Brake questions from my hangar.
 

I am making a second attempt to rebuild the Matco master cylinders on my RV-7. They were slowly dripping from one cylinder when I bought the plane but all four cylinders were weaping. I had an A&P help me rebuild the cylinders once and a couple hours later they were the same. This time it was a different cylinder that was dripping but all were weaping again. Everyone found this odd.

I've talked extensively with the Matco people and am in the process of rebuilding them again. I was hoping to be a little smarter this time but I was not able to pinpoint the leak. I wiped the dripping cylinder clean with mineral spirits and taped clean paper towel around the 5 spots I thought might be the source...the top where the shaft is, and the two ends of the two fittings (hose side and cylinder side). I then pumped the brakes very hard several times from both seats but at best, tgere was a hint of fluid on one or two of the fitting locations, both the hose side and the cylinder side. Not enough to draw a conclusion though. I also wiped the hoses a few times during the process to make sure no fluid was running down from a higher location. They seemed dry throughout. At this point I pulled all four cylinders and began to break them down and clean them.

I have steel break lines, aluminum fittings, and the thread sealant on the cylinder side of the fitting is one of the black or dark gray sticky sealants that comes in a tube. Possibly Permatex 2 Form-A-Gasket. This time I will be using Loctite 567 as the Matco guy suggested. I also used Phillips 66 X/C Aviation Hydraulic Fluid. It seems to be made to at least the same standard as the fluid Vans sells.

Here are my questions:
1. Does anyone know why I have a leak!?:D

2. The dripping fluid appeared to be "dirty" this time instead of clean red. I presumed it was just picking up some grit from the floor. But when I removed the cylinders and shook them out on my workbench, at least two of them dripped some blackish fluid out. You can see it in the picture below. I also attached a picture of some fluid that ran out of the lines when I pulled them off of the cylinders in the plane. It looks mostly clean except for the small piece of metal that may have come out of one of the lines. Does anyone have any ideas about this?

3. I was able to do a decent job cleaning the threads on the fittings with a combination of alcohol bath, a metal pick, and a wire brush. The cylinder female threads are going to be a lot more work. Are there any tricks for these threads!? I imagine I am headed out to a gunshot to buy a small copper bore cleaning brush. Is copper even safe to clean the cylinder threads with?

Thanks a TON for any help!

Andy,

Check the depth of the threads....... Matco has, in the past didn't tap deep enough. Just shorten the fittings by 2 threads to allow it to make 2 more turns if needed.

The color of the fluid could be from the fluid not being compatible with the matco parts.

Any reason you could not use a bottoming tap carefully?

These are interesting ideas! What I did find when I cleaned the parts and threads was odd. I have at least 2 different fittings, maybe more. They are different in the color of the blue anodizing, different in the length of the threaded portion (at least on the cylinder side), and the cylinder threads themselves had what appeared to be deeper of flatter sections of thread in places. That is, if you looked into the opening fitting hole and rotated the cylinder, there were several sections around the thread hole that seemed to have mashed or flatter threads. These areas were like lines about 3/32" wide from the outside to the inside across all the threads. I didn't get a picture of it though. And lastly, I have one fitting that is wider at the bend than the others. I can easily slip a 3/8" (???) wrench over them other fittings to tighten or remove them. This one fitting has to be nearly 1/16" wider and I need to use an adjustable wrench instead.

One last thing of note is there was scuffing on all of the shafts. I have no idea if it is significant but I though I'd ask. I think I am going to call the Matco guys back and see what they suggest before I begin any more home remedies. But I would still be interested in any feedback. I rebuilt the dripping cylinder and replaced the shaft but that is all the new shafts I had so the others are disassembled on my bench still. I don't mind tapping the threads but I don't want to do that until I run out of other options I guess.

Oh, and incidentally, the Matco guy thought that the fluid was acceptable.

Thanks!!

Are these not tapered national pipe threads?

This just sunk in. I will pull the clean fittings back out and check the uniformity of the thread depths. Hence the bottoming tap! I think I also need to order some more fittings so at least I know I have a uniform group. I have a set of steel fittings for this purpose but they weigh way too much for me to want to use them now.

This is all I know about the fittings.

I had a leak on one of my cylinders prior to first flight. It always looked like the leak was coming from the brake line fitting but no matter what I did, I would always find more leaking fluid the next day. I finally removed cylinder and disassembled it finding a small piece of aluminum lathe turning chip inside that damaged the cylinders piston wall and "O" ring seal. I de-burred and buffed out the groove and replaced the "O" rings. Problem solved. Basically the same thing with my parking brake valve. Persistent leak prior to first flight. No debris found there during disassemble however new "O" rings did the trick.

Thanks. I'm sure the builder tried unsuccessfully at least once to remedy this problem. And I rebuilt all the cylinders once myself with the help of an A&P, including new pistons. So I don't think I am going to be a simple fix. Also, all of mine leak. Everyone I talk to about that is amazed.:(

Yep Dummy me, no bottoming tap in NPT.