1968 Dart Brake and Booster issues?

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PinkGT

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I had a front disc brake conversion done on my '68 Dart and the brakes seemed OK for a while, the guy who did the swap pulled my Distribution Block and plumbed in an aftermarket adjustable Proportioning Valve (which started leaking almost immediately). I recently bought a new Disc/Drum correct master cylinder so I can go back to the Dist Block setup and get my dummy switch working again. The new mechanic installed it for me and plumbed the old Dist Block back in (I'd love to see a plumbing diagram for the original Dist Block if anyone has one???). The swap went fine and the front seemed to work well but the rears just won't grab to lock up when idling in gear on the hoist? There seems to be a little sigh/hiss/wheeze from the brake master after it first is depressed. So that leads to 2 areas for questions:

1) Is there some sort of plumbing issue with the Distribution Block, and can I get a plumbing map to make sure we have the lines in correctly? The mechanic can get a hard pedal when bleeding but once the car is running and the brakes are pressed the rear tires keep spinning. Is there an internal lockout of some sort in the distribution valve that is designed to isolate half the brake circuit if there is an imbalance? Where do I get a 1968 A body proportioning valve to get rid of my Distribution valve? Am I chasing my tail?

2) What is the correct Brake Booster for my application? Does Summit Racing have a part number for one which is correct for my vehicle (1968 Dodge Dart convertible had a 273 before the engine swap)? I'm in Canada so Summit seems to be the only Mopar shop up here. Could it be a check valve issue on the booster? A collapsed line on the rear? Rear pistons too large?
 
Originally the distribution block contained a safety switch that would flash a warning light on your dash if it detected an imbalance between the front and back brakes. The way that works is by grounding the attached switch. Think of the internals as a piston that moves left or right when detecting an unevenness. In the center there is no path to ground for the switch, the circuit is "open". When an unevenness occurs the piston moves and grounds the switch and blocks the offending low pressure side so no more fluid is lost. When "tripped" this way, it is often needed to disassemble the block to "re-center" it. Original distribution blocks and proportioning valves and lines are available. Sorry about being so "wordy".
 
^^What he is trying to say is, the original distro block is NOT a proportioning valve. You either need to add an aftermarket one, or find / buy one like the original 73/ later stuff. Adjustable is attractive as it allows, well, "adjustment." This could be due to tire differences, brake mismatch front to rear, or even lining type.

"Why can't" you fix the leak in the one you have? Or replace it with another?
 
67Dart273 said:
^^What he is trying to say is, the original distro block is NOT a proportioning valve. You either need to add an aftermarket one, or find / buy one like the original 73/ later stuff. Adjustable is attractive as it allows, well, "adjustment." This could be due to tire differences, brake mismatch front to rear, or even lining type.

"Why can't" you fix the leak in the one you have? Or replace it with another?
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It seems like the $35 unit from Summit is sort of weeping out every port, since original installation. Does anyone know who stocks an appropriate Proportioning valve that would plumb into the location where my current (possibly compromised) Distribution Block is located? And also have a correct output for the brake dummy light? Is this Year one or Rock Auto?
 
Get a 1 inch bore master and you won't need a booster. I can put people through the windshield at 130+

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PinkGT said:
It seems like the $35 unit from Summit is sort of weeping out every port, since original installation.
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Do you still have the aftermarket P-valve installed?
PinkGT said:
There seems to be a little sigh/hiss/wheeze from the brake master after it first is depressed.
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this is normal.

When you get so many leaks, you have to step back and think about it. I'll bet you a chewing gum wrapper that the flares were done incorrectly, or the lines used were very hard.. Double Flares are hard enough to do correctly and successfully on soft lines by experienced people. If your mechanic did not double-flare your lines, that is a problem. If he did and failed to properly inspect them afterwards, that may also be the problem.

That aside;
The rear brakes from the factory, are limited to doing about 15% of the work. And the P valve works like a hold-back valve, making sure the fronts are well into doing their jobs before the rears get activated.
Furthermore, the rear brakes cannot work until the wheel cylinders actually push the shoes out into drum contact.
Furthermore, if the radius of the shoes does not match the radius of the drums, all the force will go to a small segment across the shoes maybe a half inch wide or LESS with the tires not contacting the road and turning.

If the pedal is high and hard, and has been so since the beginning;
then chances are nothing is "wrong". It might not be optimum, but it probably is not wrong.
As to Proportioning;
My car started out with 235/60-14s on the front and 275/60-15s on the rear. With the KH 4-pistons on the front and 10X2s on the back, NO PROPORTIONING was needed. I simply used one size smaller rear wheel cylinders.
After I switched to 295s, I went back to the larger w/c's.

and done
 
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AJ/FormS said:
Do you still have the aftermarket P-valve installed?

When you get so many leaks, you have to step back and think about it. I'll bet you a chewing gum wrapper that the flares were done incorrectly, or the lines used were very hard.. Double flares are hard enough to do correctly and successfully on soft lines by experienced people.

more coming
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Aftermarket P Valve is out to try and go back to more stock setup and avoid the extra leak points. I'm wondering if a NOS Proportioning valve might be the solution?
 
After Resto on My fish I was perplexed!

All the leaks ETC. All new lines and components!

You have to tighten/Loosen all those new fittings, sometimes more than once! I was in "Terror" for all the new Paint at first but slowed down and re-worked the whole system!

Use the correct tools and have patience!

I almost wish my car was Non Power! it would have been easier!
 
PinkGT said:
I had a front disc brake conversion done on my '68 Dart and the brakes seemed OK for a while, the guy who did the swap pulled my Distribution Block and plumbed in an aftermarket adjustable Proportioning Valve (which started leaking almost immediately).
Click to expand...

Pics of the Set-UP?
 
1969383S said:
Pics of the Set-UP?
Click to expand...

I'll try and get by the mechanics in the next few days to take a picture. I live downtown and don't have access to a garage where I can have a car in pieces, so I have ended up farming out work these days. The aftermarket P.Valve was plumbed in like the old D.Valve and worked fine for a few months but then things seemed to go very Flintstone Brakes. I parked it for winter/Covid when I saw the leaking etc. I pulled it out of storage to get the tranny rebuilt this Fall and decided to tackle the annoying leaky brake setup at the same time. I sort of thought I had been sold a leaky junk unit and why not go back to having a crude warning light system? I thought swapping out the 4 wheel drum MCylinder for a Disc/Drum MC would mean I could go back to my old DValve. I suppose I would likely need that NOS style P.Valve to solve the problem though?
 
PinkGT said:
I thought swapping out the 4 wheel drum MCylinder for a Disc/Drum MC would mean I could go back to my old DValve. I suppose I would likely need that NOS style P.Valve to solve the problem though?
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The car does not care what M/C is on it, and the P-valve is in no wise matched to it. The only only differences between these two styles is that the drum-drum unit has 10psi hold-back valve installed in BOTH ports whereas the Disc-drum has it only in the port to the rear brakes.
And the DB unit has a larger reservoir for the front brakes. That is all.
You can make any Mopar M/C work on your car, whether DB fronts or not.

Post 6 is now finished,lol.
 
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AJ/FormS said:
The car does not care what M/C is on it, and the P-valve is in no wise matched to it. The only only differences between these two styles is that the drum-drum unit has 10psi hold-back valve installed in BOTH ports whereas the Disc-drum has it only in the rear.
And the DB unit has a larger reservoir for the front brakes. That is all.
You can make any Mopar M/C work on your car, whether DB fronts or not
Click to expand...

I had been told by numerous folks that the larger reservoir on the Disc side of the MC would also be set up to push more fluid to the calipers whereas the smaller wheel cylinders would get less fluid and that the MC would basically be proportioning it at the source and so the D.Block would be adequate.

Have I been steered wrong? Most of the replies to the posts seem to suggest that one still needs a P.Valve in line to the rear to dial it in.

I'd like to rule out the Distribution block as the source of the problem before going on to tackling the quite possibly leaking Vacuum Brake Booster.
 
Here's what I would do, with 235/275 tire balance, or any balance of fronts being 85% of the rear;
Skip the adjustabe P-valve. Skip any P-valve. Fix the leaks.
With a brake booster you can run any sized M/C bore that you want to.
Iv you have a Disc/drum M/C with the one larger reservoir; Make sure the hold-back valve is removed from the port at that larger reservoir, and plumb that reservoir to the front brakes.
 
PinkGT said:
I had been told by numerous folks that the larger reservoir on the Disc side of the MC would also be set up to push more fluid to the calipers whereas the smaller wheel cylinders would get less fluid and that the MC would basically be proportioning it at the source and so the D.Block would be adequate.
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Not true; BOTH pistons in the M/C are the same size and volume. In a properly working system, whatever you stroke at the pedal, Has the potential to move the same amount of fluid .
The difference in reservoir sizes has exactly one function, to eliminate the constant topping up of the reservoir attached to the harder-working, faster-wearing, front pads which are doing 85% of the work; that is all.
There is NO proportioning in the M/C.
Think about this
the only forces that act on your front calipers to separate them from the disc are; seal-retraction, pad deflection, and disc run-out. As to run-out, there isn't supposed to be any, but loose wheel bearings will always generate some. As to pad deflection, there isn't supposed to be any, but it happens. So that leaves seal retraction as the principal player.
In any case, with all this going on, you might get a piston movement of perhaps .007. On a pair of 2.5" pistons, this mathes to .0687 cubic inch or ~1.1cc. So when you step on the pedal, and if the pistons are correctly retracted, you can expect ~1.1cc to be pushed down the line. I like a 15/16" bore for this so .0687 cubic inch means the piston has to move .01 inch or a lil less than 1/8th inch . Forget about what you were told; do the math for yourself.
Here's the deal, suppose you go for a year without topping up your reservoir. Suppose that during this time 3/32 orv .09inch of your pad gets worn off. This is ..090/.007 x 1.1cc=~14cc. with your big reservoir, this is a noticeable drop. But with the smaller reservoir, it might be over half empty (I'm guessing). That is why you have a larger reservoir for the fronts.
 
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All of these aftermarket distribution and proportioning valves seem to leak and weep, find some OE stuff.
 
PinkGT said:
I'd like to rule out the Distribution block as the source of the problem before going on to tackling the quite possibly leaking Vacuum Brake Booster.
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The sound that your booster is making is normal.
Your booster has two separate chambers in it; the one side is under vacuum, while the other is at atmospheric pressure.In between them is a valve system.
When you step on the brake pedal, the linkage opens the valve and the diaphragm pushes on the pistons in the M/C in direct proportion to how hard you are pushing but amplifying your power by about 7times(IIRC). The sound you hear is atmosphere rushing in there to do this work. And rushing out when it has finished. There is supposed to be a foam-rubber muffler on that port so you don't hear it, or at least don't notice it.
To test your booster; there are a few tests.
Test-1)
With the engine off, equalize the chamber to atmospheric by pumping the pedal about 5 times. Then step on the pedal with about normal power and hold it right there, then start the car. Immediately on a stock 318, the pedal should rise and force your foot upwards. That is normal.
Test-2) idle drop
Start the engine and wait until it is idling normally. Watch your tach. step on the pedal and hold it there. You might notice a slight dip in rpm followed immediately by a return to normal. Try it several times, with varying pedal pressure, allowing a couple of seconds between each application for recharging. If it passes this test, there is nothing wrong with it.
Test-3) vacuum decay.
start the car and let it evacuate the booster for a few seconds. With a big cammed engine, to at most 10 seconds with the engine at hi-idle. Do not touch the brake pedal. Turn off the engine.
Come back in ten minutes. Put some light pressure on the pedal than start the engine. The pedal should rise just a lil under your foot, but not like when you emptied it before. If it does as before, then the atmospheric valve may be leaking but more likely is the charge line under the hood.
Test-4) This is more difficult to do accurately. You need to Tee a vacuum gauge into the charge line between the booster and the check-valve so that you can see what is going on inside the booster. You need a fitting etc, but this is a last resort test.
 
PinkGT said:
I'd like to rule out the Distribution block as the source of the problem
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The distribution block, if it separates the front from the rear, that is fine. But if it allows mixing of the front and rear systems, that is not good; it defeats the whole reason of having a dual piston M/C.
For my car I just fished out any old Combination valve from my used parts bin, and gutted the proportioning valve out of it. The front system is still separated from the rear system by the floating safety switch.

BTW, silicone brake fluid likes to leak, far more so than regular Dot 3 or 4. With good double-flares Dot3/4 is easy to seal up. But not so with the silicone stuff; I mean I think it really likes to leak just to pizz a guy off.
 
Hello Folks,

Well it seems the inevitable has occurred: The mechanic is convinced the Brake Booster is dead! So where do i go to get a factory correct booster to replace my dead one. I saw a comment on FABO about a shop in California that does rebuilds on them and sent an email but haven't heard back (Covid...Christmas..Closed??). I guess I'd like to order one ASAP then send mine off to be rebuilt so I have a backup available down the road. Can anyone point me to a supplier of a unit which will actually have the correct bolt holes etc??

Thanks in advance.
 
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