Master Cylinder cap hole?
 

I'm hopefully bleeding my brakes later today, but I have a question on the cap for the M/C.

It has a hole in it, right in the middle on top. Looks like it should be there (center and clean) but it seems wrong to me given how much effort is put in to keeping air out of the lines. I once put more fluid in, the brakes engaged a whole lot better, but it also shot fluid (like a foot high) out of the hole.


So does this have a purpose or was it one of those things that needs to be welded shut?

I know the truck was converted to air-brakes at one point, then swapped back by the original owner (worked in a gold mine for 50 years), so maybe the hole was put in for some reason with all of that. However with using fluid it just seems very wrong. I can't seem to find any pictures of caps with a hole in them, or even just a stock looking M/C cap...



I plan on upgrading this later on, but I'd like to be able to drive it around in the mean time.

Am I right in thinking this needs to be welded closed?

Thanks

That's an air vent. It you plug it up, you'll pull a vacuum in the reservoir and it won't drain into the cylinder correctly. Later cylinders had a rubber bladder in the cap that separated the fluid from the air vent to prevent the fluid from absorbing moisture but still allow the level to fluctuate. On the older vehicles, you were just expected to bleed the brakes more often to purge the moisture-contaminated fluid.

Brakes? what do you want with brakes? In that beast you won't need to worry about it, everyone will get out of your way....LOL

Yup, its supposed to be there, but, there should also be some sort of baffle in the cap as well to keep the brarke fluid from spraying up out of the hole. Typically there is a small cardboard looking disc that fits inside the cap with holes around the outside edge

Bobby

There should be a baffle in the cap to keep the fluid from shooting out as it returns to the cyl.

Yes, it almost looks like a gas cap does on the inside.

Crap, can those just be bought or do I need a new cap with one in it? lol how does that even get lost?

Ok, I went and checked it out, still has the piece inside. Seems a bit odd that it shot out that one time, as it doesn't any more. Maybe I had it too full, it seemed full when I removed it this time, but it didn't look like the cap would submerged in it. The rust looking stuff whipped off with my fingers, I'm guess it's either residue or just from whatever

http://i126.photobucket.com/albums/p...7/DSCF6822.jpg


Also, why do the brakes build more pressure when they're pumped?

They don't make more pressure when pumped. A given hydraulic system will only make so much pressure given the lever arm of the pedal, the cylinder bore diameter of the master cylinder, and the cylinder bore diameter of the wheel cylinders.
However, if there is some air in the system then pumping the brakes will seem like it does more. This is do to the fact that the air in the system will compress, depending on the amount of air in the system it might take several pumps of the brake pedal to fully compress the air. Fluids don't compress, so when there is no air in the system the initial step on the brake pedal results in the corresponding movement of the wheel cylinders.

Alot of the vendors the deal in brake components; Master Power Brakes, ECI, Speedway, etc have decent tech articles on hydraulic brakes.

Bobby

That makes sense. I'm part way into bleeding my brakes, I'm getting more air than fluid at the moment! So if I've done my job when I'm done with the last wheel I should notice no difference in the "pumping of the brakes makes it stiffer" thing correct?

The first brake was pretty straight forward, but on the second (DR rear) I just can't seem to stop getting massive air bubbles. I'm thinking it would have to either be in the drum or in the hose I've got connected, I'm going to go get a better one now...as I sorta went through like 20oz of fluid on that one and am all out, lol

Or another way of putting it is if you pump the brakes and there is no air in the system, you are basically pumping fliud into the wheel cylinder and pushing it out. As you pump they haven't given the brakes enough time to recover yet(blow th efluid back into the MC) and then you pump more in - so the pedal has less room to move because you have forced more brake fluid into the void created when you release the pedal.

It's like you are adding an extra slice of fluid with a pump or two on top of what got pumped down on the first push - the pumping has simply stacked more fluid behind the MC seals filling the volume normally left by the pedal pulling back (and allowing the fliud to flow back into the master cylinder - relaxing the wheel cylinder pistens and releasing the brakes shoes). Eventually after two or three pumps it won't allow you to pump any more - there is no more pedal travel, making it feel like the brakes are stiffer - it's an illusion related to the pedal position and stacked fluid in the MC.