BIG A#$ Brakes dilemma..
 

Hello everyone,

So I just picked up a 1984 C20 that used to be an old railroad truck. You know the ones that actually drive on the rails. (and no it doesn't still have the train wheel things on it. lol) But it does still have the inverted or reversed 19.5 steel wheels that narrow the width so the tires match the rail width.

So here is my dilemma. It has MONSTER brakes on it. The front disk's are 13" and the rear drums are 15"

It has no brake pedal pressure. I believe the master is shot. I pulled it off and it has a 1.5" bore on it. The spid has no code for brakes or even a listing of the brakes. I thought it would be the JB7 or JD7 but that only has a 1.25" bore. It is a cast iron master with the small chamber in the rear of it. It has a pretty big booster on it so i'm thinking that this is some special combo that was maybe converted by the rail company or ordered that way but not listed. I don't want to just throw a new style 1.5" bore plastic style on it and have it not work proper. Or just get a new booster and master from a JD7 and it not be enough for this Huge combo. Any suggestions as to how I would look up a master by bore size and see if I can match it? I've been all over the net to no avail. Maybe one of you guys has come across this before.
Any help would be appreciated.

Re: BIG A#$ Brakes dilemma..
 

Where’s the pic of the master and booster?

Re: BIG A#$ Brakes dilemma..
 

Nothing I could find on line from this end but what about taking the old MC apart and measuring the piston. Any numbers on the old one? pic of it.....pics of the rear drums also---never heard of a 15'' drum. What rear axle is in it? pic of that also

Re: BIG A#$ Brakes dilemma..
 

First things first.

Tell me about that Hodges Hauler it is sitting on.

After that I'll pull the brake charts out.

K

Re: BIG A#$ Brakes dilemma..
 

:lol: 1989 c3500 ext cab with a 454.

Re: BIG A#$ Brakes dilemma..
 

What are you wanting to do with the truck? If it were mine, and I was considering using it as a mobile toolbox, I would scrounge junkyards and find the correct stuff to put it back to a typical stock configuration. It depends how much you value the 'coolness' of the railroad setup, though.

Re: BIG A#$ Brakes dilemma..
 

I'm planning on pulling it out tomorrow. Letting it soak with penatraiting oil over night. I'll see then if any #'s are visible and take some pic's. The rear end is a 14 bolt with a G80 locking diff. Drums show 15" outside diameter measuring with a tap measure laying on my back underneath. 😆 the front rotors must actually be 12.5" not the 13" I thought. Measured them looking thru the rim with a tape measure.

Re: BIG A#$ Brakes dilemma..
 

Plan on first getting everything maintenance wise done. Running and stopping proper. Then MAJOR cleaning inside and out. Lots of moss growing all over it lol. Then redo the interior upto a respectful driver level. Update all the lights and clean up all the chrome and stainless. New weatherstripping and window gaskets. And if I'm not broke by then I would like to lower it about 4" all the way around. I think a lowered service truck would be kool.

Re: BIG A#$ Brakes dilemma..
 

So in other words it's just a regular 3/4 ton with 13'' drums. Looks like a semi-float rear which also came as a 14b with a 9.5 ring gear as opposed to the 10.5 full float rear. the only thing really special is the rims and tires

Re: BIG A#$ Brakes dilemma..
 

Nice.

Now that that is behind us I can focus on your question at hand.

a) It's vacuum boost? Not hyroboost?

b) Any idea of what the GVW of the vehicle is?

This might be a good juncture to remind you that if you send the GM Heritage Center your VIN (and $50) they can tell us what the original option content was, as it left the GM factory.

c) C20 pickups could be had with JB6/JD6 or JB7/JD7. Front rotors are shown as 12.5" in all cases, the largest rear drum is 13" diameter (that would be at the friction surface, not the outer most visible flange of the drum. Master Cylinder bore is 28.6mm (ie, 1.13") for JB6/JD6; 31.8mm (or 1.25") for JB7/JD7. Make sure you are measuring the bore of the piston/seal itself, not the outside diameter of the casting.

If it doesn't match that stuff then the system was changed by the upfitter, after the fact. I really can't comment or say what they might have done.

Pics of the m/c and booster, and rear drums would sure help.

K

Re: BIG A#$ Brakes dilemma..
 

Thanx for the reply Keith.
I found a master after some digging that seemed to match. It was an i.d. of 1.25. I never had to measure for one of these before. Thought I needed to measure the O.D. that went into the booster.

It is a vacuum boosted set up. I had a hell of a time finding one once I identified it. Rockauto was out of stock. Autozone showed unavailable anywhere as well as Advanced. O'reilley crossed to one that was backwards from what I needed. Ended up getting one from Amazon in the Cardone brand. Fits but seems now I have a caliper problem. Pedal still is just an air ball even thou I can see fluid moving in the master. I think a caliper is stuck. That will be the next adventure. Probably do new calipers and new wheel cylinders to make sure everything is good. Just gotta wait on some help to move the truck safely to where I can work on it with some air tools that reach. No brakes is hard to move around by yourself. lol

Re: BIG A#$ Brakes dilemma..
 

A quick thing to check is that the front calipers are installed on the correct sides. They're almost identical, except for the bleeder hole location. If installed left-on-right and vice-versa, the bleeder will be at the bottom of the caliper piston chamber instead of at the top, and won't allow you to bleed all the air out. Edit: That's on the 1980's K20 trucks with JB7 brakes. I can't say for sure if the C20 trucks of the same era have the almost-interchangeable calipers as well, or not.