65 66 GT350 proportioning valve adjustment

[Bob Gaines]

I am tying to get a consensus on the number of threads showing on undisturbed 65 66 GT350 proportioning valve adjuster. The valve was adjusted differently then a regular Mustang because of the larger rear wheel cylinder and brake shoes used on a 65 66 GT350. A regular Mustang typically had 12 threads showing for example.

[GT350Lad]

Bob, when I got 6S1276 the valve was in the trunk and looked untouched. Date code on the end was correct. It had 7 /12 threads showing
Keen to hear others

[texas swede]

What I have been told is that the GT350 valve showed 8 treads and was painted black.
The one that came in my car (5S275) when I bought it in Feb.1988 is dated 39-5.
Never checked the treads on it when replaced it in 1990. Don't remember if it was black or just rusty.
My Swedish friend rebuilt the original and I painted it black and adjusted it to 8 treads.
Haven't put it into the car yet.
Texas Swede

[68blk500c]

I have a C5ZZ boxed nos just like Bob's image with 12 threads, plus another installed on 65 Mustang GT that is the same.

Also, have a used valve from a swap meet (unknown origin) that is painted black and has 8 threads.  It has an interesting attaching bolt with head marking of X.

[Bob Gaines]

A regular Mustang proportioning valve was painted battleship gray. Earlier 65 GT350 proportioning valve would also be that color because it was adjusted at SA. It was noticed that the color of the valve on the later cars was painted black instead of the typical gray as on the disc brake Mustang. It has been postulated that when SA got Ford in a little later 65 production to start adding Shelby specified different components on the Ford assemblyline it included a already pre adjusted (for 2 1/2 shoes)proportioning valve.These Ford assemblyline installed parts saved SA labor.The Ford installed 2 1/2 brake adjusted proportioning valve was painted black, I and others have assumed for identification purposes. Interestingly the 65 and 66 Corvette also used the same proportioning valve which coincidentally was also painted black. The NOS Corvette service valves that I have seen were pre adjusted the same as the regular Mustang painted gray versions. That would conflict with the identification assumption on the GT350 valve however.     

[Bob Gaines]

Unfortunately in regards to the color the vast majority of original installed valves are typically rusty and consequently causing paint to flack off and making it hard tell what color they were originally.

[J_Speegle]

I am tying to get a consensus on the number of threads showing on undisturbed 65 66 GT350 proportioning valve adjuster.

Not all om those that I have pictures of are at the right angle to count the number of thread but here is what I have on file. The number of visible threads likely vary based on what side your viewing from




Just 1966 models below in a fairly quick pass through the pictures

1- Approx 10 threads or 3/4" from base to end two additional thread past the base of the reliefs in the sides

2- Approx 10 - one additional thread past the base of the reliefs in the sides

3- Approx 15 (later car)

4- 4 threads pass the relief in the side of the threaded

5- Approx 15 (later car)

6- Approx 11 (later car)

7- Approx 2-3 past the relief

8- Approx 12

9- Approx 3 past the end of the relief PS

10- Approx 3 past the end of the relief PS - Carry over car

Hope this helps

[GT350Lad]

Thanks all, I correct mine to 7 1/2
It's hard to see now it's in car and it does depend on angle. Taking a photo with phone and zooming is key
I will look at 6S373 this weekend

[Bob Gaines]

I am tying to get a consensus on the number of threads showing on undisturbed 65 66 GT350 proportioning valve adjuster.

Not all om those that I have pictures of are at the right angle to count the number of thread but here is what I have on file. The number of visible threads likely vary based on what side your viewing from




Just 1966 models below in a fairly quick pass through the pictures

1- Approx 10 threads or 3/4" from base to end two additional thread past the base of the reliefs in the sides

2- Approx 10 - one additional thread past the base of the reliefs in the sides

3- Approx 15 (later car)

4- 4 threads pass the relief in the side of the threaded

5- Approx 15 (later car)

6- Approx 11 (later car)

7- Approx 2-3 past the relief

8- Approx 12

9- Approx 3 past the end of the relief PS

10- Approx 3 past the end of the relief PS - Carry over car

Hope this helps

I am tying to get a consensus on the number of threads showing on undisturbed 65 66 GT350 proportioning valve adjuster.

Not all om those that I have pictures of are at the right angle to count the number of thread but here is what I have on file. The number of visible threads likely vary based on what side your viewing from




Just 1966 models below in a fairly quick pass through the pictures

1- Approx 10 threads or 3/4" from base to end two additional thread past the base of the reliefs in the sides

2- Approx 10 - one additional thread past the base of the reliefs in the sides

3- Approx 15 (later car)

4- 4 threads pass the relief in the side of the threaded

5- Approx 15 (later car)

6- Approx 11 (later car)

7- Approx 2-3 past the relief

8- Approx 12

9- Approx 3 past the end of the relief PS

10- Approx 3 past the end of the relief PS - Carry over car

Hope this helps

All of those observations indicate a regular Mustang adjusted proportioning valve regardless of if in a original car or not. Any threads after the relief on the side of the adjuster represents more then 10 threads. Possible replaced like many have with a service unit which is adjusted to regular Mustang specs from Ford.

[J_Speegle]

All of those observations indicate a regular Mustang adjusted proportioning valve regardless of if in a original car or not. Any threads after the relief on the side of the adjuster represents more then 10 threads. Possible replaced like many have with a service unit which is adjusted to regular Mustang specs from Ford.

Appears by my count that there are six examples I posted that are adjusted for Mustang.

Looks like I did list possibly three examples that appear to be in the 10 thread range (depends on what side your counting I would guess. Three of the examples are either low mileage and reported to not have been replaced by the original owner and all three are unrestored. Two of the three still has some black paint on the main body and not on the adjuster.

Just sharing as r  equested

[Bob Gaines]

With the 65/66 cars being close to 60 years old a lot of the proportioning valves have been replaced. One way to tell if a proportioning is most likely replaced with a service unit is by looking at the top surface where the fittings are. It seems like that starting in the later 60's the mfg machining process changed. The later proportioning valve had that top machined flat. The assemblyline and probably early service units had a machined circle centered around each fitting opening . The circular machining left a ridge around the outside perimeter of the circle which is visibly pronounced. The picture shows the later NOS service valve with flat top on the left and the assemblyline unit with the pronounced witness ridge from the circular machining process on the right. 

[s2ms]

To add to Bob's comments, I've seen a few early valves where the ridges are deep cut and not beveled. IIRC this valve had a late 1964 date code...