76 f100 brake fluid leaking from master cyl cap... why ?.. i installed a new.... not a reman. master cylinder with new lid and gasket. its power brakes with the steel bail that holds down lid. no matter what i do, brake fluid seeps from under cap and is wrecking my paint on the frame etc. any ideas ? ive bled all brakes .. i bench bled master before i put it on..

 

First, is the reservior overfilled? If not, is one reservior overfilled but not the other? If it is, does the one that's not overfilled seem to be losing fluid.
If that's the case, my guess is a bad master cylinder (even though it's new). If that's not the case and the m/c isn't overfilled, I'd replace the gasket and see what happens.

 

I just installed a re manufactured unit and it did the same thing. Check for rust on the metal (sand down if necessary), if not, get a new gasket for the lid and that should help. When you pump the brakes it will squirt fluid on top of the lid and if the seal is faulty it will leak out.

 

I had the very same issue when i replaced my MC, most replacement units come with a non-vented cap, i swapped my old vented cap on and issue was resolved.

 

Thats a great idea, mine has a vent but I bet it's plugged with paint when they re mand it. I will check tomorrow.

 

On new units I have found the "bail" or wire retainer is not always bent right. It can put inadequate pressure on one side or the other. Check if it is equally snug front and rear. Some tweaking with pliers/visegrips can fix it.

 

I recentered the gasket on mine and it quit.

 

I've seen many reports of this. Cures tried have been sanding the bowl flat to using screw clamps to hold it down. Two tie wraps cured mine and was able to remove them after about a month. Just poor quality like everything else anymore.

 

Look at the top edge. If it looks smooth and machined then it's probably new. If it looks like it has been in a tumbler style cleaner I doubt it.

The rebuilds these days are all banged up and you're lucky if they seal. I put up with one for a year before exchanging it for a new one; and the top had machine marks. No more leaks.

The non-vented cap sounds promising!

Note: Don't sand the top while it is installed. Only clean and upside down; then make sure there isn't any residue left from the sanding.... Return would be a better idea.
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Thanks for adding that. Maybe one day I'll learn to be specific. Sanding/flattening top of MC should be done removed, upside down, on a completely flat surface.

 

thanks guys. i think the vented cap is the answer. the old one was vented and the new one isnt. maybe i can drill a very small hole in the new one. the whole reason i installed a new one and not a reman is because i wanted the new cad plated cap for looks. the reman ones always come with a sand blasted cap. tumbled etc. thanks again for all the replies.

 

You could just drill a 1/8" hole and serve the same function as the vent, or just paint your old vented cap and use it.