'69 Brake junction block questions.

69427

The Artist formerly known as Turbo84
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Clinging to my guns and religion in KCMO.
Just for kicks, what all is inside one of those things? Anything to wear out, or look at if I buy a used one? I'm trying to decide whether to try to unscrew all the lines from mine (knowing that none of them have been touched in almost 40 years), or just get another used one that's been taken off another C3. I'm using a C5 m/c and Wilwood calipers on the "new" frame, so I'm going to have to custom cut and flare most of the system anyway. I'm just trying to save myself some disassembly labor time and if possible not tear up the current chassis stuff that's going to be rolled over in the corner for storage, anyway.

Thanks,
Mike
 
I think the block just has a port between the front and rear brake lines. The port has a plunger in it that operates a switch if the front or rear brake pressure drops. My limited experience with these is that they don't seal very well if you replace the lines. I guess the new flares don't seal on the seats of the re-used block.


.
 
Yes, it only has a differential pressure switch. The later 74? and up ones also have a prop valve in the rear circuit outlet.
 
Yes, it only has a differential pressure switch. The later 74? and up ones also have a prop valve in the rear circuit outlet.

I thought that prop valve, that no one ever proved was in fact ever done, as the outward appearance was the same to ME....as I recall, anyway.....well supposedly that didn't appear untill the later 70's....78?.....

at any rate, I found the prime leakage point on my switch to be at the switch itself....I replaced the orings and fiber seal, and a while later it started leaking again....

at that point, I ran the lines to a simple splitter in front, like the rear, and directly to the m/cyl....hell with the switch....this was before the final fix with the HB install...
 
I appreciate the feedback, guys. I'll have to take a peek at some parts books to see if the numbers change over the years, and perhaps also take a look under the hoods of some later C3's if I have the opportunity.

TT: What junction block is/was in your '82?
 
My 82 had the one with the prop valve. it's very distinct, it has a round cylinder on the back where the rear line goes in. Same for the 77.

PR101.jpg


See the round cylindrical piece on the right side? That's the prop valve right there.

This is the valve on the 69, see it's standing straight up on a bracket? The later ones sit almost horizontal with the rear line going out the back. This to make room for the added prop valve on the back.

249354a0f4ed34.jpg
 
TT,

Thanks for the info and pictures. Do you know any details about that proportioning valve? Is it fixed (non-adjustable) from the factory? Right after I posted the last post, I got to thinking about any C4 or C5 units that may be interesting to use, but given the use of ABS, my first guess is that a similar item doesn't exist. My (non-ABS) '84 is stored offsite, so I can't go take a peek at that for a while.

Thanks again.
 
Yes, it's fixed. You can't even take it apart without breaking the whole thing.

The C4 has it inside the brake master, it's on the front )the C4 master has 2 individual ports for the front brakes, one outboard, one inboard)
 
Bird, that's an excellent pic, shows precisely how the prop valve screws in the back of the junction block. Nice that it shows the differential pressure piston and switch exposed too. Very clear how it works by the tapered sections off center in the piston will push the pintle of the switch up and trigger the light (ground connection).
 
Do they wear out???? Stick??? Corrode???

How did you finish your brake system Mike (69427) ? Did you use the old valve??? I may have missed your post, and was wondering if there was any good feedback of the proportioning valve and it's reliability. Mine is a '77 and wondered how it,s behaving due to the dash light coming on once a week.http://www.vettemod.com/forum/showthread.php?t=4246

I do feel it has a value even though it's a modified block, I like the heads up it gives in my case...
 
How did you finish your brake system Mike (69427) ? Did you use the old valve??? I may have missed your post, and was wondering if there was any good feedback of the proportioning valve and it's reliability. Mine is a '77 and wondered how it,s behaving due to the dash light coming on once a week.http://www.vettemod.com/forum/showthread.php?t=4246

I do feel it has a value even though it's a modified block, I like the heads up it gives in my case...

I ended up reusing the original block. After seeing the asking price for new ones, I couldn't justify the expenditure.
The older ones like mine are just a junction block (no proportioning valve). So far, even with a totally different brake system, the balance isn't too bad. Once I get the car dialed in more, I'll see if a proportioning valve would be useful.
 
How did you finish your brake system Mike (69427) ? Did you use the old valve??? I may have missed your post, and was wondering if there was any good feedback of the proportioning valve and it's reliability. Mine is a '77 and wondered how it,s behaving due to the dash light coming on once a week.http://www.vettemod.com/forum/showthread.php?t=4246

I do feel it has a value even though it's a modified block, I like the heads up it gives in my case...

I ended up reusing the original block. After seeing the asking price for new ones, I couldn't justify the expenditure.
The older ones like mine are just a junction block (no proportioning valve). So far, even with a totally different brake system, the balance isn't too bad. Once I get the car dialed in more, I'll see if a proportioning valve would be useful.

I am tossing out a weakly held opinion here to see what others say....

Proportioning valves are for drum rear, front disc cars....I don't recall having on 4 wheel DRUM brake cars....the way I undersand it is that drum brakes are somewhat self actuating, so the rears have to be limited in application pressures to prevent them from locking up...allowing the front discs to work .....

with the small pistons/pads in the rear, but the same essential design, I think that f/r proportioning is already done in the design of the car....

and I think proven with the fact that the switch in the early sharks worked fine for years....

I dunno why they went to a proportioning valve in the shark....

for that matter my '87 had ABS, so I doubt an proportiong valve would be needed.....
 
Funny I have been planning on adding an adjustable proportioning valve to my 74. I finally got the brakes working good after fixing a rotor run out issue and now it will lock up the rear tires way to easily. There is definitely a reason GM added the part though.
 
My 77 locks up all four wheels pretty evenly at speeds up to 75mph if you really stand on the pedal. Well at least it slides straight.

It is my understanding that the proportional part of this system doesn't even come into play unless a high amount of pressure is applied.
Under normal conditions the bias is controlled by the different piston areas of front vs rear.

I think true proportioning can only be done with two master cylinders and a bias bar like the big track racers.
 
As reported earlier, I believe '77 was the first year for the proportioning devise being added to the 'block' :quote:
I only believe with pressure testing and the engineers reasoning for it will help us understand it's introduction in this year.

As far as the calipers for '77, the rears (Rear; Bore = 1-3/8") have a different piston size than the front (Front; Bore = 1-7/8") The 69 is the same and in '87 Front; Bore = 54mm, Rear; Bore = 1-5/8" and the proportioning valve is in the MC.

All work and let us never forget that DRUM brakes would still be the best choice as they work better, just the damn production simplicity and fade issues.

I do believe the 'Nader' word has a lot do with the changes. Some are for the better and some we just say :twitch::rolleyes::huh::surrender:

No doubt,The key word next to safety here is "MOD"!!! Make it better and better and...:idea::thumbs:
 
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