Info needed: M38A1 vent tube

[Les Kovacs]

It has just come to my attention that many M38A1's have a vent tube from the valve cover to the oil fill tube. I do not have this on my 67 CDN2....what was this for and can the lack of this vent tube cause high oil consumption?

Thanks in advance,

les

[rob love]

For air to be drawn out, air has to be drawn in. Yes, you should have the vent line from the intake hose to the filler tube. Perhaps your jeep has a different vent from a CJ? Seems to me there used to be a tube hanging from the side pan on the CJs, and I think I recall seeing CJ valve covers with an inlet on them as well. It's been a lot of years since I dealt with the civilian engines mind you.

[Les Kovacs]

Hi Rob....the only rubber tube going to the oil filler is from the upper fording valve between the air cleaner and the carb. The only other place where the engine may be venting is at the side valve cover to the PCV valve and lower fording valve both of which are still there but open....is this enough engine venting or do I have to find a valve cover with a vent hole and a oil filler with two vent holes?

Please let me know.....maybe the lack of a valve cover vent is what is causing the high oil consumption?

Regards,

les

[rob love]

The fording valves are to defeat the PCV system. The size of those tubes is all that is required.

By closing the two valves, you shut off the ventilation system thereby causing some level of crankcase pressure due to the normal blowby (and about 15,00 miles into the life of the engine, the abnormal blowby) of the pistons. This pressure, which may only be a few pounds, helps seal the engine from the ingress of water. Of course, it can also cause seals to blow should it be applied for too long a period of time when there is no need for it.

Personally, if you are not leaking oil, I would live with the problem. One possibility is that the rings have not yet set in on a freshly honed engine...it can be a few hundred miles up to a thousand miles before they fully seat, depending on the tolerances followed.

Another possibility is that the rings may not have their gaps spaced around the piston. I have opened up engines before where the OCD mechanic lined up all the spaces, which is a very rooky mistake.

[Les Kovacs]

Hi Rob....again, thanks for replying to the venting/oil issue.

I was told that and as per jpeg dated pictures by the rebuilder that the build was completed in 2011....but the seller that I got it from bought it in 2015 from the builder....I am not sure what the builder did with it from 2012 to 2015....the first buyer only put 600 miles (1000kms) on the jeep as per the safety. I have since put on another 800 kms and still the oil consumption....maybe he did not hone the cylinders? The jeep was not meticulous by any means even though it was advertised as bolt to bolt restoration....it was re-built more for resale to somebody else so there compromises as I am finding out now.....if the oil consumption continuous I will live with it for a few years then rebuild.
Right now I will deal with other issues as fixing the front crank seal & cover gasket as there is a small leak there but the oil gets onto the belts and they spray the oil everywhere.

Next questions....is it easy to replace a master cylinder?....he replaced it too in the 2011 rebuild (not rusty at all) but its leaking at the rubber boot big time. Yes I do have the heat shield installed.

thanks,

les

[rob love]

Master cylinder isn't terrible, but it is a little tight working in that location. Remove the vent line (if it even still has one...most of them are long gone). Remove the banjo bolt at the back of the master cylinder (if memory serves the brake light switch is on the banjo bolt, so remove the wires from it before turning the bolt), then remove the two 3/8 bolts that go thru the master cylinder with a 9/16th wrench and you should be able to slide it away from the brake pedal. A ratchet wrench will be worth it's weight in gold for that last pair of bolts.

Leave the pushrod alone even though you may get a new one with the new master cylinder.

By this time you should be wearing a little brake fluid along with whatever crud the jeep has been into for the past decade. Put the new master in place in reverse order of removal, with the new boot attached to the rod. Check for freeplay at the pedal (you should have something....even a 1/4" will be enough, 1/2" will be ideal). Fill the master cylinder and bleed the brakes. While bleeding, or when the cylinder is empty, be careful not to bottom the brake pedal into the cylinder or you can damage the rubber cups.

If you have the vent line, great, reinstall it. If you don't, you cannot merely plug the vent hole. You can install a breather of some form, or your new master cylinder cap may be the type with the little vent holes built in. Not ideal if you ever want to drive through deep water, but OK for everyday use.

Master cylinders are cheap. Napa part no 2796 is a little pricey at 151.28 retail or 92.99 trade



however rock auto in the US has CENTRIC 13063003 for $38.13 CDN each.

[Les Kovacs]

Hi Rob....thanks a million....sound fairly easy but messy.....I am assuming that brake fluid will puke out and empty all four lines. Also, are these master cylinders poorly built? As I mentioned in my paragraph the one I am replacing is not rusty (but pukes fluid out of the plunger side) at all and dude said that he replaced it during the rebuild in 2011 so there should be only about 2000 kms on the unit.

PS....I bought a replacement cylinder for about $70.00 CDN delivered from either Eriks Military or Walcks Jeeps.

Regards,

les

[Wayne Hingley]

Les; I have found the biggest killer of these master cylinders (and the wheel cylinders), is lack of use.

If they sit for lengths of time they tend to start leaking.

[rob love]

Originally Posted by Les Kovacs

Hi Rob....thanks a million....sound fairly easy but messy.....I am assuming that brake fluid will puke out and empty all four lines. Also, are these master cylinders poorly built? As I mentioned in my paragraph the one I am replacing is not rusty (but pukes fluid out of the plunger side) at all and dude said that he replaced it during the rebuild in 2011 so there should be only about 2000 kms on the unit.

PS....I bought a replacement cylinder for about $70.00 CDN delivered from either Eriks Military or Walcks Jeeps.

Regards,

les

They won't empty, since the lines are kind of at their high spot at the master cylinder. But you will end up with some minor loss, so you will have to bleed the lines. There are only two lines at the master cylinder...they each split up at the diffs to make 4 lines. You will most likely get away with just bleeding one front and one rear wheel, but quite frankly fresh fluid to all 4 wheels is a good thing.

As Wayne says, it is the sitting that kills these things. As well, if the boot is not on properly, it can fill with water depending on when you drive it, and that water will sit there rusting at the front of the cylinder. But also, brake fluid itself (Dot3 and Dot 4 at least) are hydroscopic, so will absorb moisture from the air. So if there is no vent line, water or moisture in the air will get into the master, be absorbed by the fluid, and can start rusting the cylinders. Ideally, you flush the fluid every few years, although nobody does. As well, a cylinder installed in 2011 is very near it's end of service life anyway......I usually count on 7 years of service for this older stuff.

That said, my own master cylinder on my Jeep is probably 10 years or older and still working just fine. But it is driven all the time in the summer months, and held in deep freeze storage the remainder of the year.

[Les Kovacs]

Thanks Rob & Wayne....I do not drive the jeep in the rain, summers only as this year and it is now parked in a cold but dry garage. Once replaced, I will try to pump the brake a few times in the winter months to exercise the master cylinder.

Thanks for the tips,

les