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Downdraft tube education...
Saw this downdraft and was curious if it was to keep oil splatter down.
http://i1060.photobucket.com/albums...0Uploads/20170408_115322_zpsxa74hwv5.jpg

That is a factory-installed PCV valve system. It was standard equipment on some of the heavy duty rigs, and optional on others, I believe. It eliminates the road draft tube altogether, and recycles crankcase vapors through the intake manifold.
Jerry

The PCV change over is a great upgrade to the 235s, I've done this on my motors, also help with crankcase/rear main leaks etc.

thanks. did not know it was a factory-installed option.

Here is a picture of the accessory install kit.
https://www.flickr.com/photos/advance-design_parts_co/8460085515/in/album-72157631868452754/
This is one installed on a '54 2-ton I parted out.
https://www.flickr.com/photos/advance-design_parts_co/8129417263/in/album-72157631868452754/
This is a picture from a 1948 vintage filmstrip showing the install on a Dubl-Duti.
https://www.flickr.com/photos/advance-design_parts_co/8665388167/in/album-72157633290718528/

The advantage is that the road draft tube uses vehicle speed to induce a light vacuum in the tube, which doesn't work at all when idling and slow traffic.

Interesting.. And Thanks for the info, I've not ran across one before.

Just don't forget to clean the internals- I took mine off and it was completely blocked- no air could be forced through the tube packing....I'm SURE that this would cause a leaky engine!

Additionally, my 261 had that exact same PCV system as in your pics when I bought the truck. If you drive the truck regularly, it's kind of a bummer to have a metal vacuum line going over the valve cover that has to be removed every time you want to adjust the valves... if I decide to re-install it, I'll be routing the pipe clear of the valve cover...

I have been wondering about the whole PCV thing, though- if you make any modifications to the engine, such as a camshaft change, then you will be changing the idle vacuum, which will, in turn, change the PCV valve requirements. So the factory PCV might not be calibrated for your new idle vacuum....and will not work properly, right?

Why does the PCV help the rear main leak?

Not doubting you, just curious.

The PCV keeps the crank case under a negative pressure so anything st the seals or gaskets is being drawn in rather than pushed out.

Makes sense.

I put a PCV system on mine using a modern valve for about the same displacement engine. I also have a bypass filter. I change the oil at 3000 miles and it doesn't darken any faster then a modern car.

...I a PCV valve on my 261 using the info provided by... I think Dave. I havent' driven it on the road but it seems to work great and Dave...or maybe it was Deve, even suggested the PVC number to use and why.
Yes, it all makes sense.

EDIT: and I just want to give a shout out to Brad Allen for all the info and great stuff he provides for us. THANK YOU BRAD.

If you are already using the single port on a Fenton intake manifold for a power brake booster, how would you choose to plumb a PCV system into it? Is there a way to set it up so that you don't have to drill another hole in the manifold?

I used a brass 'T', one side to PCV other to vac wipers or what ever.

JiMerit,

I thought about that, but was concerned that I would run into erratic/low vacuum at the booster from sharing a port with something that is actually "flowing" , or that I might get oil vapor working it's way into the booster (which is mounted lower, under the floor). Perhaps that should not be a concern, I don't know.

Loop the vacuum line up near the top of the firewall before routing it to the booster under the floor to prevent oil from getting where it doesn't belong. The PCV has a pulsating flow and it shouldn't interfere with the vacuum going to the booster. Most brake boosters have a one-way check valve built into the fitting that goes into the vacuum chamber, so there's at least one brake application with power assist available in case of an engine stall.
Jerry

Even though there's lots of talk about converting over to a PCV, I would still like to keep my Road Draft Tube. What is the best way to clean out a clogged tube?
Thanks
DougR

Thanks, Jerry- I will give that a try.

Doug- I usd kerosene. Popped off the top and let it soak overnight. I had to use a pick to clean out some of the baffle holes. Lots of filling and flushing.

If you've got an old gas grill, pull the tube out of the engine,put it in the grill, fire up both burners as hot as they'll go and close the lid. Come back in 3-4 hours and turn the heat off. When everything cools down the sludge will be ash that can be shaken out or purged with an air hose. Don't try this procedure with a grill you're still using for burgers- - - - -it makes 'em taste a little funny afterwards!
Jerry

Use your neighbor Ned Flanders's grill, there Homer?
Okely dokely neighbor...