How to: NBS master cylinder swap for firm brake pedal

[saif najd]

mine does that think it may not be too good. will replace with new one eventually tho

Thats the way I like it and how it should be thank you for confirming

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[Nak]

There's another option for a MC that should be considered as well. (For the Vacuum boost setup.) The bore sizes for different MCs are:

OBS Vacuum Assist MC: 28.6mm
OBS Hydroboost MC: 31.8mm
NBS Vacuum Assist: 34mm.

AND a ACDelco (19176354 ; 18030606; 18M364) Three part numbers for the same thing it seems. The bore for this MC is 31.8mm. Plus, GM lists this as the appropriate MC replacement for a '92-'94. The original was 28.6 in those years, but the ACDelco replacement is 31.8 for vacuum assist. Looks just like all the OBS MCs and has the proper fitting thread sizes. I've not tried this MC yet, but it seems like it might be a good choice as well?

Also, I just completed swapping in a 14 bolt SF rear end. This bumped my '92 Blazer from 10" drums to 11". My brakes have always been typical OBS sponge before, but now they feel great! I couldn't get the ABS to activate on pavement before even with both feet on the pedal. Now, moderate pressure with one foot and the ABS goes nuts and I'm stopping a LOT faster than before. I've knocked at least 10 or 15 feet off my 40-0 distance. The brakes no longer go nearly to the floor. Maybe there was air in the lines before, but the brakes felt squishy like that since the truck was new. My first thought on the new brakes was that the rears weren't engaging strongly enough, possibly causing the ABS cycling the fronts. But there's no sign of that after repeated tests. Stopping distance is much improved, hardly a sign of the rears not engaging. Also, there is no "nose dive" tendency at all, and there should be if the fronts were doing all the work.

Anyways, I though this information might help someone looking at improving their OBS brakes.

 

[nomad]

NBS Master Cylinder Conversion

After reading all the success stories of the nbs MC in an older Tahoe I thought I would try it also. Before I get into the details let me describe my configuration.

97 2 Door 4wd Z71 with 4 weel anti lock Vacuum assisted brakes, 14 bolt SF diff with 11 inch drums front, standard disks with standard roters and calipers. Standard tires and wheels, not lifted. The new MC was for a 01 Silverado and matches all the specs mentioned in this thread.

Since I put all new everything in the rear brakes when I did them approx 5k ago I thought I would redo the ones in front at the same time I did the MC upgrade. I turned the front rotors, put in new semi-metallic pads, replaced the MC, bled the system and went for a test drive.

My results were less than stellar, true the pedal was much harder but the vehicle did not appear to stop as well, in fact it seems it requires more pedal pressure to get the same stopping force of the old MC.

So, whats the problem? Every thing is new including the fluid. Should I have drilled the brass adapter? I really dont understand it.

One thing I wanted to mention is that I measured the inner diameter of the new MC, it was 34mm as listed in a prior post. Now the interesting thing, the old MC measured 40mm. Granted I did not completely remove the piston, just checked it from the outside.

Any Ideas?

 

[SunlitComet]

Possible the vacuum assist is different for the nbs m and maybe not strong enough or something is wrong with existing one?

 

[saif najd]

Possible the vacuum assist is different for the nbs m and maybe not strong enough or something is wrong with existing one?

I am extremely happy to see you around

I think (from what I am facing right now) ur problem is the ABS!

I did not do the this mod (I am waiting for final results) but I tell what happened with me

I did the new pad change and caliber fix for both front ends and flushed the brake fluid and replaced the vacuum and the unexpected happened!

The truck was extremely heavy and after checking we found out all 4 wheels are locked the brake was engaged!! We checked every thing and later found it was dew to the ABS!!!

still we didn't solve it 100% some times the ABS engage it self on low speed!! And when I apply the brakes I feel the truck dancing right and left.

So advice check the ABS

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[nomad]

Now questioning if it was worth it

I tested the Vacuum assist unit this morning and it is working as it should. It really bothers me about the 40mm bore size of the old MC. I was expecting to see a really small piston. This really makes me wonder if perhaps the Z71 used a larger MC then the standard one. If this is the case I doubt the NBS MC will ever work correctly.

As far as the 4w ABS unit is concerned it is not giving any ABS error lights on the instrument panel and I did verify the unit was active by jacking all 4 wheels of the ground, placing the trans in drive and activating the brakes.

Later today I am going to hook up the GM Tech 2 tool to the Tahoe cycle and diagnose the ABS unit . Maybe there is some air in the unit but I would think that would give me a mushy pedal instead of one that is hard but has limited braking.

If I get no where with that I am going to re-bleed the system and drive it for a week or so to allow the new front pads to wear in before trying some agressive braking.

Other then that if all else fails I plan on putting the old MC back in place and looking at a Hydroboost setup down the road which from my estimates will cost me about $400. for all new or rebuilt parts.

 

[SunlitComet]

I tested the Vacuum assist unit this morning and it is working as it should. It really bothers me about the 40mm bore size of the old MC. I was expecting to see a really small piston. This really makes me wonder if perhaps the Z71 used a larger MC then the standard one. If this is the case I doubt the NBS MC will ever work correctly.

As far as the 4w ABS unit is concerned it is not giving any ABS error lights on the instrument panel and I did verify the unit was active by jacking all 4 wheels of the ground, placing the trans in drive and activating the brakes.

Later today I am going to hook up the GM Tech 2 tool to the Tahoe cycle and diagnose the ABS unit . Maybe there is some air in the unit but I would think that would give me a mushy pedal instead of one that is hard but has limited braking.

If I get no where with that I am going to re-bleed the system and drive it for a week or so to allow the new front pads to wear in before trying some agressive braking.

Other then that if all else fails I plan on putting the old MC back in place and looking at a Hydroboost setup down the road which from my estimates will cost me about $400. for all new or rebuilt parts.

Air in the ABS module or jammed solenoid in there is a strong possibility. Cycle them like you said. If one is stuck that may fix it.

I will be installing a used hydro booster from a 96 k2500 bar this weekend and for now it will use a new MC for the same application.

 

[nomad]

I will be installing a used hydro booster from a 96 k2500 bar this weekend and for now it will use a new MC for the same application.

Please keep us advised on your results, I might have to go the same route and really want to know if it was worth the effort.

Thanks for the reply!

 

[saif najd]

Air in the ABS module or jammed solenoid in there is a strong possibility. Cycle them like you said. If one is stuck that may fix it.

I will be installing a used hydro booster from a 96 k2500 bar this weekend and for now it will use a new MC for the same application.

May I ask why the hydro booster? Not vacuum

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[SunlitComet]

Faster linear movement of the brake rod, higher force pressure in depressing the MC and the brake booster rod is longer so it contact the MC piston quicker.