OBS to NBS Master Cylinder Upgrade REPORT

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Coveman

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1998 Master cylinder piston:


As to more pressure, I would need the piston diameters to tell. Larger master piston will exert more pressure on the calipers and rear brake slave cylinders for the same amount of stroke on a smaller piston.

Basic hydraulics.

:)

Not so fast! A smaller diameter MC creates higher pressure at the caliper. :smile:

Two hydraulic cylinders interconnected
Cylinder C1 is one inch in radius, and cylinder C2 is ten inches in radius. If the force exerted on C1 is 10 lbf, the force exerted by C2 is 1000 lbf because C2 is a hundred times larger in area (S = πr²) as C1. The downside to this is that you have to move C1 a hundred inches to move C2 one inch. The most common use for this is the classical hydraulic jack where a pumping cylinder with a small diameter is connected to the lifting cylinder with a large diameter.

The OBS truck had a step, or two sized (something like 40 and 28mm) bore in the MC. This NBS MC swap gives us a single, or straight bore (33mm in my case; 1 5/16 inches).

On the step bore OBS MC, the primary bore pushes high volume up to a certain pressure then transitions to the smaller secondary bore and begins to develop pressure at a more rapid rate.

When CAFE standards for the OE's forced them to design more towards fuel mileage, low drag calipers became the norm. Low drag calipers have chamfered seals that retract the pistons further than necessary to make sure that they don't drag on the rotors which was supposedly good for 1-3% better mileage. Since the pistons have further to travel before making contact with the rotors, the quick take-up MC achieves this with the primary bore. The secondary bore gives us higher pressure but with a longer throw--That's the spongy feeling we had.

I'm still trying to figure out my next step, but ditching the low drag calipers seems to make sense. A larger caliper like from the heavy half-ton should create more pressure to the pads. The 1 5/16 MC that I installed gives me a very firm pedal, but I'm no where near locking up the wheels yet :think:
 
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nobears

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I did this in my 99 Suburban. Didn't help, and I went through literally a gallon and a half of fluid. Any ideas? Still feels spongy and my brakes are getting worse. Might add it has 1 new caliper, both wheel cylinders are new, new vacuum booster, new pads/shoes all the way around, new disks and drums. Thinking I'm going to warranty this master cylinder and pick up a new one, then swap 3/4 ton calipers on the front and put a new hardware kit in the back. For that matter is there a lo buck rear upgrade for the JB5 brakes? Bigger wheel cylinder or anythin

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great white

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deleted by OP.

Don't feel like getting into typing a novel on hydraulics.
 
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98_k1500

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Looks like coveman hit it pretty good as far as a basic understanding of the problem and how to fix it.
 

98_k1500

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But actually you are both right. 2 pistons of different sizes, moved the same stroke. The larger piston will create more pressure on the caliper, but will also require more effort at the pedal.
 

Coveman

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Nobears,

In addition to some squishy pedal, the dual bore MC on our trucks can be a pia to bleed. I swapped to the NBS master and did achieve a firmer pedal, but not sure it stops any shorter than before. I noticed that the newer trucks use a vacuum pump to assist the booster, currently I'm looking into that.

In the meantime have you seen this write up? http://www.gmt400.com/forum/showthread.php?17805-NBS-Front-Brake-Swap-for-OBS-Trucks/page1
 

nobears

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I glanced at it. Searching for the parts right now. Ended up disconnecting my ABS this morning due to a very weak pedal and even worse brakes since it kicked on my way home from work and it helped a little

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Coveman

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So the NBS MC was a little tricky to bleed. But after re-rebleeding the MC and lines I finally got to a good pedal. I think the adapter I got was partially to blame, it was a zinc coated steel jobbie and it made seating without stripping the MC a little tense, so I don't think it was tight enough and allowed some air in. I'd recommend a brass adapter for sure.

I also waned to share that I went ahead and did the Rocky Thorton NBS knuckle/rotor/caliper swap and BOOM, really good brakes! I have 18 inch wheels so I could fit the nnbs rotors as well, and for the money and a day and half work (...not that bad, I did my whole front end, balljoints, shocks, the works) its the first time this Burb could lock them up on pavement.

Since I had a leaky caliper and would have changed the rotors anyway this was basically a hundred bucks more (for the knuckles) than a stock brake job. If you're on the fence, the nbs MC and front brakes are the best upgrade you can get!
 
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Dakota_Don

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what vehicle did this master come from? a 2001 4x4 1500? which should be this fenco unit from the zone..Fenco New / Master Cylinder (Brake System)
Part Number:
NM3089
Warranty:
Lifetime Warranty
Application:
With JC5 brake systemNotes:
With reservoir
Master cylinder comes with a new reservoir installed.
With vacuum booster.
 

LiftedVictory

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Rich, good to see you still playing with your 'hoe. I never saw the swap done, glad to hear it works well. I'm gonna consider the swap if I don't sell the truck. Got a baby on the way and it's just not baby friendly.
 
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