85 k20 master cylinder??

[tigwelder]

replacing  master cylinder on my 85 k20. called several auto parts stores and they say i should contact GM to find out what it had on it from factory because they list 4 different  part numbers.has anyone tried calling a dealer and ask for info on a truck that is older then new? it has the cast steel on it now and they all say its hard to tell what it originaly came with.

[bake74]

     That is why I am so glad I found "dan the man" ( is what I call my local auto parts guy I buy from ), he knows what he is doing and will take the time to research what I need.  I ordered the rebuild kit for my 77 k10, he got it and it was the right one.
     Is there any cast numbers in your pump that you could cross reference it too with the auto parts store ?

[PromiseKeeper]

are you fortunate enough to still have the sticker on the glove box door? That should help.

[snibook7537]

This might have to do with the GVWR of the truck the smaller rating would have smaller brakes, bigger rating larger breaks.  I know there is somewhere on the Internet you can look them up. The Hydroboost masters are JD and vacuum boost ones are JB. Then the number is associated with the GVWR.  I believe the higher the number the more they are designed to stop.

The Vin options from a   truck for GVW's are as follows:
C=4001-5000
D=5001-6000
E=6001-7000
F=7001-8000
G=8001-9000
H=9001-1000
The letter to designate the GVW in the vin is the fourth place and third number if yours is a chevy it would start 1GC_K24... the blank spot is the GVW, this is my understanding of how the brakes work.  I think you can go ahead and eliminate any of the ones with a JD at the parts store if you have vacuum breaks and then maybe look the rest up online.

I found this on a different website so maybe it will help
JB1 BRAKE, MANUAL, DISC/DRUM, 4300 LBS
 JB3 BRAKE, POWER, DISC/DRUM, 4600 LBS
 JB5 BRAKE, POWER, DISC/DRUM, 6400 LBS
 JB6 BRAKE, POWER, DISC, DRUM 7200 LBS
 JB7 BRAKE, POWER, DISC, DRUM 8400 L85
 JB8 8RAKE, POWER, DISC, DRUM 10000 LBS

here is the site that I found it : http://tlentz.oldsgmail.com/rpotext.txt

Hope this helps if anything is wrong I'm sure someone will let you know

[thirsty]

Can you take yours to the parts store and match it up with them?

[snibook7537]

Did you ever figure this out, tigwelder?