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(actually Slim Jr now)
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Discussion Starter #21
22GT thanks for checking this out. I was concerned that I was mis reading this. Since my back and legs have gotten so bad I drive extremely cautiosly. If the differential shuttle is stuck giving maximum pressure to the rear brakes I wouldn't have noticed. I do notice that the brakes are very sensitive. When the weather gets nice and there's no salt on the roads I'll go with our son, with him driving and do a couple panic stops. If the rear tires easily slide I'll know which way the shuttle is stuck. If it's stuck that way then I know which way it has to move to center it.

Thanks again,
Slim
 

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Easiest way to test this is on a gravel road. Gradually increase braking power until the wheels start to skid. If the rear locks before the front, you have a problem.
 

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As long as we are discussing distribution blocks and shuttle valves, I would like to bring up something I've been mulling over lately - doesn't the presence of the shuttle valve and its seals make these hydraulic systems not truly independent?

For example, I have a rear hydraulic failure, forcing my shuttle valve over. That valve and those seals have been in intimate contact with one another for the last 45 years, so one of those seals fails as it shuttles, all of the sudden both resevoirs of my master cylinder are pushing fluid out of the same hole! Am I missing something? Better yet, as long as I don't have a pressure imbalance front to rear, those seals could be completely missing, and I wouldn't know it until it was too late.

I understand the shuttle valve is necessary to provide the brake failure signal, and I'm being chicken little here with my multiple simultaneous failures scenario, but honestly, I'm considering eliminating the whole dist block and just running my front brakes from the master through a standard T fitting, and the rears from the master directly into the external prop ('68) and be done with it.
 

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As long as we are discussing distribution blocks and shuttle valves, I would like to bring up something I've been mulling over lately - doesn't the presence of the shuttle valve and its seals make these hydraulic systems not truly independent?

For example, I have a rear hydraulic failure, forcing my shuttle valve over. That valve and those seals have been in intimate contact with one another for the last 45 years, so one of those seals fails as it shuttles, all of the sudden both resevoirs of my master cylinder are pushing fluid out of the same hole! Am I missing something? Better yet, as long as I don't have a pressure imbalance front to rear, those seals could be completely missing, and I wouldn't know it until it was too late.

I understand the shuttle valve is necessary to provide the brake failure signal, and I'm being chicken little here with my multiple simultaneous failures scenario, but honestly, I'm considering eliminating the whole dist block and just running my front brakes from the master through a standard T fitting, and the rears from the master directly into the external prop ('68) and be done with it.
You got it.

You can distrbute with a t just as well as with the "Distribution block". Same E-Like-E.

Then run the rear line in out of Proportioning valve straight to rear brakes.

Dan @ Chockostang
 

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(actually Slim Jr now)
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Discussion Starter #25
As long as we are discussing distribution blocks and shuttle valves, I would like to bring up something I've been mulling over lately - doesn't the presence of the shuttle valve and its seals make these hydraulic systems not truly independent?

For example, I have a rear hydraulic failure, forcing my shuttle valve over. That valve and those seals have been in intimate contact with one another for the last 45 years, so one of those seals fails as it shuttles, all of the sudden both resevoirs of my master cylinder are pushing fluid out of the same hole! Am I missing something? Better yet, as long as I don't have a pressure imbalance front to rear, those seals could be completely missing, and I wouldn't know it until it was too late.

I understand the shuttle valve is necessary to provide the brake failure signal, and I'm being chicken little here with my multiple simultaneous failures scenario, but honestly, I'm considering eliminating the whole dist block and just running my front brakes from the master through a standard T fitting, and the rears from the master directly into the external prop ('68) and be done with it.

Quite a while back there was a long discussion thread about the differential pressure warning light shuttle. The shuttle moves in the direction of pressure loss. It does not seal off that end. If the leak is fixed and the shuttle remains off center the brakes still work normally.

I brought this subject up again because in reading the 68 shop manual it looks like the 68 (and 69 apparantly is same) is different in that a front failure results in full pressure at the rear, bypassing the proportioning valve. It would stay that way if not re centered.

Going to a T or a couple Ts would eliminate the warning light. So you would have separate front and rear systems with no knowledge of failure until you lose control of the car!
Same as the current set up (except 68/69 difference) if you just unplug the warning light!
 

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Discussion Starter #26 (Edited)
IMO Ford engineering thought they had "a better idea" with the 68/69 distribution/pressure differential valve design. With a front failure the proportioning valve is bypassed and 100% pressure goes to back brakes. This gave better stopping for the surprized driver than the normal 40% pressure directed to the rear.

My wild guess is the very thing we've been discussing is the reason the 68/69 design was dropped.

I have a 70 Torino with power disc brakes. 10 years ago I junked my badly rusted 70 Torino with power disc brakes. Wish I would have saved the pressure differential and proportioning set up. I'd install it on my non concours 68! According to the 70 shop manual the shuttle is self centering on the 70 design.

I need to keep in mind the shop manuals aren't intended to be instruction manuals. You need to very carefully read and examine figures to determine the operation of something, sometimes a very important feature is obscure!

Another example is hook up/routing of spark plug wires/cable on V8 with firing order that has 7,8 sequence. Guess a good experienced mechanic just knows this!

Love this forum. The Muppet Frogy sang "it's not easy being green...". Hey on here we odd balls that like lime gold are allowed to easily post and state opinions!

Slim
 

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Centering methods

Slim, what have you tried so far to center your shuttle valve? I would think you could loosen a bleeder on one side (say a caliper) press the pedal with the key on (and warning lamp on) until it goes out. If you use up all your travel and the lamp is still on, you just chose the wrong circuit. Tighten up your caliper bleeder, loosen a drum bleeder and repeat. Maybe I am oversimplifying.

As far as having no knowledge of a failure if you disable the warning lamp, that may be true, but I've had my share of burst brake lines, and you don't need the lamp to tell you something went terribly wrong, the extremely long pedal travel and stopping distance in concert with the empty reservoir tell the tale :) !
 

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(actually Slim Jr now)
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Discussion Starter #28 (Edited)
Slim, what have you tried so far to center your shuttle valve? I would think you could loosen a bleeder on one side (say a caliper) press the pedal with the key on (and warning lamp on) until it goes out. If you use up all your travel and the lamp is still on, you just chose the wrong circuit. Tighten up your caliper bleeder, loosen a drum bleeder and repeat. Maybe I am oversimplifying.

As far as having no knowledge of a failure if you disable the warning lamp, that may be true, but I've had my share of burst brake lines, and you don't need the lamp to tell you something went terribly wrong, the extremely long pedal travel and stopping distance in concert with the empty reservoir tell the tale :) !

I've tried both circuits and the light stays on. I've owned and driven many miles with very old


cars incuding my 37 Ford (mechanical brakes) in 1975 as regular driver. In my over 50 years of driving have never rear ended someone or hit an object although I've had brake lines rust out. Yes with the Mustang I noticed the light after the failure!

I want it to work because it's a feature on the car...I'm an engineer...can't help it!

I'm pretty sure, right now I'm getting full pressure to the rear (drums). That to me is not good for regular driving!
 
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