Re: Restoring Rusty
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I feel bad about that, since I recommended it. I have never had that kind of delay from that supplier.
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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.
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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.
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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
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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 |
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gawr frt 2946 rear 2946 will this Service Parts Identification sticker help? What's a J55 code? |
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That's either JB3, or optional HD power brakes JB5. The JB5 MC (18000996) would handle either. |
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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. |
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"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. |
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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? |
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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. |
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