85 C20 Master Cylinder replacement - what brake system?

[mysticdragon72]

Hello all,  I am new here and have a question about replacing the master cylinder on my Scottsdale.  It's an 85 C20 3/4 ton and has been sitting for about 7 years... the master cylinder was leaking before we took it off the road so needs replacing but when we went to the parts store they asked us what brake system is on the truck?  The guy told us that the info would be on the sticker inside the glove box... well the only info in there is all about towing capacity and max. load ratings.  I have done a search on here and the one thread I found gave me a little info but I'd like to clarify with y'all first.  According to the info in the thread I found the truck should have a JB7 brake system but on the master cylinder it has 1 1/8 which I'm assuming is the bore size and according to the parts listing online that is for the JB6 brake system.  Could it have the wrong size on it?  No history on the truck other than it was a farm truck before we bought it soo...   Any advice?
TIA for any replies.
Mystic.

[81GMCBILL]

i believe there wanting to know if its a "conventional" or "hydro-boost" system.

[bake74]

     First off welcome to the site. 
     1985 is the first year they started having hydro-boost brake systems on our trucks.  Unfortently we do not have a 85 manual in our tech section.  But here is a link to where you can look up the 84 and 86 manuals, look in section 5 of both manuals and it will be very clear by comparing to yours if you have mechanical or hydro-boost.

[Irish_Alley]

diesels come with JD3, JD5, JD6 and JD7
gassers come with  JB3, JB5, JB6 and JB7
now here's the kicker for gassers if its a plastic reservoir its a  JB3, JB5 or JB6
if its a cast reservoir its a JB7

If your a gasser you dont need this info but if its a Diesel then read on
if you reservoir looks like this

then you have a JD3, JD5 or JD6
if it looks like this

then its a JD7

JD7 (for diesles) and JB7 (for gassers) use the same cylinder
other than that gassers use plastic besides the JB7

[mysticdragon72]

Thanks for that!! 

We were thinking it was a JB7 but weren't 100% positive.

Our beast has the cast reservoir so I guess that's it?  Why would the numbers in the bottom of the master cylinder have 1 1/8 though?  Isn't that the bore size? 

One other stupid question if y'all don't mind.  In one catalog it says we have to measure the rear brake shoe to tell what brake system we have? 

Thanks again for all the replies!
Mystic.

[Irish_Alley]

you can measure also to confirm but at the parts store they should be able to tell what size it is on their screen. but since the cast reservoir  was only used in the jb7 on a gasser then it has to be it.