Brand new master cylinder/cap leaking

[71 SE3834V]

Any ideas why a brand new master cylinder and cap would leak? This is my 3rd one I've tried and the 2nd brand (tried 2 different brands). These are brand new not remanufactured. All 4 brakes are bled. Brakes function fine for stock 43 yr old design.
I'm stumped and tired of wiping up messes and seeing my brand new engine bay paint coming off.

[ws23rt]

What I would try is to take the gasket out of the cap and rest the cap on the mc. See if it rocks corner to corner. It could be just a bit twisted. The way It is held in place requires it to be flat with the housing. The only other thing it could be aside from a hard gasket is a flaw in the mc casting.  Look very close where the leak comes from.

BTW remanufactured stuff can be faulty in the same way and for the same reasons as a brand new one.

[71 SE3834V]

I will try that. I know that the rubber is good and I have visually checked the mc surface before mounting because of the previous issues.
The first one leaked in the front. I ordered another and tried the cap from that one. Leaked in the same area. Put a straight edge on the surface and didn't see any issues. On that one the rubber looked like it wasn't sealing completely in the area it leaked.
The current one seemed to be making contact all the way around. Made sure of that as that was what I was sure was the problem on the first one.

Now that I think of it it seems like the clip is easy to remove/install by hand. Seems like it should be harder. I seem to remember back in the day always needing a screwdriver to pry them open. Should I try bending the clip tighter?

[ws23rt]

I will try that. I know that the rubber is good and I have visually checked the mc surface before mounting because of the previous issues.
The first one leaked in the front. I ordered another and tried the cap from that one. Leaked in the same area. Put a straight edge on the surface and didn't see any issues. On that one the rubber looked like it wasn't sealing completely in the area it leaked.
The current one seemed to be making contact all the way around. Made sure of that as that was what I was sure was the problem on the first one.

Now that I think of it it seems like the clip is easy to remove/install by hand. Seems like it should be harder. I seem to remember back in the day always needing a screwdriver to pry them open. Should I try bending the clip tighter?

I think you might have it there.   The spring should be hard to remove.

[charger Downunder]

The bale clip bend it, plus don't over fill it with brake fluid keep the level a little lower.

[Just 6T9 CHGR]

I think you might have it there.   The spring should be hard to remove.

Yes you should need a screwdriver to pry the bale clip off..not by hand.  Bend it tighter like suggested above

[71 SE3834V]

The bale clip bend it, plus don't over fill it with brake fluid keep the level a little lower.

Ok so what's everyone opinion on how full it should be? I was of the opinion (and it seems I was taught this way back when) that air, with moisture in it, was the enemy and you want to keep as much of it out. I usually fill it to about an 1/8" to 1/4" from the top. Certainly not to the brim. The first m/c I was dealing with I dropped the level down to at least 1/2" below the brim maybe more.
Pics of the first one, that was leaking from the front, where you can clearly see it wasn't sealing completely across the front. Figured they were sending out caps that were warped (tried 2) so I went to another brand.

[ACUDANUT]

1/4 inch below the cover.

[71 SE3834V]

Ok, checked things out. I was at 1/4" below the brim and the cap doesn't seem to have any warpage. Maybe a slight amount of rocking but not enough the the seal shouldn't over come. The clip was way too loose. I could easily move it off and on with my arthritic hands if any of you out there know what I mean. Bent it a bit in the vise and now I could only get it halfway up the peak before having to use the screwdriver. Will have to take it out for a test run if the rain holds off.

[71 SE3834V]

Well that didn't work. Still leaking.
Ran it by Oreilly's and they ordered another one. Went home to see if I could use my old Mopar cap on the new m/c. No go, different shape.
I put a straight edge along the sides of the reservoir and both side are a little high in the middle. Lowered the fluid about another 1/8", replaced the clip with my Mopar one off the old unit and out for another ride this evening. A little better but still seeping. I will have to check out the straightness of the new one before mounting that one.
At this point I've wasted so much time I could have cleaned my original one and rebuilt it. Anyone know of a good brand? Auto Zone = junk, Oreilly = junk, Napa sells Cardone (don't get me going on Cardone), Advance Auto sells Wagner at twice the price and who knows where it's made? I think I might disassemble the old mopar one and put it in the dunk tank tomorrow and see if it'll clean up.

[PlainfieldCharger]

My has finally stopped leaking after about 2 years. I think I painted that MC 3 times and the fixed the paint on the shock tower...funny thing was it leaked with a new repoped from cardone and leaked with a rebuilt original...must be something with the gasket material?

[Dino]

Any chance you can level the rails on the new mc before trying anything else?

[71 SE3834V]

Any chance you can level the rails on the new mc before trying anything else?

I could probably take a file to it and dress it up but 2 things come to mind. 1) getting metal shavings in the fluid can't be good and 2) I don't want to do anything that might mess up my ability to return it although these places seem to take anything back no matter what.
And one final thing, I shouldn't have to jerk around with something I bought new. Yeah, sometimes it's a tough debate on whether to take the time to "fix it" or take the time to return it and start over.

[70-500-SE-EXPORT]

I had the same problem...brand new paint ruined. Tried three new masters, same problem. I tightened the clip and that didn't help. So I kept lowering the fluid level till the leaking stopped, this was about 1/2 below lid.

[charger Downunder]

I had the same problem...brand new paint ruined. Tried three new masters, same problem. I tightened the clip and that didn't help. So I kept lowering the fluid level till the leaking stopped, this was about 1/2 below lid.

yep thats about right

[71 SE3834V]

Here is an interesting piece of info. Last fall when I stripped the engine bay to paint it I was trying to save a little time and pulled the mopar mc off but left the lines connected. Bent everything out of the way and the mc ended up 90* vertical while I cleaned and painted (maybe 2-3 wks). Didn't leak a bit outta the cap. It was only after repositioning the mc horizontal that it started leaking out the back (cylinder) of it. I was like "crap" I shoulda just took it off and figured on replacing it.
Now I can't seem to find a replacement that doesn't leak. I have now painted and installed my 3rd one (I ordered another last year just to try a different cap). Only filled it halfway. Checked the surfaces and touched them up with a file.
The store had ordered both a new and reman this time just to compare things. Both came with a different seal that fills up the cap better and has some ribs on the sealing surface that might seal better. Another difference is the seal is mounted in the cap opposite of what you normally see w/the "bellows" pointing down into the reservoir, w/the words "this side down", instead of up into the cap. I will post pics later.

[charger Downunder]

Just a thought on the cap top front reservoir at the back of the bulge is there a slit to let the air out when you apply the brakes maybe it is blocked or partly closed.

[71 SE3834V]

Just a thought on the cap top front reservoir at the back of the bulge is there a slit to let the air out when you apply the brakes maybe it is blocked or partly closed.

The vent is clear and open. I've always assumed the vent is to let air in to allow the bellows to move down to compensate for the lower fluid level as the brake linings wear causing the caliper pistons and wheel cylinders to move out farther. I've noticed only the front chamber has a vent on the new one whereas the old mopar cap had vents in both chambers.

Ok so the latest update:
I install another new mc Fri. Fill it up to 3/4" from the top. Bleed the brakes Sat. Go out for a 10 min. run Sat eve up to a local car show. Park and check for leakage. Sure enough there are 2 runs 1 each from the front corners. Wipe it off and see more running out. Wipe again and it has stopped. It hasn't taken the paint off yet just stained it.
I hang around for an hour then drive a mile down the road to Oreillys and talk to the guy I did before about this. We try to find a rebuild kit for my original mc to no avail. No leakage at this stop.
I go to pull out of the lot and see a '69 grill coming down the road. I decide to wait till it passes and see that it's a General Lee. He blows his dixie horns at me as he passes.  By the time traffic clears and I pull out the GL is about 3/4 mile ahead of me. I push the go pedal to try and catch up. It takes me about 3 miles to weave thru traffic before I can pull along side the GL. We exchange waves and I decide to turn towards home. I pull into a gas station to check for leaks and to my suprise there is no leak after all that hard driving. I drive another 10 miles to reach home and check again. NO LEAKS! 
The weather hasn't been the best for driving classic cars (storms almost every day) but I did get out for a short run Sunday and still no leak.
I don't know what to think. Why did it leak once then not again this time. At this time I'm gonna cross my fingers and hope for the best. The next test might be Friday where, if the weather is good, I'll make a 10 mile run to another local weekly car gathering.