First hand experience with changing to the NBS master cylinder WOW!

[400sbctruck]

Thank-You!!! Awesome thread, I have driven many of these trucks, and this always bothered me.

This WILL happen ASA I need another M/C.

Curious about the NBS 13" vs slotted replacement(I can drill
And/or slot at work).

[speedygonzales]

Ok I have JB5 system and lookup says they are 6400 Lb.
Here's a picture
http://i1371.photobucket.com/albums/...g?t=1403961476

Only thing I see is the "DA" sticker. So what do I have? And how do you figure it out? I didn't see anything that explains the types of diaphragms. How does the types relate to the RPO code?

I see JB8 says 10000 Lb That must be Hydraulic.

I found this on a bulletin board
JB5= front disc 11.86X1.29 rear drum 11.15X2.75 Vacuum-Double Diaphragm

JB7= front disc 12.50X1.54 rear drum 13.00X2.50 Vacuum-Double Diaphragm

JB8= front disc 12.50X1.54 rear drum 13.00X3.50 Hydraulic-Hydro-Boost

But how do you get part numbers for these boosters.

Then there is this:
http://www.oreillyauto.com/site/c/de...1305&ppt=C0066

It says it's 10.75" and used for JB5,6&7???? I don't get it.

Do I want the one that is 13" x 2.5? Do you look for the booster from a 1 ton truck?

[CDR]

Hey Speedy

I have JB6 brakes and believe I have the single diaphragm booster...With an HA sticker on it

Vacuum-boosted:
JB5 = Power Brakes, Disc/Drum, 6,400 lb
JB6 = Power Brakes, Disc/Drum, 7,200 lb
JB7 = Power Brakes, Disc/Drum, 8,400 lb
JB8 = Power Brakes, Disc/Drum, 10,000 lb

Hyrdaulic-boosted:
JD5 = Dual Power Brakes, Disc/Drum, 6,400 lb
JD6 = Hydraulic Power Brakes, Disc/Drum, 7,200 lb
JD7 = Hydraulic Power Brakes, Disc/Drum, 8,400 lb
JF9 = Hydraulic Power Brakes, 4-Wheel Discs

I believe the JB7 called for the larger DD booster you linked to. There is also a SMALLER DD booster available... Is that what you have on your JB5's?

[speedygonzales]

Originally Posted by CDR

Vacuum-boosted:
JB5 = Power Brakes, Disc/Drum, 6,400 lb
JB6 = Power Brakes, Disc/Drum, 7,200 lb
JB7 = Power Brakes, Disc/Drum, 8,400 lb
JB8 = Power Brakes, Disc/Drum, 10,000 lb

Hyrdaulic-boosted:
JD5 = Dual Power Brakes, Disc/Drum, 6,400 lb
JD6 = Hydraulic Power Brakes, Disc/Drum, 7,200 lb
JD7 = Hydraulic Power Brakes, Disc/Drum, 8,400 lb
JF9 = Hydraulic Power Brakes, 4-Wheel Discs

There is also a SMALLER DD booster available... Is that what you have on your JB5's?

I have no clue All I saw was "DA" and it measures best I can see at between 10 and 11 inches.

Now above you say JB8 was vacuum but all the information I saw was it was Hydro. If it is vacuum, I want it.

[CDR]

Originally Posted by speedygonzales

I have no clue All I saw was "DA" and it measures best I can see at between 10 and 11 inches.

Now above you say JB8 was vacuum but all the information I saw was it was Hydro. If it is vacuum, I want it.

And I think you're right about the JB8, a little more looking and I found:

JB8 76 88 BRAKE SYSTEM,HYD PWR DISC/DRUM

So they were hydro-boost systems installed from 1976-1988. Sorry about that other RPO lookup, I got that from another post online.

[Tom]

Yes all JB8's were hydroboost. I have the JB5 and have the dual diaphram stamped 'HA' like rockauto said I would. Not sure where the single diaphram is used....

[CDR]

Originally Posted by CDR

I have JB6 brakes and believe I have the single diaphragm booster...With an HA sticker on it

Strike that… it has a DA sticker on it. It's the 10.5" dual diaphragm I believe, it's 10.5 inches wide by 5.75 inches thick from the MC to the firewall.