I beg for help to fix this ATF automatic transmission fluid leakage.

I own a very dear old 1979 F150 truck. I use it infrequently......perhaps 6 times per year and only locally so very few miles per year.

As it sits over a couple of months it bleeds a few cups of the red fluid beneath it on my garage floor.

If I do not top it up every time, to replace that loss, it slips and is totally slow to engage. In fact I even have to give it hard pedal for what seems like forever to convince it to move. If I top it up to replace what is lost it is fine. After it heats up from about 4 miles of driving it is also fine.

How can I correct this to where it retains its fluid??? And am I damaging it by letting it slip when/if I do not top it up? I do not want to abuse this very fine old truck. Also if I were to replace the tranny how much would that cost and what is the best way to go.....rebuild, replace, repair. Money is an issue.

Any Help will be greatly appreciated. The rest of the truck incl engine is as good as the best for this age of truck.

 

It is the original C6 tranny. Only the rear wheels are driven. Not a 4x4.

 

Are you able to get under the truck to identify what is leaking? I would plan to never put the truck in Drive without doing a cold check of the fluid level via the dipstick. You will probably already be too low to measure. Then, top it off and you should avoid the slip.
It could be an easy leak to fix, so don't consider a rebuild yet.

 

Check the dip stick tube, more than likly thats where its coming from when the fluid drains back from the converter, and yes on the damage to the tranny slipping it. If it get's that low do top it off, but let it sit and idle before you check it because you want it up to temperature. When you know the fluid is correct shift back and fourth from R to D a few times and make sure its going into gear fine before you step on the gas. It maybe slipping because fluid is to high.

 

My '77 was leaking some like that from the front seal a few years ago as I was only using it a few times a year and found myself adding fluid. It has just over 105,000 miles now, original trans.

In 2010 I retired from a career and even though I bought a newer truck, I found I was hanging on to the '77 ..... I put it up for sale .... but everyone wanted me to give it to them. So I decided to use it some, and one thing I did was I added one quart of Trans Stop Leak. It was a grey bottle?

Since then I make it a habit to use it at least once every couple weeks, and some weeks I drive it to and from work several days in a row in my part time fun job. I don't know if it was the quart of trans stop leak or if it's the use or a combination ..... but I haven't needed to add any trans fluid in two or three years now. Trans works perfectly.

I also had added a shift improvement kit back in 1986 when I bought it, only had 35,000 miles then ..... and I have always since then had an extra cooler inline with radiator cooler.

I also have made other improvements and find I really enjoy driving the old girl still.

My transfer case leaks a smidgeon, it's a NP205, I just check it every year and top up if needed along with rear axle.

 

Also check the shift shaft seal. (Not sure if the design is the same as a C4, but mine was leaking badly after it sits for a few days with the converter drain-back.) It's on the side of the tranny where the shifter linkage hooks to. The linkage turns a shaft that changes gears (PRND1.)

 

I also had the same leak at the shift shaft It took a while for me to find it since I had replaced the pan gasket and suspected it was the leaker.

 

BudMud.....can you explain more on the dipstick tube leak. Is where the tube is bolted down to trans housing where it leaks. Is there a gasket or o ring there???

 

Again I'm assuming the C6 is designed the same as a C4. On a C4, the dipstick tube just pushes into the trans fill-hole and is sealed with a O-ring. The mounting bolt/bracket higher on the trans positions it straignt and keeps it from moving. All you *should* have to do is remove the mount/bracket bolt, and then with a little wiggle and twist you should be able to pull the tube up and out. (If you fluid level is high enough you will have a spill, so put a pan under it. ) If the O-ring is old and hardened - it could certainly be a cause of your leak... Good luck!

 

If the dip stick tub is leaking Due as markintexas suggested but also add just a bit of black RVT around the oring. A lot of times there is a dent in the tub and the RVT will fill it in. If it slips when its full but cold then you have hard internal seals in the unit. The reason it works fine when warm is as the unit warms up the seals inside soften and then the pistons can aply the clutches. I am a trans builder for a living. We call this morning sickness and the colder it is out side the worse it will be. There is very little you can do but to rebuild the unit. If your handy enough to pull it any decent tranny shop should be able to build it for about 800$ or less as this unit is as easy as they come and very rarely need any hard parts. I have never understood why some one will put 3-5k into there motor but expect a junkyard or worn out trans to put up with all the extra power. Then when it cost 800$ to do the trans right they cheap out. I dont care how well your motor runs if you cant put the power to the ground your not going anywhere. Ok sorry for the rant. Check it in the morning with it full. If it give you issues well then you know how I feel about it. You can try some of the products on the market the swell the seals but the issue with all of those products is they are not selective meaning they dont just swell the rubber you need but they swell all the rubber and every part of that trans uses rubber seals. They swell and swell until the blow out. Then your left on the side of the road. Of course this is all JMO

 

I am so grateful for all suggestions to date. What an amazing group of generous helpful fellow human beings. I now have some great shared experience and guidance to start troubleshooting. The dipstick tube and the shift shaft are on my list plus everything else also. I will do a thorough cleaning and use paper towels etc. to track and locate the source. Hopefully I can find a single smoking gun reason. Once again thank you all so much and if any new or current posters wish to add more please feel more than welcome to do so.