C4 is a major bog

[9t3svt]

Also make sure it is not trans fluid in the line, indicating the diaphragm is already bad.

[bbyrnes1]

When I take the vacuum modulator out of the transmission, will trans fluid come out?

And also can comeone please verify that originally the hard line threaded directly into the intake manifold and that the only rubber is to connect the other end to the transmission?

Thanks for all the input guys,
Brandon

[Donnie]

If you jack up the right side of the truck, most of the fluid will move to the other side of the pan, put a safety stand under the frame & take a small gallon coffee can with you...BE CAREFUL here as there is a 1/8 dia pin that may fall out. It is a little over an inch long & connects the diaphram in the can to the" real" valve that is in your trans. You will not lose a lot of oel. No need to drain the pan.
AS far as a direct steel line, some had a thread in connection, but others used a short "rubber" connection at the manifold.. If you have an option to screw in a metal line, get a brake line from your parts store & bend it to fit...DON'T forget what I said about creating a loop in the line that is HIGHER than your carb. You should have a 90 degree "rubber" elbow at the trans..
You do not want a "rubber" hose from the engine to the trans===no good
REMEMBER A TIRED ENGINE = low vacuum=late shifts ....................also "rubber" mentioned throughout this thread should say"NEOPRENE"
If you ask your parts store for "Bundyflex" brake line, you can make some pretty sharp bend's in it..A young counter man will not know what you are asking for ..find an old NAPA store .........donnie

[bbyrnes1]

Donnie,
What's the advantage of having the line looped above the carburetor? I don't understand how gasoline could get down that line. Any vacuum pulling the air/fuel mix from the carburator would be the engine pulling it into the combustion chambers to my understanding.

Is there a way that the tubing mounts to the side of the bellhousing or transmission, or is it left hanging like the cooler lines?

Thanks again,
Brandon

[Donnie]

Your assumption is 90 % correct...Here in lies the answer: Suppose your accelerator pump takes a dive & your engine backfires thru the carb?
Now your vacuum in the intake manifold is no longer being pulled into the cylinder, the force of an engine backfire thru the carb is actually the compression pushing the fuel mix back into the intake, ever see a back fire flash from the carb, shooting fire into the air a foot above the manifold? Think about it !
About the metal line to the mod can, there should be a bracket/ hook type affair attached to the rear extension housing bolt right above the mod where you can clip it into
HTH.....Donnie

[bbyrnes1]

That is a big help Donnie. I'll look for the bracket to mount it to the transmission. Afetr a moment of "DUH" I got to thinkingh that this line was made out of an old fuel line, so I'm probably getting my smell from it's former purpose and probably not getting gasoline down the line.

What was the original diameter of the modulator line?

Thanks again,
Brandon

[Donnie]

I believe it was 1/4 inch........

[bbyrnes1]

Just want to update. I replaced the metal line, put a new piece of vacuum hose, and replaced the vacuum modulator. It now shifts almost imperceptably at regular acceleration and mildly aggressive at high throttle. Looks like that was the source for my garbage shifting. I had to grind down a wrench to get at the modulator. Thanks for the help guys.

Brandon

[ADE ROWBERRY UK]

I am attempting to rebuild my C4 (for a 1979 F100 Custom) at the moment.
I noticed that when removing the trans, the vacuum line was connected to the modulator also via a short length of rubber hose _ this was easily pulled off the modulator - is the use of a short rubber line (3-4 inches) completely out or with hose clamps is it OK?
If this is leaking, could it - in part explain why my forward clutch has burnt out?
Hope you don't mind me crashing this conversation!

[Donnie]

Ade, there should be a 90 degree "rubber" elbow at the end of the metal line coming down from the trans . It is a tight press fit to both the metal line & the modulator can ....no hose clamps are used...if it was soft, either the diaphram in the can, (modulator) is leaking or fuel is getting into the line..........most likely the mod. is faulty....If your unit was experiencing faulty/ slipping shifts in foreward gears, most likely you will find the rubber seals on the piston in this drum are brittle (hard) thus allowing fluid to bypass & not apply your foreward clutches, thus resulting in clutch wear.....The seals in this drum are square cut & are prone to wearing out from normal movement & there is some heat transferred to this drum thru the input shaft which is directly connected into the converter where 90% of the heat is generated...Heat & age take their toll on these seals.....You should find that they will snap like a pretzel.
It is falsely thought that the band apply & the clutch apply cause the most heat...This is only true if they are SLIPPING when applying..
Fluid shear in the converter is the heat generator...The higher the stall speed of the converter====more heat is generated....

And no, you didn't crash the thread, Brandon is on the road.....the saga of the C4 continues..................HTH.....Donnie