Venting master cylinder

[Jay Turnbull]

Yo!
Replacing master cylinder on a M43. Vent line from the side of the top plate on MC was cut and tucked under frame edge. . God knows how long this was like this.
Reviewed manuals and confusing as to where the original line went to.
New MC has no vent aperture. Checking out the other trucks, the lines appear to connect to the carb inlet.
I can switch the top plates and make up a whole new vent line as it is missing. This will be an ENDEAVOR. Do these lines have to have the double crimp as well? These vent lines on my other trucks are smaller diameter than the brake lines.
What value this line in a system that is to avoid moisture?
BTW, I am updating to silicon brake fluid, new lines,wheel cylinders,etc,etc.
Yes, even the brass connectors and flex lines.
What if I leave the master cylinder as is and unvented?
Previously, I rebuilt or replaced my other M43s MCs with vent lines in place, simply reconnected fittings, etc. Kept DOT 3 in those.
Thanks in advance.
Jay

[gusbratz]

I think it has to be vented because as the brakes are applied and the shoes go out the fluid that is displaced to make that movemet happen needs to be replaced with air. Originally it was vented to between carb and air cleaner as these trucks were waterproof and deep water fordings would contaminate the brake fluid with water. the carb was vented to the atmosphere via a snorkel..you should see everything vented to that carb. gen,dist,fuel tank. i took all that stuff off (except for the dist) because it was in my way for the things I do with my truck.

[Charles Talbert]

You have to have a vent. If you look closely, you will find your new M/C has a vented fill cap. It's fine like that if you plan on keeping the M/C above water, otherwise you'll need to hook the tubing back up. Yes double flaring is required for use with inverted flare type fittings.

[Kaiser2boy]

I went about 9 rounds with the brakes on my M37 and finaly won. You do need a vent or a vacuum will build up in the MC.
Good luck bleeding the system. Remember pump slowly to bleed. Fast pumping just makes millions of tiny bubbles.
Note The book I have , TM9-8031-2 shows the MC having a shut off valve on the breather vent. Not just sure why its there. I eliminated the valve so it runs directly to the carb elbow.

[Jay Turnbull]

Venting master cylinder.

Thanks Team!
Yes, the new MC has a vented fill bolt.
I did locate a better than average copy of the TM diagram showing vent lines and brake lines. What a route to vent!
This rig will not see high water in my ownership . The maintenance log for this truck will CLEARLY show the self-vented MC and that I opted out of the vent line...for now.
I traced the vent line from the carb elbow to a crimped and rolled dead-end behind the engine. Plug it I will.
Yeah, I learned about the fast pump fiasco when I asked my spouse to help by simply s-l-o-w-l-y pressing the brake peddle. Needless to say, she wanted to get out shopping that day. (what's Don Ho's favorite song "Tiny bubbles..", only the WHINE was heard and not sipped.)
Next step is dropping the fuel tank along with new fuel lines, rebuild the pump,the filter and the carb..BUT ONLY IF THE BRAKES WORK!
ANY value to adjusting the brakes tight and then bleed the lines and cylinders? Then adjust them out to specs?
Thanks again.
Jay

[Kaiser2boy]

Right. Going is good but stoping is nice also. If you are anything like me I was all head up to get her up and running. I knew the brakes needed some looking at but I figured what the heck how bad could they be.
Well I did get it up and running real nice but soon found that stopping was a real concern. Not just stopping now and then but stopping all the time.
Like I said prior I spent a few monthes off and on fooling with the breaks. For such a simple system it sure can be a PITA..
Interesting idea about adjusting the shoes fully out. If your drums and shoes are older doing the by the book system may not totally work. It will be a meld of by the book and feel to get them right.