Re: Restoring Rusty
 

not yet, it was last seen here

Re: Restoring Rusty
 

I feel bad about that, since I recommended it. I have never had that kind of delay from that supplier.

Re: Restoring Rusty
 

Yup, looks like a project for this weekend. By the way how come this power one from AutoZone looks opposite from mine, on mine the smaller chamber is in the front and on this one it is in the back, my old one looks like the ones for manual brakes, could the P.O. have used the wrong cylinder?!

Re: Restoring Rusty
 

No worries bro, you have helped me so much, this time frame is a first for me too.

Re: Restoring Rusty
 

OK, this one took a while with the parts book. It turns out that it depends.

The little one in the front and the big one in the back is for "1974-75 C-3 (JB8 w/P.S.) (292) HYDRA-BOOSTER PUMP" among other things.

The one with equal reservoirs is for "1973-78 CG-1, BLAZER, JIMMEY (JB-3) SINGLE DIAPHRAGM VACUUM CYLINDER (MORAINE)" and "1973-78 CKGP-1, 2, 3, BLAZER, JIMMY (JB-5, JB-6, JB-7) TANDEM DIAPHRAGM VACUUM CYLINDER (MORAINE)"

The one with the big one in the front and the little one in the back is for "1973-78 C-1 BLAZER (C), JIMMY (C) w/JB-1, JB-3 BENDIX MASTER AND VACUUM CYLINDER"

Some notes. C means 2WD. K is 4wd. G and P are vans I think. 1, 2, 3 means 10, 20, 30. So "CKGP-1, 2, 3" means C-10, C-20, C-30, K-10, K-20, .... JB1, JB3, JB5 are RPO codes for power brakes and should be called out on the glovebox sheet. My 1978 K-10 calls out Z84 Cheyenne Equipment, but doesn't say which PB code it contained. Probably JB5, JB6 or JB7, which went by GVW. You probably have JB5.

Got all that?

Given all that, I just went out and looked at mine, and it's big one in the front and small one in the back, so go figure.

Which kind of makes sense. Drum brakes take a lot more fluid movement than disks, which hardly move at all. So if the front reservoir is for the rear brakes, I would expect the front one to be larger on a disk/drum setup.

Re: Restoring Rusty
 

Looking deeper, I can nail it down exactly if you tell me the GVWR on the door sticker. Add the front and rear axle ratings, I just need the total.

Re: Restoring Rusty
 

Here is a writeup I found elsewhere that really brakes down a lot of the common brake setups.

http://goo.gl/m0Zh1I

Re: Restoring Rusty
 

Yeah, my truck should have a JB5 setup, but when the garage replaced the original master cylinder because the cup was age-hardened and passing fluid (pedal would slowly sink to the floor as you held it), they put the big, honkin' JB7 master cylinder on it, which is fine. Going to a LESSER duty part would NOT be good.

Re: Restoring Rusty
 

Oh, and note that both of those pictured have the big reservoir in the front.

Re: Restoring Rusty
 

G = Standard Vans
P = Forward control chassis
1,2,3 = Chev 10,20,30 GMC 15,25,35
JB1 = manual (no power)
JB3 = light duty Vacuum 1/2t
JB5 = heavy duty Vacuum 1/2t this is standard with a V8 and A/C and certain other packages
JB6 = Vacuum light duty 3/4t up
JB7 = Vacuum heavy Duty 3/4t up
JB8 = Hydraulic booster 3/4t up

Re: Restoring Rusty
 

Yeah, with the '74 C-10, he could have the JB1, JB3 or JB5, but it's probably the JB5, listed as "Optionally Available HD Power Brakes".

The correct GM part number master cylinder for JB5 in 1974 is #18000996. NOTE THAT YOU DO NOT WANT AN NOS PART. It will be junk, due to age-hardening of the rubber seals and cup.

That part number cross-references to a bunch of stuff:
http://www.yoyopart.com/oem/11454860/gm-18000996.html

Large reservoir in front on the Bendix cross-reference, no longer available:
http://www.summitracing.com/parts/ben-11944

Summit has an equivalent:
http://www.summitracing.com/parts/su...del/c10-pickup

Re: Restoring Rusty
 

Wow Rich you so went the extra mile for me on this one, THANK YOU SO MUCH. So I just went out there in the dark with a flash light to look at my brake master cylinder and follow the lines. Ok here are the findings.

My front smaller reservoir has the bigger brake line going to it and is for the rear drum brakes

My rear larger reservoir has the thinner brake line going to it and is for the front disc brakes

this don't seem right, I tell ya mine looks like a manual master cylinder instead of power, like this one (Part # 10-1581) from O'Reillys

Re: Restoring Rusty
 

gvwr 5300

gawr frt 2946 rear 2946

will this Service Parts Identification sticker help? What's a J55 code?

Re: Restoring Rusty
 

You should have a prop valve on the front cross member where the brake lines go to.

Re: Restoring Rusty
 

Yeah, that's wrong. Drums require more fluid movement. Disks are all about pressure, not fluid flow. Bigger reservoir should be front on the MC, going to the rears.

Re: Restoring Rusty
 

That's either JB3, or optional HD power brakes JB5.

The JB5 MC (18000996) would handle either.

Re: Restoring Rusty
 

Seems perfectly correct to me.
The larger reservoir is for the disc brake side of a disc/drum setup always.
the caliper pistons displace more volume than drum wheel cylinders.

Re: Restoring Rusty
 

Oh o, you two are gonna have to arm wrestle for this one! I also found this which sides with Titomars

"A dual master cylinder designed to work with a disc/drum setup will feature two different reservoir sizes. As disc pads wear, the caliper pistons will move further outward toward the rotor, causing the reservoir fluid level to drop more than it would for a drum brake reservoir. As a result, a disc/drum master cylinder needs a larger reservoir for the disc brake circuit to keep more fluid in reserve."

Master Cylinder Selection

Re: Restoring Rusty
 

One difference between JB3 and JB5 (I think) is that JB3 is 2" drum and shoes in the back and JB5 is 2.75" drum and shoes in the back.

enaberif is right, all 1973-78 (years of my parts book) had proportioning valves to control the rear brake pressure.

Re: Restoring Rusty
 

yes that is correct, your point being?

Re: Restoring Rusty
 

Nope not happening. I'll pass on my 45 years of auto mechanics and auto dealer experience. you guys can do what you want with it, but I am not here to get into a pissing match with anyone. I am here to help if needed or wanted and to enjoy the shared love for our machines.

Re: Restoring Rusty
 

And we appreciate that, I did go on to say "I also found this which sides with Titomars" as in you are right sir.

Re: Restoring Rusty
 

I checked a couple other sources. I stand corrected. The larger reservoir is for the discs, because they use a lot more fluid as they wear and the pistons move out. The higher flow *in operation* is to the drums. So master cylinders on some makes apparently have bigger lines from the smaller reservoirs to the drums. The (original?) hard lines on mine are the same size. I haven't followed the plumbing all the way over, under, and around, to see what goes where, but they all go to the proportioning valve originally.

BTW, that's probably why your smaller front reservoir is empty. Wanna bet it's plumbed to the front brakes, and as they wore they sucked it all up?

Re: Restoring Rusty
 

All truck or car brake systems with a dual reservoir has a proportioning valve.
JB3 = 1 inch thick rotors and 11x2 shoes
JB5 = 1.25 inch rotors and 11.15 X 2.75 inch shoes
Both use the same master cylinder bendix to bendix or moraine to moraine and both use the same prop valve

Re: Restoring Rusty
 

Now going back and looking at your pics, you have different sized lines, but they are off the front of the master cylinder, which I think now should be to the fronts.

I'm confused.

Discs as they wear use a lot of fluid. Got that. The actual flow to and from the rears as you apply and release the brakes is higher, because the disc pistons hardly move in and out at all, whereas the rear pistons do. Got that. Larger reservoir is for the fronts, so as they wear and the pistons move out and the calipers have to fill up with fluid, they don't suck the reservoir dry. Got that.

But your setup looks confused. Or is it just me? Does your proportioning valve have a colored stripe on it? That is a GM identifier for which one it is. I think you should also track down which lines go where. Getting this right is important.