OBS to NBS Master Cylinder swap discussion

[bowtiefreak]

Ok fellow GMT 400 owners. I want to start yet another thread about the infamous master cylinder swap / upgrade / downgrade whatever you want to call it. I would like to keep back and forth arguing out of this, no one needs to read through that crap.

What I'd like to accomplish is some feedback from the guys who have done this and how they currently feel about the swap. I'd also like to hear legitimate reasons not too.

Now I own two 1997 trucks. A tahoe and a 1500 suburban. Both have identical brakes other than the burb has the 11" rear drums. The tahoe stops much better than the burb. We have done a lot of chasing on the burb trying to figure out why and I haven't touched it in a while and the weather is warming up so time to start working again.

So, please chime in and lets get to talking about this. My goal is to get that big SOB to stop better. Rear brakes were just rebuilt, new front pads and rotors are due in next week....again. I go through some brakes around here.

---------- Post added at 08:17 AM ---------- Previous post was at 08:06 AM ----------

Mods please move, no idea how I put this here.

 

[laredo1307]

May I suggest new Brake hoses. If they are getting old and expand under pressure this could be causing some effects. I replaced my front pads and master cylinder/booster a while back. Even after 3 brake bleeds I don't think my brakes are working up to par. I just recently ordered everything to rebuild my front-in including new rotors and brake hoses so we'll see what that does.

 

[bowtiefreak]

I have all SS braided line already

 

[Gzes]

Did like with used Mc off a nbs. Works pretty good. I haul cars a lot so have the extra weight of a dovetail and car and it stops ok. It's not as powerful as I'd want it to be but might be that it's old and needs a new Mc. Pedal goes to middle then starts braking but isn't soft like it would be with obs

 

[bowtiefreak]

Good to hear, keep them coming

 

[Nak]

First off, I love your truck!!!!

Seems like you have two intents with this thread: 1. Discuss the NBS MC swap, 2. Diagnose your brake problem.

1. You know my feelings on the NBS MC swap. It's a Disc/Disc MC and won't work well on a Disc/Drum setup irregardless of brake feel.

2. So if I read this right, you're happy with the 2 doors brakes and want the 'Burb to do as well? Is it just that the 'Burb has a longer stopping distance, or is there a brake feel issue as well? Obviously the 'Burb will require somewhat longer stopping distance in a perfect world as it is heavier. OTH, the 11" brakes require both less fluid volume and pressure to activate compared to the 10" drums.

The absolute best thing to do would be to check the brake pressure at all four wheels. Unfortunately, I don't have access to the proper ratio at various pressures. If you can find that information, you can check the balance of your brakes quickly and objectively. Failing that, you could do some drive tests.

The first thing I'd do is test the brakes in a variety of conditions. Ideally on a deserted wet road. Be very careful, obviously. From 25 mph, apply the brakes hard enough to start a skid on one wheel only, the ABS should activate. Which wheel started to skid first? It should be a front wheel. Try the same test going downhill. It should be the front wheel still, but much closer to the rear going first. If it's a very steep hill you may even see the rear skid first. What you're checking is the balance of the brakes. The front brakes should skid before the rears. If the rears skid first your balance is off and your rig is dangerously unstable. Going downhill, you have more weight transfer to the front wheels, hence more traction. So the fronts should have less tendency to skid first. If the hill is steep enough, the rears are going to skid first, as more weight has transfered to the front wheels. If you disconnect the ABS, be very careful you don't spin the truck end for end...

What could be wrong? You could have the wrong bore wheel cylinders for one thing. You could also have the wrong combinatrion valve. The factory could have screwed that up, as the only marking is a sticker to differentiate different models. The ABS unit could be bad. Have you pressure bled your brakes following the factory procedure? I bled my brakes manually several times, still my brkes sucked. After pressure bleeding via the factory procedure my brakes are now rock solid and stop the truck exceptionally well... You could have a bad MC.

If your issue includes bad feel, try this: plug the output lines coming out of the combination valve. (It look like it is part of the ABSunit on your year, but is in fact replaceable.) If your brake pedal doesn't feel rock solid now, you have either a bad MC, a bad ABS, or a bad combination valve. Plugging the output of the MC would further narrow it down to either the ABS/Combination valve or the MC.

 

[SunlitComet]

would go with better pads and rotors first, then hydroboosting. if still not happy then upgrade to ssbc calipers and rear disc conversions but price will go high quick with much less gain as compared to first two options.

 

[bowtiefreak]

Thanks guys. Would the master off a 2500 with the JB7 brakes fit? They are set up for disc / drum right? Just larger calipers and rear wheel cylinders?

Mike can you tell me more about the bleeding process you used that seemed to help?

New pads and rotors are here. It's raining, so later this week I will throw them on real quick

 

[Nak]

I replied in the other thread about the MC...

The service manual has a procedure where you clip open several valves on the ABS and Combination valve, pressure bleed and use a scan tool to cycle the ABS. It's the only way possible to effectively bleed the ABS unit. I have a '92 which is a different ABS than yours, you'll need the procedure out of a service manual for your year. I don't have a scan tool, so I took it to a shop that did. They completely flushed the brake fluid and pressure bled the system. I made a copy of the procedure in the service manual for them to follow. My brakes immediately went from crap to brakes that would throw you against the seatbelt. I found out then that my C-valve was wrong as my rears locked up with very little brake pedal pressure. I changed the C-valve, had the brakes bled by the shop again, and now I have the best brakes of any truck I've ever driven.

 

[SunlitComet]

the kelsey-hayes ebc-310 used in our vehicles do not have such valves on the abs module that are held open. a depressing tool is used on the combo valve if pressure bleeding or nothing if using the two man method. both involve bleeding then four functional tests while holding the pedal then bleeding again.