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OK... so I'm finally finishing hooking up the new wiring from Vintage. Testing the
brake light circuit by pushing the brake pedal... no brake lights in back. Checking
brake fluid in master cylinder; it's low and I re-fill the chamber.
Brake lights work now but every time I push the brake pedal, brake fluid squirts out
on top from the (breather?) hole in the master cylinder screw plug.

What is the problem? Any help appreciated.
Jerry

"hole in the master cylinder screw plug"? The what? I don't have a hole in any of my MC plugs. Actually, aside from the filler plug, I only have the switch at the top rear, the hydraulic line going out the rear to the brake system, and the vent line at the top outside going to the air cleaner elbow. Do you have another hole somewhere or is something not connected?

Look at page 343 of TM 9-8030 for a good photo.

Ditto Mike's note-I have a solid Master Cylinder fill plug in mine. I would think you'd have to have the same, as a breather would be the path of least resistance for the fluid, forcing it up and out instead of powering the wheel cylinders. Can you try a solid plug, Jerry? Surely your wheel cylinders aren't locked up....
Please keep us posted on this one,
Gary

Sorry Mike.... my terminology was poor. It's the filler plug. The tiny hole in the filler plug looks stock.

Gary: The hole doesn't make any sense. Why have a hole in the plug and also a copper
gasket to seal the plug to the MC.

I'll try to post a pic of the plug.... hopefully showing the hole.

Jerry

Jerry It sounds like I have the same master cylinder as you. I got mine from Charles . The only difference between mine and the original one it replaced is the cover .It doesn't have an threaded opening to attach the vent line to the carb elbow but vents through the filler plug. The covers are interchangeable.

Hi Jerry,

There are indeed two different MC covers out there, I've also run across them. The correct cover, as has been stated, does not have a vent hole in the filler plug, but has a boss to attach the vent line. I think the flat cover you have was an early attempt at a reproduction of the original master cylinder.......but I'm not 100% sure on that. Regardless, having a vent hole on your MC that sits that low to the ground is going to let water into the MC if you do any playing in the water or rain. I'd suggest you get a hold of the correct cover and vent piping if you're going to get wet.

But all this aside, you still shouldn't be getting fluid coming out of your vent, whether it's a hole or a pipe to the carb. Without seeing it I'd venture to say you've got a bad seal on the MC piston. You might want to take that puppy apart and give here a rebuild.

regards,
bob

Sal wrote:Jerry It sounds like I have the same master cylinder as you. I got mine from Charles . The only difference between mine and the original one it replaced is the cover .It doesn't have an threaded opening to attach the vent line to the carb elbow but vents through the filler plug. The covers are interchangeable.

Sal: Yes my MC has no place for a vent line to the carb and appears to vent through the hole in the filler plug.
So the question is: Why is the fluid coming out of the vent hole when the brake pedal is depressed?
The MC is a replacement by a previous owner and seems to have been working fine for years.

Bob: You're probably right. Will have to r and r the MC. Something is stuck, causing fluid to come out of
the breather hole in the filler plug when the brake pedal is depressed.

Mike: I cked TM 9-8030. My MC does not have the breather tube to the carb or the boss for the fitting.
Jerry

I think Bob's right its time for a rebuild or a new one. Do you know what caused the low fluid level in the MC in the first place?

Sal wrote:I think Bob's right its time for a rebuild or a new one. Do you know what caused the low fluid level in the MC in the first place?

No apparent leakes. Backing plates are dry as well as brake lines and hoses. Have not pulled drums to
see if wheel cylinders are dry. Since the truck is still in the restoration stage, I don't drive it much.
The brakes were working fine the last time driven.

I'm assuming now that the low fluid level came from the fluid being ejected out of the breather hole in
the cap/plug.

I'll tear into the MC. Probably put a kit in it if the bore looks good.

Wil report back when I get into the MC.
Thanks to all who have contributed.
Jerry

Hi Jerry,

Just a thougth......if you get that MC apart and it's not looking so good.....or you're just lazy like me.........brandy spanking new ones (with the correct top) are only $70 on ePay.

regards,
bob

w30bob wrote:Hi Jerry,

Just a thougth......if you get that MC apart and it's not looking so good.....or you're just lazy like me.........brandy spanking new ones (with the correct top) are only $70 on ePay.

regards,
bob

Bob:
Thanks for the E-bay tip. I'll pull the old one off and take a look at the bore.
$70 for Ebay is probably not much more than a kit.
Check that...... MC rebuild kits are $18 from Vintage.
JB

A big thankyou goes out to John P. , fellow AMVCC and g741 member who brought a NOS master cylinder,
unannounced, to the club meeting for me last night.
A great example of MV folks reaching out to others for help and assistance.
Thanks a heap John.
Jerry

Kudos to John P.!

Yes, I too have a master cylinder with the top vented plug and fluid comes out as I pump up the brakes. Had a leaky wheel cylinder and replaced, now trying to bleed and fluid comes out the tiny hole in filler plug. Took plug to hardware store to try to get one that is not vented, could not find anything that would screw into MC. Do I need to have it vented or not? Or should I replace MC with different top? It makes sense that fluid would come out the hole because when you pump up the brakes it puts pressure in the lines and wheel cylinders. What to do?

Squeeky,

Love that username......the M37 master cylinder needs to be vented, as all master cylinders do. Normally in automobiles the vent is in the fill cap, but to make the M37 waterproof there's a hard vent line that connects to a boss on the master cylinder top plate. It's then vented up into the engine compartment, etc. The master cylinder has to be vented because it's a reservoir of brake fluid whose level changes as you hit the brake pedal and as the shoes wear. If it didn't vent it would build a vacuum, preventing fluid from flowing to the wheel cylinders. The vented reservoir is never pressurized when you hit the brake pedal, it's just a reservoir at ambient pressure.........so if you're getting brake fluid coming out the vent you have a problem in the master cylinder. They're really simple, and cheap, to rebuild........but make sure the bore is in good shape or you're wasting your time. New ones are pretty inexpensive also.

regards,
bob