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Hey guys, whats the purpose of the master cylinder vent line? I don't know if the petcock should be opened or closed. I'm planning on replacing my master cylinder but the vent line fitting and tubing are so corroded I doubt I'll be able to get it off without destroying it. Where does this vent line actually vent to and do I really need one. I heard that some guys have master cylinders that don't even have a vent line provision. Still trying to bleed the brakes but still no pedal. Replacing the M/C is my last hope. ARG!!!

Bill,
The vent line goes up to the intake elbow and allows clean air in and out of the master cylinder when the fluid level changes. It should be run open. If you are having trouble getting a pedal try pressure bleeding the brake system with the vent line CLOSED. I've never been able to get a good pedal by manually bleeding the system in the M37.

Andy

I used a Motive power bleeder. Bought from Jegs Racing a 1/2 inch NPT plug, with a threaded hole in the top, into which I screwed (with teflon tape) another NPT with a 5/16 nipple on it. Then ran a clear plastic hose from Home Depot to the Motive, turned the T on the vent closed, and pumped up the Motive, and then bled all around, this after making all new brake lines, with new bleeder screws. No leaks, and a hard pedal.
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Thanks guys for the ideas. Decided to make my own power bleeder out of a short piece of 1 1/2 pipe with all the adapters to fit the master cylinder on one end and an air compressor fitting on the other. It probably holds about twice the amount of fluid then the M/C. Used only about 10psi and bled all the brakes in minutes. But to my surprise still no pedal. Decided it might be a bad M/C. But when I finished removing the tubing fitting at the M/C the other end of the tube going to the frame rail just fell off. It was cracked almost all the way around at the fitting. It didn't leak while bleeding but must have been sucking air. Part on order and I'll update when I install it. Thanks again !

that's a pressurized fitting, it should have been leaking fluid during bleeding if that's the issue. sounds like it needed replaced regardless.

I put together a motiv bleed setup just like Nam vet's. When I was done, I put a filter on the hose end which connected to the power bleeder and coiled the hose up ... it's still there seven years later. I don't have the vent line on the truck

Finally got a good pedal. Ordered a master cylinder so put it in anyway. Changed the cracked tube from cylinder. It seems my problem was that the brake shoes were not adjusted properly resulting in too much pedal travel. Nice hard pedal but still stops as fast as an aircraft carrier, lol.