JB5 JB6 JB7 JB8 Size Thread

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chipskittles

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The quick take up master was used on the JB5 and JB6 brakes with the 75mm dia piston calipers (low drag). The master for the JD6,JB7,JD7,jd8 are all the same Vacuum or Hydroboost and should work fine with the 80mm 3/4 ton caliper those calipers should be convention calipers i think the JD6 system used the low drag 75mm calipers . The whole low drag calipers and quick take up master has been used on chevy stuff since the early 80's my 83 caprice wagon had it but the brakes were very good and would throw you thru the wind shield if you looked at the pedal the wrong way not sure why it didn't work out on these trucks.

Many attribute the poor braking to the poor rear wheel anti-lock(RWAL) system that was used up until about 95 or so when they went to all-wheel ABS. There's more to it of course but I will say from personal experience that RWAL system does not age well and removing the system can make a world of difference to pedal feel/real world braking performance.
 

magnumb

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  • I have 1995 and it uses the 4WAL Anti-Lock system. Just changed out MS to a JB7 series without the quick uptake and JB7 calipers and the pedal is rock hard...crazy amount of difference.
  • The bench bleeding took about 1hr and found the trick was to push the cylinder at different angles and for whatever reason this got the last of the air out and the play is about 1/16. Without pushing at different angles i had 3/16 or better of play and almost threw it in like that.
  • Glad i didn't go with NBS master as i don't think it would have worked well for rear drums and i didn't have to use any adapters.
 
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shamrock246

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  • I have 1995 and it uses the 4WAL Anti-Lock system. Just changed out MS to a JB7 series without the quick uptake and JB7 calipers and the pedal is rock hard...crazy amount of difference.
  • The bench bleeding took about 1hr and found the trick was to push the cylinder at different angles and for whatever reason this got the last of the air out and the play is about 1/16. Without pushing at different angles i had 3/16 or better of play and almost threw it in like that.
  • Glad i didn't go with NBS master as i don't think it would have worked well for rear drums and i didn't have to use any adapters.
How is the braking?
 

magnumb

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Should have come this way from factory. Well worth the effort.
Put on about 90 miles today and they work great. Heat cycled them a few times and rear just starts to lock up when they are hot....but not full lock up. Although this might be due to my rear drums being warped. Trying to sort this out...new drums came warped and might have them cut is 2nd set is also warped.
I would say they probably stop 5-10ft shorter and pedal feel is a lot better. Seems like a little more foot pressure is needed, but not much.
This is my 4th GMT400 and should have done this on all of them.
 
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chipskittles

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I would be curious what your fuel economy does over time. Gm changed the calipers to begin with for fuel economy but does it really make that much of a difference? I don't expect much out of my truck in the way of fuel economy but I try to get as much as I can out of it as it is my daily driver.
 

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I thought of that also. I averaged 12.7 before the swap. I'll start recording my MPG again.
My quess is that they didn't use low drag calipers on the 3/4 & 1 ton because the EPA doesn't require the MPG to published.
 
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95C1500

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  • Glad i didn't go with NBS master as i don't think it would have worked well for rear drums and i didn't have to use any adapters.
Some 800s came with rear drums. I'm sure it would've worked fine as long as you bought the disk/drum mc
 

shamrock246

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This is a good read. He did not have good luck with the 3/4 calipers because it looks like he still had the Quick take up unit on there but once he put the master for the JD6,JB7,JD7 on there it was fine. From my research it looks like that master was used on calipers up to 85mm so it should handle the 80mm 3/4 with no problem. Also it looks like the Hawk SD pads made a nice difference.

http://www.gmc4x4.com/topic/119-spongy-brakes-testing/
 

evilunclegrimace

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You left out the JB3 systems that were standard light duty brakes on the 88-91 K1500 trucks. I'll see if i still have specs and post them later.
 
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