Brake upgrade help

[Garyg]

Ok guys, I need some help. I read all the info on the forums and went ahead with the NBS master cylinder upgrade. I have a 1988 GMC C1500 5.7, 700r4. I purchased Dorman M630031. I bench bled it and installed. I also installed all new brake lines in the front and rear, also replaced both rear wheel cylinders, brake rotors, pads, wheel bearings, rear brake shoes. I also bypassed the abs module. I bled everything again. Brake pedal is definitely firmer. No more sponginess feel. However if you do a panic stop, the brakes will not lock up no matter how hard you stomp on the brakes. I went over everything again, re bleeding etc. Still won’t lock up the brakes under hard braking. I’m stumped and look to the GMT 400 group to put me in the right path.

 

[df2x4]

FWIU the NBS master cylinder "upgrade" can actually be a downgrade for many people, for exactly the reason you mentioned. Improved pedal feel but decreased stopping power. I don't recall the science behind it but I believe it's something to do with a difference in bore sizes and how they interact with the "quick take-up" calipers that some of these trucks are equipped with.

Paging @Schurkey, he knows the details.

 

[Supercharged111]

The only 400 trucks I've driven that will lock the tires had hydroboost.

 

[df2x4]

Both of my '97s still have the ABS intact so they won't lock up per se, but I've had the ABS engage and chirp the tires on dry pavement in both of them. JB5 and JB6 brakes respectively, no hydroboost.

 

[Garyg]

FWIU the NBS master cylinder "upgrade" can actually be a downgrade for many people, for exactly the reason you mentioned. Improved pedal feel but decreased stopping power. I don't recall the science behind it but I believe it's something to do with a difference in bore sizes and how they interact with the "quick take-up" calipers that some of these trucks are equipped with.

Paging @Schurkey, he knows the details.

Just thinking out loud, do you think the brake booster is bad? Should I put the old master cylinder back on?

 

[df2x4]

Just thinking out loud, do you think the brake booster is bad? Should I put the old master cylinder back on?

Not sure about the booster, but if you still have the old master cylinder and it was behaving properly when you removed it I would try reinstalling it for sure.

 

[Garyg]

Not sure about the booster, but if you still have the old master cylinder and it was behaving properly when you removed it I would try reinstalling it for sure.

So what is the cure for spongy feeling brakes?

 

[1998_K1500_Sub]

I ... went ahead with the NBS master cylinder upgrade. I have a 1988 GMC C1500 5.7, 700r4. I purchased Dorman M630031. I bench bled it and installed. I also installed all new brake lines in the front and rear, also replaced both rear wheel cylinders, brake rotors, pads, wheel bearings, rear brake shoes. I also bypassed the abs module. I bled everything again. Brake pedal is definitely firmer. No more sponginess feel. However if you do a panic stop, the brakes will not lock up no matter how hard you stomp on the brakes. I went over everything again, re bleeding etc. Still won’t lock up the brakes under hard braking. I’m stumped and look to the GMT 400 group to put me in the right path.

@Schurkey can likely help you best.

If you happen to have "quick take-up" calipers on your truck, that may be your issue as @Schurkey will likely clarify.

I have the NBS MC as well but use larger-piston RPO JB7 front calipers which are not a "quick take-up" design. This is an option you may consider.

See attached .pdf for one explanation of the "quick take-up" issue.

For the record, I also have JB8 1.1875" rear cylinders on my 11" drums, using my Suburban's stock JB6 vacuum boost, ABS and combination valve. This combination IMHO works very well.

 

[df2x4]

So what is the cure for spongy feeling brakes?

In my '97s the thing that helped the most was a full bleed of the system including the ABS module using a Tech2 scanner, but considering you don't have ABS anymore that's not going to help you.

You said you installed new lines, did you use braided stainless replacements for the factory rubber soft lines? If not, those will help your pedal.

 

[Supercharged111]

The guts of the ABS pump can supposedly cause a spongy feeling so I trashed it entirely. I never had a spongy pedal after that, a flush, and a brake adjustment, but the brakes always sucked. They sucked less when I put Hawk HP+ pads up front, they sucked less when I put a 14 bolt out back, they sucked less when I did the 8600# gvw calipers, less when I did the GMT800 master, and finally good when I hydroboosted all of that with the matching GMT400 master for the non-quick take-up brakes.