Correct Master Cylinder

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Treb

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Anyone know what is the correct upgrade master cylinder for my 88 GMC K2500 (7200lbs) with 6 lug set up with front disc and rear 11"drum set up. I am still going to keep the rear drum brake setup with the 1 3/16" wheel cylinders and the 3.15 upgrade front calipers. I also going to delete the rear antilock brake set up.
 

Schurkey

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Anyone know what is the correct upgrade master cylinder for my 88 GMC K2500 (7200lbs) with 6 lug set up with front disc and rear 11"drum set up. I am still going to keep the rear drum brake setup with the 1 3/16" wheel cylinders and the 3.15 upgrade front calipers.
Define "correct". Defne "upgrade".

I expect there's more than one master cylinder that can work, but only if you don't have low-drag calipers.

Sounds like you started with JB6 brakes originally, then installed JB7 calipers? Is that correct?

I also going to delete the rear antilock brake set up.
Why?
 

Treb

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I installed a master cylinder from a 2002 Chevy Silverado 1500 on my 88 GMC K2500 6 lug (7200lbs) work out great.
 

Schurkey

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I installed a master cylinder from a 2002 Chevy Silverado 1500 on my 88 GMC K2500 6 lug (7200lbs) work out great.
JB6 brakes?

If so, the OEM front calipers on that vehicle are a low-drag design, intended for use with a three-chamber, "Quick Take-Up" master cylinder. You've installed a non-compatible two-chamber master cylinder with a too-huge cylinder bore.

You've thrown away hydraulic advantage you'd need if the booster fails.

Wild Guesses:
1. The front calipers are sticky, require more fluid pressure than they should to operate.

2. Problems in the ABS unit (or elsewhere) affecting the efficiency of the rear brakes.

The new master cylinder puts a band-aid over both problems, works better than the original master cylinder. But it doesn't work better than the original master cylinder if you'd fixed the REAL problem(s) before "upgrading" to an incompatible master cylinder.
 
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1998_K1500_Sub

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Even for those who have JB6 brakes on their truck, if the calipers aren't original you have to ask yourself "Do I really have Low-drag calipers on my truck?" (as noted in the attached Bendix / Allied Signal document), and, if you really do, "Are they working as intended?", which may not be the case. Finally, ask "If I buy JB6 replacements, will they actually be 'Low-drag' calipers?" (see attached document).

These are a few reasons why I simply replaced the JB6 front brakes on my Suburban and installed components (NBS / JB7) which were not, by design, "Quick take up / Low drag".

However, the NBS MC has a larger bore than another alternative MC, which is the GMT400 8600# MC. Both MCs are non "Quick take up". Using the latter would mitigate the loss of hydraulic advantage that @Schurkey mentioned above.
 

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  • Low drag calipers - Quick Take-up.pdf
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