The Stovebolt Site Forums The Shop Area Driveline Leaking master cylinder set screw port

I did a site search on this and didn't find this aspect of brake leakage problem addressed before.

In bench bleeding the new reproduction master cylinder there is a leak at the set screw area on the side. I tried tightening with no results. Then tried a thin application of a copper flake sealer, being careful to keep that away from the leading threads. Still no joy. Next I tried replacing the set screw with a 3/8" fine thread cap bolt, cut to length, cleaned and applied with a copper washer. That seemed to stop all but less than a very slight amount of leakage. I suspect the MC is defective, but before I replace it would like to get opinions here.

One guy suggested using Teflon tape as a sealer, but I think I recall others strongly advising not to use the stuff, especially on brake fittings?

Any ideas?

This is for a '46 half ton.

Thanks.

I have not seen a set screw on a master cylinder. Now, on my 38, there is a tapped hole where the brake light pressure switch mounts. Does your master have a pressure switch? If so maybe they protected the hole with a screw.

A photo of the leaky port is posted here.

I assumed, maybe in error, that port was to be used in bench bleeding the MC?

The switch, I believe should be a mechanical, at least that is what is attached to the floor board.

You can try Teflon tape, or use loc-tite red.

I think you need a male plug with a tapered thread used in pipe fittings. Did your old master have that hole?

The MC was missing when I got the project.

I've heard using Teflon tape or other stuff on any brake fittings can be dangerous. Is that true?

The only place that the tape is used is on pipe threads and usually not on automotive. I think the exception might be on something like a heater hose valve. I don't own a 46 truck but from the manual, I don't think that hole should be there. I am guessing that the master you have there is for an earlier model. On my 38, there is a port where the pressure switch mounts, and it uses a pipe thread. I would not depend on tape on a pressure fitting like that and I would imagine that the tape would dissolve anyway. Maybe someone will chime in here.

I have seen those tapered plugs used a lot on hydraulic valves and lines. Use some teflon tape and see if it stops the leak.

If you must use Teflon, which I would not, maybe you can put in a short set screw, then another on top of the first one with tape on that one or any sealer. The first set screw should stop migration of the sealer material. If it is pipe thread get another plug as said. Argentina? metric pipe thread? with a SAE plug? =leak, No bueno.

Thanks guys!

I have not checked the internal threads of the suspect port, but have on the screw. They, as those for the mounting studs and nuts and the brake line fitting bolt and nut are SAE.

I will look for a plug, as suggested.

If the remedies suggested fail I wonder if using a short set screw and then plugging the top several threads of the port with solder would work? Before doing that I'd disassemble and clean the MC. I want to avoid piston damage and a small exciting fire otherwise.

Another option could be to drill and tap for a replacement bolt.

The tape idea could work, but from what I've read if a bit of that gets into the system it could plug a line and cause the brakes to fail.

Hy 46Sparky, I think that hole/port is a 1/8" pipe hole for mounting a hydraulic brake light switch. 1/8" pipe thread is 27 threads per inch, 3/8" u.n.f. bolt thread is 24 threads per inch, pipe thread is tapered and the more you tighten it the better the seal. I can't see them using a regular bolt thread anywhere that intersects with the hydraulic system unless there is a flat machined surface for a copper washer to seal on, hope that helps.

I think your wording "set screw" is throwing us off. Set screw won't seal and screw threads won't receive a switch (if it's for that). So see if the "set screw" will thread into a nut or will it thread into a female pipe fitting. It's very hard to see a taper on a short plug. Soldering cast iron and steel plug? You MUST determine the thread before you "screw" something up.

Looks like I got it fixed?

The MC came with a set screw installed as stated previously. It did have SAE threads. Why...I don't know. I cleaned the threads with a tapered tap and used a brass 1/8" pipe plug. The leakage seems to have stopped. I'll watch it over the next few days to be sure. This project is a long ways from being drivable so, I've got plenty of time to get this right.

Thanks again.