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Replacing MC and it looks like I can remove the connections from the old one and instal them into the new one and use my original line hook ups. But, it looks like Bubba put in some sort of valve in the one line and I don't know if this was just what he had in the toolbox to make a right angle connection? Or, is this valve some sort of proportional set up to fine tune the psi in two different lines? Not a petcock, per se, no fluid or air comes out the handle, it just opens and closes fluid going to one line.
Is there any benifit to removing this or leaving it in place? Wondering if this set up would allow the valve to be closed when working on brakes fed by the other line and not having to bleed the entire system as it would close off any air getting to the line that goes through this connection.

My first brake job on the M37 and everything but the lines is being replaced. Got the old shoes relined for $22 a shoe which I thought was not bad. Getting the three drum screws off with welding on a nut to each has been the hardest part so far. But I haven't installed the new rubber hoses yet.

Helps to post the frickin picture I took...

Looks to me like the stock vent valve on top of the master cylinder cover. Here's a screen capture from TM 9-8031-2 Powertrain Body and Frame M37. If you don't have this manual, it would be very helpful, along with the companion engine/clutch and parts catalog.
The valve is in the vent line (not the output pressure line) for the reservoir. The valve is for use during deep water fording.

Extremely good and useful information. I was worried that I messed up the compression nut and now knowing that this is only vented air, I'm not going to lose sleep over a tight fit. Question . . . would this valve normally be in the closed position and only opened to vent through the line that I presume is connected to the air filter or snorkle set up? Or is it normally open and only closed when fording?

Normally open.

Otherwise, no venting occurs.

PoW

There is also a one-way valve available that some guys install to avoid a spongy pedal. I saw one advertised on "Trucks" years ago.
Gary

If you use a pressure bleeder, be sure to close the valve, or you will pump out the new fluid into the vent line. I seems to always forget this when I bleed and flush my brakes, Hal in UpState SC

Thanks guys. I'll leave it open while I manually bleed the system with pumping and opening and closing the wheel cylinder bleeder balves and see how I do. Then leave it open all the time unless going under water, which is not on the agenda.

TO THE WORKSHOP!!!