Vintage Mustang Forums banner

1 - 20 of 28 Posts

·
(actually Slim Jr now)
Joined
·
24,597 Posts
Discussion Starter #1
This has been discussed more than once so please be patient. A leak occurs on either the front or rear brake system. The shuttle in the pressure differential valve shifts and the warning light goes on. Sometimes the shuttle sticks and wont return to the center, light off position. Most, including me agreed that the shuttle turns on the warning light but the brakes are fine with it shifted. We can ignore the light, unplug it fix the valve later!

Looking at the system more closely and reading the shop manual carefully it seems the above conclusion insn't entirely right, at least on a 68 with factory front disc brakes. Apparantly if the shuttle is shifted because of a front leak it by passes the proportioning valve allowing/providing full pressure to the rear (drum) brakes. I'm hoping someone explains that I'm wrong.


Slim
 

·
Registered
Joined
·
35,892 Posts
Nope. The 68 proportioning valve is a separate item. (65-69, actually). The prop valve is inline on the rear brakes, and is after the differential valve, so it cannot be bypassed.

Other than the size of the fittings, the 67-69 disc differential valve is identical to the drum brake version:



In 1970, Ford changed to a combined unit on disc brake cars, but the two functions were still separated:

 

·
(actually Slim Jr now)
Joined
·
24,597 Posts
Discussion Starter #3
Nope. The 68 proportioning valve is a separate item. (65-69, actually). The prop valve is inline on the rear brakes, and is after the differential valve, so it cannot be bypassed.

Other than the size of the fittings, the 67-69 disc differential valve is identical to the drum brake version:



In 1970, Ford changed to a combined unit on disc brake cars, but the two functions were still separated:

Look at 68 shop manual 2-11 Fig3. Read page 2-10 and examine Fi19 page 2-22. What is shown at the bottom is exactly what's on my 68. The proportion valve runs internal to the to the prtessure differential valve. The proportioning valve mounts at the front of the pressure differential valve. The inlet and the outlet connect to the differential valve body.

To quote page 2-10:
On disc brake equipped vehicles, the pressure differential valve will move to the low pressure area of the front system. (this is with front leaking/failure). This movement uncovers the rear brake system outlet passage and provides a direct passage from the rear inlet passage to the outlet passage, BY PASSING THE PROPORTIONING VALVE. This provides full hydraulic pressure to the rear brake system.

I apologize for using CAPS above but it's a lot of typing/reading.


Slim
 

·
Registered
1967 Mustang GT fastback
Joined
·
4,019 Posts
FWIW, my 67 Ford Shop Manual (page 2-11) states: "In the event a front brake system failure should occur, greater pressure from the rear brake system during brake pedal operation forces the valve (ie. the valve piston which moves fore and aft and usually is stuck on our cars...) forward, moving the switch plunger upward onto the valve ramp to light the brake system warning lamp. However, failure of either the front or rear system does not impair operation of the other brake system." (Ford's own boldface in the shop manual).

Hope that helps a bit.
 

·
Registered
Joined
·
3,725 Posts
This has been discussed more than once so please be patient. A leak occurs on either the front or rear brake system. The shuttle in the pressure differential valve shifts and the warning light goes on. Sometimes the shuttle sticks and wont return to the center, light off position. Most, including me agreed that the shuttle turns on the warning light but the brakes are fine with it shifted. We can ignore the light, unplug it fix the valve later!

Looking at the system more closely and reading the shop manual carefully it seems the above conclusion insn't entirely right, at least on a 68 with factory front disc brakes. Apparantly if the shuttle is shifted because of a front leak it by passes the proportioning valve allowing/providing full pressure to the rear (drum) brakes. I'm hoping someone explains that I'm wrong.


Slim
Simply stated, the late 67 "Combination/Portioning valve" is the very same thing as the common H Block used on the Drum cars, 67 Disc cars with P Valve installed above the rear end.

Why??? The loop in the late 67-69 Disc brake p-Valve is the same Cushion in the rear brake circuit as the P-Valve in the H block Disc brake system. A Loss if Sudden Pressure using a spring, and plunger. Remember all these are seperate systems after 66. Rear, from From Seperation.

So, yes, if a failure is indicated in the light, it means a pressure imbalance has occured, pushing the plunger in the center piston ,which makes the contacts in switch connect.

Not good to Ignore Light, Some Folks need to. Of course hitting the tree is a good clue as well to them.

Dan @ Chockostang
 

·
(actually Slim Jr now)
Joined
·
24,597 Posts
Discussion Starter #6
I quoted right from the 68 Ford shop manual in the Mustang power disc brake section.
Not my opinion, typed word for word. I don't want to be right. No one has yet actually adressed this part of the shop manual. The line quoted by jfstang is also in my 68 shop manual. I'm aware the 70 design was simplified. I have the 70 Ford shop manual and a 70 Torino GT vert with power disc brakes. The side loop 68 proportioning valve is confusing. The standard drum brake differential pressure...warning light set up is no problem. Easy to see how the brakes work fine after repair even if the shuttle remains shifted.
 

·
Registered
Joined
·
35,892 Posts
I quoted right from the 68 Ford shop manual in the Mustang power disc brake section.
Not my opinion, typed word for word. I don't want to be right. No one has yet actually adressed this part of the shop manual. The line quoted by jfstang is also in my 68 shop manual. I'm aware the 70 design was simplified. I have the 70 Ford shop manual and a 70 Torino GT vert with power disc brakes. The side loop 68 proportioning valve is confusing. The standard drum brake differential pressure...warning light set up is no problem. Easy to see how the brakes work fine after repair even if the shuttle remains shifted.
Now I'm gonna have to look at the reference you noted tomorrow. I can tell you for a fact that the 67-69 proportioning valve is external to the differential valve. In this photo, the brass cylinder toward the front is the proportioning valve. As you can see, it is entirely not part of the differential valve. If somehow the proportioning valve could be bypassed, sending full pressure to the rear brakes, these brakes would lock at the first hard stop, the car would swap ends, and you'd fly backwards off the road. Not exactly a safety feature. More like a sudden death feature.

 

·
(actually Slim Jr now)
Joined
·
24,597 Posts
Discussion Starter #8
To clarify: The brakes work great on my 68. It bothers me that I haven't been able to recenter the pressure differential valve/shuttle. I've read on this forum about attempts by people to repair the differrential valve. Seems like this (the pressure differential valve) and the proportional valve are important parts that seem to be going begging!
 

·
(actually Slim Jr now)
Joined
·
24,597 Posts
Discussion Starter #9
Now I'm gonna have to look at the reference you noted tomorrow. I can tell you for a fact that the 67-69 proportioning valve is external to the differential valve. In this photo, the brass cylinder toward the front is the proportioning valve. As you can see, it is entirely not part of the differential valve.


Yup! Almost precisely as I described mine and what's pictured in the 68 shop manual.
 

·
Registered
Joined
·
35,892 Posts
Yup! Almost precisely as I described mine and what's pictured in the 68 shop manual.
Then you're reading it wrong. Bypassing that valve is totally impossible. Follow the tube from the rear brake, to the proportioning valve, to the differential valve. There is simply no route for the fluid to go, except through the proportioning valve.
 

·
Registered
Joined
·
3,725 Posts
Now I'm gonna have to look at the reference you noted tomorrow. I can tell you for a fact that the 67-69 proportioning valve is external to the differential valve. In this photo, the brass cylinder toward the front is the proportioning valve. As you can see, it is entirely not part of the differential valve. If somehow the proportioning valve could be bypassed, sending full pressure to the rear brakes, these brakes would lock at the first hard stop, the car would swap ends, and you'd fly backwards off the road. Not exactly a safety feature. More like a sudden death feature.

That is exactly correct, Just a loop that done the same thing (A inline circuit done back through Glorified H Block) This done the Sudden reduction.

Dan @ Chockostang
 

·
Registered
Joined
·
3,725 Posts
To clarify: The brakes work great on my 68. It bothers me that I haven't been able to recenter the pressure differential valve/shuttle. I've read on this forum about attempts by people to repair the differrential valve. Seems like this (the pressure differential valve) and the proportional valve are important parts that seem to be going begging!
The Plunger inside has jammed itself.

Once this happens it is sometimes tought to get if to slide back to center.

Dan @ Chockostang
 

·
Registered
Joined
·
35,892 Posts
Since the shuttle seals are available, I'm gonna try something. I'm gonna make a brake fitting plug with a zerk on it, and use grease pressure to push the shuttle out.
 

·
Registered
Joined
·
5,046 Posts
I simply use the blunt end of a drill bit that is slightly less in diameter than the orifice in the end of the distribution block and tap with a hammer. Has always worked great.
 

·
Registered
Joined
·
3,725 Posts
I simply use the blunt end of a drill bit that is slightly less in diameter than the orifice in the end of the distribution block and tap with a hammer. Has always worked great.
That will work recenter if the Schuttle was push from high pressure on the Rear brake side. Yes the schuttle will move back to center because it needs to move in pushing through the left front brake port.

If the schuttle was moved off center from high pressure on the front brake side (Schuttle moved to rear brake side) the Drill bit method won't work. The schuttle doesn't need moved further in, it needs moved back, toward the left front brake port. Tapping would only further drive it in tighter.

So, here is a fix. Determine the exact center of the rear brake side (No port side). Mark, and drill a 1/4" hole, or smaller, your choice, through the wall of block. Insert your device, punch, bit, what ever, Tap the schuttle completly out of the block. Proceed to rebuild, clean up, etc. Insert making sure free movement is noted.

Tap the hole you drilled, to insert your choice of threads. insert plug using JB weld. Set 24 hours/.

Install.

I kinda like 22's idea, let us know.

Dan @ Chockostang
 

·
(actually Slim Jr now)
Joined
·
24,597 Posts
Discussion Starter #16
That will work recenter if the Schuttle was push from high pressure on the Rear brake side. Yes the schuttle will move back to center because it needs to move in pushing through the left front brake port.

If the schuttle was moved off center from high pressure on the front brake side (Schuttle moved to rear brake side) the Drill bit method won't work. The schuttle doesn't need moved further in, it needs moved back, toward the left front brake port. Tapping would only further drive it in tighter.

So, here is a fix. Determine the exact center of the rear brake side (No port side). Mark, and drill a 1/4" hole, or smaller, your choice, through the wall of block. Insert your device, punch, bit, what ever, Tap the schuttle completly out of the block. Proceed to rebuild, clean up, etc. Insert making sure free movement is noted.

Tap the hole you drilled, to insert your choice of threads. insert plug using JB weld. Set 24 hours/.

Install.

I kinda like 22's idea, let us know.

Dan @ Chockostang

Like you said above. I don't know which way the stuttle is shifted. Was bleeding the brakes all around. Did it with no concern about the shuttle/warning light DUH! Get done, fire it up to make sure/check the power booster and it works fine. One problem, everything works great but the warning light is on. That was over two years ago. Took the switch out and confimed it's good. As last resort disconnected the warning light.

I have no idea which way the shuttle is shifted. Have tried the recentering suggestions to no avail. Not anxious to risk butchering my perfect (looking) original pressure distribution valve. I'm on the look out for an original I can futs with then use to replace mine.


Slim
 

·
Registered
Joined
·
35,892 Posts
I simply use the blunt end of a drill bit that is slightly less in diameter than the orifice in the end of the distribution block and tap with a hammer. Has always worked great.
Sure, but if it's too far in, you can't pull with a hammer.
 

·
(actually Slim Jr now)
Joined
·
24,597 Posts
Discussion Starter #18
Thanks!
 

·
Registered
Joined
·
35,892 Posts
Mea Culpa

Look at 68 shop manual 2-11 Fig3. Read page 2-10 and examine Fi19 page 2-22. What is shown at the bottom is exactly what's on my 68. The proportion valve runs internal to the to the prtessure differential valve. The proportioning valve mounts at the front of the pressure differential valve. The inlet and the outlet connect to the differential valve body.

To quote page 2-10:
On disc brake equipped vehicles, the pressure differential valve will move to the low pressure area of the front system. (this is with front leaking/failure). This movement uncovers the rear brake system outlet passage and provides a direct passage from the rear inlet passage to the outlet passage, BY PASSING THE PROPORTIONING VALVE. This provides full hydraulic pressure to the rear brake system.

I apologize for using CAPS above but it's a lot of typing/reading.
I got copies of the quoted sections, and checked out the diagram.

I seems the above analysis, despite my earlier comments, is correct, concerning 68-69 differential valves.

If front brake pressure fails, the shuttle valve illuminates the warning lamp, and bypasses, via the cross-connected rear brake line, the proportioning valve.

This provides the maximum possible pressure to the rear brakes, so make stopping using the rear brakes only as easy as possible. This is a good idea, since the proportioning valve will still be trying to provide proportional pressure, in this case relative to zero. While the rear brakes will still get pressure, it does make stopping difficult. If you've ever had a proportioning valve failure in a 65-67, you'll know what I mean.

Allowing full pressure to the rear brakes will allow maximum use of the rear brakes in a front failure. Of course, this make it easier to lock them, requiring some judgment on the part of the driver.

This is the differential valve diagram slim referred to:

 
1 - 20 of 28 Posts
Top