt as the full size calipers.
     
    Last edited: Nov 22, 2005
  2. divorced

    divorced 3/4 ton status

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    It was posted recently that the 3/4 ton calipers have a slightly larger piston, but not by much. I used front hoses for a '79 Chevette when I did mine. They are about 12" long and the same part number for both sides.





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  3. WARP1

    WARP1 Registered Member

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    I just put this in another thread. I can't remember if I got 3/4 or 1/2 ton calipers. I used hoses designed for the front brakes ('74 3/4 ton 4x4). Napa and AutoZone had them in stock. They were too long but I just used a zip lock tie to keep them out of the wheels.
     
  4. cybrfire

    cybrfire 1 ton status Vendor GMOTM Winner

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    Actually there are two different part #'s for the 3/4 ton caliper. Difference being piston size. I believe the smaller piston is identical to the 1/2 ton part #. That being said I always use the smaller of the two pistons to help avoid the need of a proportioning valve.
     
  5. beater_k20

    beater_k20 Banned

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    so when converting, if you use 3/4 ton rotors, 1/2 ton calipers, and 1/2 ton pads, you dont need a different proportioning valve? :confused:
     
  6. ntsqd

    ntsqd 1/2 ton status

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    I keep reading that there are two different sizes and I couldn't recall seeing them in my Raybestos brake parts book. So I looked it up. Bear in mind that my books stop at '93.

    '71-'91 2 & 4wd Blazers are all listed as having a 2-15/16" caliper piston.

    '72-'91 C, G, P, & K10's all have a 2-15/16" caliper piston. There are some exceptions with the K15's in the '88-'91 years. No size listed for 71's

    '71-'93 V, P, G, C, & K20's and 25's are all listed as having 2-15/16" caliper pistons EXCEPT for those 13x2.5" rear drums (JB7) which have an 80mm (3-3/32") caliper piston. The '89-'93 K25 w/ 8600lbs GVW and the '92-'93 K25 Sub's also have the 80mm piston.

    There is a catch with the 80mm piston calipers, though, and that is they use a different pad. All caliper pad backing plates are issued an ID number, the FMSI number. The pad used in the 2-15/16" caliper is a "D52", the pad used in the 80mm is a "D153." So then I dug out my ancient Autospecialty catalog as it has illustrations of the different FMSI numbered backing plates. Their illustration of the D153 plate looks nothing like the D52 plate. Unfortunately their D52 illustration looks nothing like the common GM pad so I've no conclusion. A search for the illustrations online has yet to return anything of use. Because of this I have no info on if the 2-15/16" caliper and the 80mm caliper interchange or not.

    Most heavy stopping is done by the front brakes unless you have a heavy load in the bed. Too large of a rear caliper piston will cause the rear brakes to lock up b4 the fronts do. To cure that you can either dial out some rear brake operating pressure with a proportioning valve, or you can start with a caliper that has a smaller piston than the front caliper pistons.
     
  7. 79k20350

    79k20350 3/4 ton status

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    not that you dont need it just that you might not need it. the smaller piston will help with reducing clamping forces therefore in some applications reduce the need for a proportioning valve
     
  8. cok5

    cok5 Registered Member

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    Not trying to hi-jack here but what other brake lines did you guys use in the rear? The Chevette ones look nice!
     
  9. TruckNutzDude

    TruckNutzDude 1/2 ton status

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    Thanks for the information guys. I was going to use the 1/2 ton calipers (smaller bore) for the same reasons you stated but I wasn't 100% sure it was the right way to go. I appreciate the help as usual. :D
     
  10. ntsqd

    ntsqd 1/2 ton status

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    The 1/2t & 3/4t calipers are mostly the same in bore size. I don't know what others have run successfully, but my inclination would be to use the "Metric" caliper with it's smaller 2.5" piston. Though that takes a different bracket than the 1/2-3/4t calipers use. My reason is about brake balance.

    One thing I've noticed is that short WB vehicles are more tolerant of near matches in piston size. The longer the WB, the more intolerent of too big a rear caliper piston they get.
     

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