How to: NBS master cylinder swap for firm brake pedal

[Nak]

Hey Dan!

No worries.

My 14 bolt swap went like this:
1: Swapped the rear end out. Woo Hoo! Love the Trutrac.
2: Realized the back brakes weren't working well at all.
3: Bled the brakes again.
4: Still no rear brakes.
5: Got the truck pressure bled. Brakes were now rock solid, but the rears were overpowering the front.
6: Researched the hell out of my brakes.
7: Replaced the Combi valve, pressure bled the brakes.

The brakes are great now. Nice and solid; much less effort to apply the brakes than the originals.

To be honest, I may even yet go with a smaller wheel cylinder in the rears. I need to get out on a slick surface and test the brakes. Pro's would be more braking stability and less pedal pressure required. Con's would be more wear on the front brakes, more heat in the front brakes, and less total braking capability while towing a heavy load. Go too small in the rear and you reduce total braking capability even under normal driving. Ideally, the fronts should lock up just before the rears under all driving conditions.

 

[nomad]

Hey Nak,

Checked the rears today with the drums off as you suggested. They appear to be working, bolted the drums back on and did a little power braking with the rears off the ground and vehicle in drive. They seem to hold very well. Of course this doesn't simulate a 5k vehicle trying to stop. Bled again, got no air. They do stop but dont feel like the should. Actually took it out in the snow today and locked them up, fronts locked well before the rears and ABS took over.
They still suck.

Got a handle on the prop/combi 12548265, It should be the correct part for my configuration and the correct part for either Vacuum or Hydroboost. I think I am going to order it.

I really wonder if the original mc is bad, this really has me thinking if its the source of the mushy pedal more then anything else.

Side note, strike 3 trying to find a complete hydro boost setup. I can get pieces and parts but not a matched setup. I found a complete system from a 2000 K3500 HD but it doesn't have ABS and is a tandem axle truck. I passed on it.

It would have been nice to find a complete system (with the abs unit) a nice bonus is that it would have had the correct valve.

Cool that you have the Trutrac rather than the Eaton Posi. In retrospect if I could do it again I would hav gone that way instead of the Eaton with carbon clutches, still beats the hell out of the SuckLock!

The saga continues...
Dan

 

[SunlitComet]

Tandem axle? That I have got to see. The never came out of the factory like that. Only medium duty trucks and bigger do.

Nomad base your component acquisitions on an 1996-1999 OBS k2500 burb with diesel or big block options that had a 4l80e tranny on them. I need to replaced the hydro I have now becuase of leaks but otherwise a complete match is done. I will likely not change cylinder size in the rear because it will go disk eventually.

 

[Nak]

Something is wrong still. I'd say get a couple of pressure gauges to check pressure at the bleeders. They're actually pretty cheap on Amazon. I seem to recall seeing them for around $30?

Did you ever get pressure bled? I didn't have any air coming out either, but my brakes were crap until I pressure bled them. If that doesn't fix it, it has to be either your Master or your ABS. I wouldn't change the Combi valve until you track it down, but I guess if you're going to flush & pressure bleed anyway you might as well.

 

[nomad]

Tandem axle? That I have got to see. The never came out of the factory like that. Only medium duty trucks and bigger do.

According to the RPO the HD3500 I mentioned was originally a big dually with a snow plow option that had been modified. Either way no abs and it would not work for me.

Nak,
I am going to look at picking up a pressure guage today. A friend mentioned to me a "pressure bleeding kit" was available from Harbor Freight, doubt I will try it, HF stuff is usually crap and falls apart the first time you use it.

Dan

 

[SunlitComet]

Dual rears wheels? Okay that is different then. Tandem axles are two individual axle assemblies one in front of the other as seen on bigger trucks.

How do you know that a k3500 non-abs parts would not work. You will be surprised to find that the MC and boosters are often alike thru most apps. Even the hydro-block of the abs is typically the same. The difference on them is what control box is mounted over it. So you can likely get used parts from that first truck and look for another k3500 with a different body but same weight classification and just get the abs control box while missing everything else. Therefore building a set.

---------- Post added at 07:22 AM ---------- Previous post was at 07:19 AM ----------

I should note that it is not really the size of vehicle you need to get parts from it is the rpo code for the brake system you need to research. jb6 jd6 jb7 jd7 jb8 jb5 jd5 for example.

 

[nomad]

Dual rears wheels? Okay that is different then. Tandem axles are two individual axle assemblies one in front of the other as seen on bigger trucks.

How do you know that a k3500 non-abs parts would not work. You will be surprised to find that the MC and boosters are often alike thru most apps. Even the hydro-block of the abs is typically the same. The difference on them is what control box is mounted over it. So you can likely get used parts from that first truck and look for another k3500 with a different body but same weight classification and just get the abs control box while missing everything else. Therefore building a set.

---------- Post added at 07:22 AM ---------- Previous post was at 07:19 AM ----------

I should note that it is not really the size of vehicle you need to get parts from it is the rpo code for the brake system you need to research. jb6 jd6 jb7 jd7 jb8 jb5 jd5 for example.

I understand all of that, but here are the issues, the junk yard provided the vin and I ran it and came up with JB8 and dual rear wheel 13inch drum ff14 rear. This 3500HD also WAS NOT equipped with ABS which I need. I ran all of this info to my Chevy dealer. He also confirmed the 3500 JB8 used a different Hydro boost unit then the JD6.

What I wanted was the complete setup, hydroboost, MC, ABS unit, pedal, rod and from a vehicle with RPO code JD6. This way all the components (including the combi valve) would be the correct combination for my configuration. Short of having to possibly rebuild some of the component I would be golden. Considering the junk yard told me "tandem axle" with a snow plow this just made me dismiss this donor vehicle quicker. Don't know if they were confused, used the wrong term or what but considering it had no ABS and was a JB8 it really made no difference to me, I didnt want it.

I have not given up on the hydro boost option, I am going to continue to look for the setup I need. At the same time I will see if I can get the stock system to work correctly. At this point I think I have a good handle on what I need, I just need to find it!

Dan

 

[SunlitComet]

Understood. Mine will have for now wheel cylinders based on the stock jb6(corrected)11" drums, booster based on jb8 and MC based on jd5, 6, 7 and jb8. You can intermix if you do it right. The ABS will be left untouched as I don't want it to think it weighs more then it actually does. Do need to replace a sensor tho when the rain stops.

 

[saif najd]

I am sorry to tell you all that I well not do this mod because I don't think my Tahoe well survive

I am sick and tired Of this brake problem I am facing

Thanx and sorry

Sent from my GT-I9505G using Tapatalk

 

[SunlitComet]

How so?