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Did I buy the wrong master cylinder?

TheRealDavid

TheRealDavid

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So a few months ago I bought a brake master cylinder at O'Reilly's I'm starting to wonder whether I bought the wrong one. Or maybe defective?

Issue: Flare nut doesn't seem to lock correctly, I have reset it several times and it always leaks, it seems pretty well aligned. The flare nut closest to the firewall is fine.

I went back to the Oreilly site and noticed that they have 2 different masters which are almost identical except for the damn thread size... it seems like that could be it and after some googlin' not sure what the deal is. Or maybe the part is defective and the flare simply can't reach the inside to site flush??

part#NMC11373M I bought this one:​

Screenshot 2024-08-09 161646_mine.png

This other one available part#NMC11373:
Screenshot 2024-08-09 161739_do_I_need_this.png


Also the old master is made in USA so maybe I should just rebuild that instead of figuring this out. I may be screwed out of 40 bux since I don't have the receipt for the new one.

Old one, it says lockheed and made in USA:
20240809_154057.jpg
 
Do you still have the old one to measure?
 
TheRealDavid said:
@Cranky I'm up to shenanigans already by doing this work myself instead of throwing money at a professional so I guess I can buy a thread gauge. Are 9/16-20 and 9/16-18 close enough that it would almost fit? Was looking at this thing to better my brain... https://boltdepot.com/Fastener-Information/Printable-Tools/US-Hex-Bolt-Sizes.pdf
Click to expand...
The threads need to be exact.....like R/T says. A bit off is not good enough. Metric included. Having the old MC is good to at least take a good hard look at and compare it to the new one but even looks ain't good enough in lots of cases. Depth of the threads is another thing too.
 
R/T Boy 67 said:
I have only seen 1/2-20 and 9/16- 20 on mopar master cyl.
Click to expand...
Thing is, what are ya gettin with a 'rebuilt' or a new one from China? The rebuilt one should have the right threads but it's no telling on one that comes in from off shore.
 
So, this may sound odd, but it works.... You can use a smaller bolt with the same thread pitch as a thread pitch gauge... When cutting threads 20 threads per inch is the same spacing (20 TPI) & the same thread depth (.031) whether it's 1/4-20 or 1/2-20.... So grab a 1/4-20 bolt & see if the threads match your ports... If not grab a 5/16-18 & try that...

Just to get an idea of how the threads will self align, grab two 1/4-20 bolts & lay the threads together, you'll see they grab one another... Now try one 1/4-20 & one 5/16-18 together... They won't grab instead they will slide past one another...
 
The line does not seal on the threads of the line nut. it seals on the tube flair. If your tube nut screws in then check your tube flair or look in the master to see if the seat is damaged or missing. Also check your tube flair itself , as it may be damaged.
 
It's gotten to the point ( actually has been for awhile ) that rebuilding a good core is preferable to taking a chance on chinese $hit/crap/junk.
 
I just went through that with an Orieleys MC. https://www.oreillyauto.com/detail/...r-cylinder-new/ohh0/nmc11218?q=NMC11218&pos=0
This master cylinder fit perfect and the fitting's screwed in easy but I kept getting a drip on the line for the rear brakes. I cut the line and reflared (double) the flare but it still leaked. I removed the MC and looking in the ports I could see that one seat had a clear impression of where the line sealed against it and the one that was leaking had no impression at all. One odd thing about this MC is that the seat was not the usual brass seat but was aluminum(?) cast in when the aluminum cyl was cast. and the seat was NOT smooth or machined at all. I took the cyl back and they exchanged it only to discover the new one leaked too!!! I then decided that the threads were not cut deep enough and wouldn't allow the seat to seal. I decided that they should not be hard to cut deeper, since it was aluminum. Now you can't us a regular tap as there is a cast in seat in center of the fitting hole so I took the line off of the car and cut four slots in the fitting such as you would see a self tapping screw. I reinstalled it, put a little cutting oil on the threads and tightend it down, TIGHT, then I tightend it some more, backed it out and tightened it down 1/ 4 turn more. Bled them out and NO LEAKS!!! Been working fine since March!
 
Last edited:
WileERobby said:
It's gotten to the point ( actually has been for awhile ) that rebuilding a good core is preferable to taking a chance on chinese $hit/crap/junk.
Click to expand...
Yeah try to find a rebuild kit today!!!!!!
 
I bought a chineseum raybestos unit from rock auto for my 67 R/T. last fall. So far no problem. I kept the original to rebuild someday. The original would bubble too much when bench bleeding compared to the new one (which I had second thoughts on using), but the new one bled good and mated up well to the original lines.
 
So I ended up using the piston from the new master and swapping that into the old master. I cleaned up the old master, didn't notice any pitting, I didn't remove the old piston that is deeper into the cylinder unfortunately, but even the old piston didn't seem into rough of a shape. I think part of the problem I was having was that clip that holds the piston in wasn't there at all so it may have been moving out a bit and leaking. Now the pedal feels nice, sitting up high and returning just fine.

I think the issue is that the flare did not want to sit, not 100% sure this was the issue. They do look slightly different, you might be able to notice if you zoom in.

New Master:
20240811_133413.jpg


Old Master:
20240811_133418.jpg
 
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