'91 k2500 NBS master cylinder swap

[ryanguy426]

Question about NBS master cylinder swap:

I have a '91 k2500, vacuum boost, disc/drum, 6 lug. Current MC needs replacing. Reading online, I need a master cylinder from an NBS of the same type of truck, correct? So I should get a master cylinder from a 2001 Silverado 2500 and it should bolt right up?

Just a little confused, as the bore diameter of the stock OBS master cylinder is different than OBS. Just want to make sure it'll fit. I know I'll need an adapter for the lines.. purely asking about the master cylinder.

OBS 2500 bore diameter: 1.125"
NBS 2500 bore diameter: 1.456"
NBS 1500 bore diameter: 1.338"

 

[Gibson]

Somebody with more knowledge of the finer details will probably chime in, but IIRC, the NBS K2500s all used a Hydroboost system instead of a vacuum booster, and they use 4-wheel discs.
And of course the combination valve and the way it interacts with the ABS is different.
What might you hope to accomplish by changing the MC to a later style that was never intended to be used on a disc-drum system.

 

[ryanguy426]

What might you hope to accomplish by changing the MC to a later style that was never intended to be used on a disc-drum system.

Same thing every other person who does the MC swap is hoping to accomplish - Fix the squishy brake pedal/impossible to bleed brakes.

 

[Schurkey]

1. Stop with the "OBS" and "NBS". NBS becomes OBS when the NNBS comes out. How about GMT400 and GMT800?

2. No one has been able to tell me what the magic difference is between the GMT400 and the GMT800 master cylinders. They look different, how, exactly, do they function differently?

3. Does your truck have low-drag front calipers? Do you have a Quick-take-up master cylinder? Low drag calipers and a standard master are a very bad combination. Low drag calipers need a large "gush" of low-pressure fluid to take up the clearance between pad and rotor, with the actual braking done with high-pressure, low-volume fluid as normal.

Low drag caliper has a step in the casting towards the rear; the booster end of the master has an enormous bore and similarly-huge piston to move a lot of fluid at low pressure. If that piston seal leaks, the pedal is going to be low.

You must be registered for see images attach

4. A larger master cylinder bore makes for a higher pedal...but less hydraulic leverage. Before you screw with the hydraulics, you need to ask yourself if Mom or daughter ever drive this truck.

5. Given that this is a K2500, I'm assuming that you don't have the terrible K1500-style leading/trailing shoe rear brakes. Are the rear brakes adjusted properly? Not worn-out?

6. Does the '91 have the same Kelsey-Hayes RWAL system that my '88 has? Have you ever bled the anti-lock itself? Does the anti-lock actually work? Solenoid valves in the anti-lock can be stuck or leaking, leading to low, squishy pedal.

 

[98 Nitro]

I thought it was just the 95+ trucks that had the crap pedal. My 94 has much better pedal feel than my 98 and stops a little better too.

 

[Schurkey]

NONE of them have a "crap" pedal...if the brake system is working properly.

 

[Markmartin01]

01 MC will not work on 90's model, I have a new 01 MC on the shelf from a failed attempt. For that to work you have to also change to the 01 calipers. I ended up with a hard pedal and no brakes. I ended up going back with a Cardone Mexican reman MC since the new chinese ones are garbage. I also went with the small bore since it's a light duty half ton. Removed the ABS mess and threw that away. Also put in an 01 disc rear end. Brakes work great now.

 

[east302]

01 MC will not work on 90's model, I have a new 01 MC on the shelf from a failed attempt. For that to work you have to also change to the 01 calipers. I ended up with a hard pedal and no brakes. I ended up going back with a Cardone Mexican reman MC since the new chinese ones are garbage. I also went with the small bore since it's a light duty half ton. Removed the ABS mess and threw that away. Also put in an 01 disc rear end. Brakes work great now.

Same experience here, including a MC sitting on a shelf. I had a higher pedal but had to really press on it to hold the truck on a downhill incline. I went back to the stock master cylinder.

 

[98 Nitro]

NONE of them have a "crap" pedal...if the brake system is working properly.

Ya I agree, I need to get a scan tool that does the ABS bleed for the 98.

 

[RI Chevy guy]

Question about NBS master cylinder swap:

I have a '91 k2500, vacuum boost, disc/drum, 6 lug. Current MC needs replacing. Reading online, I need a master cylinder from an NBS of the same type of truck, correct? So I should get a master cylinder from a 2001 Silverado 2500 and it should bolt right up?

Just a little confused, as the bore diameter of the stock OBS master cylinder is different than OBS. Just want to make sure it'll fit. I know I'll need an adapter for the lines.. purely asking about the master cylinder.

OBS 2500 bore diameter: 1.125"
NBS 2500 bore diameter: 1.456"
NBS 1500 bore diameter: 1.338"

Let me start by saying I have a '96 C1500; I know the brakes changed around the time your truck was built and were differentiated by the front rotor thickness (old=1" rotor thickness, new=1.25" rotor thickness), that being said if you have the newer brakes I know the MC swap will work. I should also include when I did my brakes I did everything; '01 2500 mc, '96 3500 calipers, '96 2500 pads and retained the stock rear drums (for now) and a fully functional abs system (we have inspections here). The 3500 caliper bolts right in and has a greater piston surface area and takes advantage of the higher fluid volume from the 2500 mc, the 2500 pads fit the rotors on our trucks but the spring tabs on the back will need to be spread to fit snug into the caliper piston bore. I've included the info on the front components to make it clear I don't know that just the mc swap will solve the problem. The 800 mc has a larger single bore instead of the 400 stepped bore which increases the fluid volume to both front and rear, but know that the proportioning valve will keep the pressure to the rears under control to not blow out the wheel cylinder. Inversely, if you planning the rear disc conversion, the proportioning valve will need to be modified to allow the greater pressure to the rears as well, again taking full advantage of the rear calipers. Hope this helps