Master Cylinder Question

[doubletrouble]

I am trying to get my dads suburban back together, been a few years since its been on the road. I am replacing the master cylinder, not sure if it has the correct one on it now, it is not the original. The parts store computer shows 2 different ones. It is a '76 4x4 half ton front disc rear drums suburban. One has the bigger threaded hole in the front and one has it in the rear, one has equal size reservoirs and one has bigger reservoir in the front. What do you think? I will get some pics later.

 

[diesel4me]

I think the rear brake shoe sizes (which have different sized wheel cylinder pistons possibly?) is what determines which master cylinder it uses..been awhile since I looked up brake shoes but I think 1./2 tons has either 11 x2" shoes or
11-5/32 x 2-1/2....the parts stores will want to know the "JB" option number like JB-6 or JB-7 ,you may be able to determine that by calling a GM dealers parts counter and give them your VIN number..some parts stores can too...if your options list is still intact on the glove box or inner front fender,that will have the info listed there..

I'm pretty sure the smaller rear brakes were the JB-6 option and the larger were the JB-7,but dont consider that gospel as I haven't been behind a parts counter for a long time...just going by what I've dealt with on my own trucks...

My two trucks have the square looking master cylinders with one chamber much larger than the other for the fluid--I have an '85 K-10 Suburban and an 82 K2500...

I tried using a master off a truck I had that was the oval looking one with two equal sized chambers on my '82 ,it fit ,but like you noted,the lines wont fit in the same places they did on the original,but did fit by swapping them front to rear...now my truck skids the rear wheels way too easy,so I'm pretty sure the lines must be hooked where they were originally in ofder to have the rear brakes work correctly...I'm thinking of buying adapters so the lines can be put where they went before,but since I noticed the master leaking some ,I might as well just get the right one instead..

 

[doubletrouble]

I did'nt see anything on the glovebox sticker that looked related to anything on the brakes. The parts store computer said one is for the 6400 gvw and the other is for the 7200 gvw but the suburban says its 7300 gvw. I forgot about measuring the rear shoes. Its hard to tell since you can switch the lines from front to back. I'm tired of not having a good feeling pedal.