I am having a problem with a leaking "welch" plug on an installed master cylinder that I purchased in a group buy from a vendor here. We all know what a pain it is to change a master cylinder so I am hoping to avoid that process. I see that bbrock had the same issue and was able to fix outside of the car. Has anyone fixed the welch plug in the car with any success? I really don't want to take it out.
Thanks for any good ideas.

I think you might be able to do it without taking it out. A C-clamp with the right-sized socket could work to press the welch plug in tight followed by filling the top of the recess with JB Weld. I don't think the JB Weld is necessary after the welch plug is pressed in tight, but I liked the extra insurance. 1,300 miles on my MC that have included many hard-ass stomps on the peddle now with no issues or leaks.

Thanks Brent for the advice. I didn't know how hard you squezed yours in the vise but I'll give the clamp a try. i really don't want to take it out. Have you really put that many miles on this summer?

What size socket @bbrock ? Something small to press the center? Or larger to push around the perimeter?

@rudedude it does take a fair amount of pressure but I think a hefty c-clamp could provide it.

@iankarr I don't remember for sure what size I used but it was probably 12mm which is what Eric Shea recommends for the welch plugs on the rear calipers. I remember thinking the MC welch plug was about the same size so probably stuck with 12mm. I think the idea is to use a size that presses the domed center to flatten out the plug so it expands the diameter.

@rudedude , Having the same issue on a new 19mm MC. It was a very very small leak. It would mainly fill the plug hole and over time/use, drip off onto the protective plate. I ended up just using JB Weld. Wish I had seen this post yesterday. I would have tried a C-clamp first. Haven’t tried my fix out yet to tell you if it worked or not.

I put the second one I bought at the same time in a vice like bbrock did and the plug seemed to settle into place. I then used the same small piece of dowel with the c clamp in the car. Its much harder to tell if the plug moved but i am waiting to see if it leaks. I'll check tomorrow. I'm hoping it works.

I would not risk it. I would buy an Ate master cylinder and get your money back. If that blow out the results could be disasterous.

@rudedude

Yes I have learned in the past to only trust original parts but sometimes I have wavered to try to help out others helping us with out projects. I have ordered a new ATE and must endure the process of changing it out again. Thanks for your input.

FWIW, the welch plug in question is not under pressure when the peddle is pushed. It only "sees" the brake fluid as it drains via gravity from the reservoir. I don't think I'd trust any welch plug to hold hydraulic pressure.

Still, it represents a significant safety hazard in that it could leak all of your brake fluid without your knowing it. In my case, it was easy to see that the welch plug was never pressed into place on assembly, and also easy to see it had properly flattened and sealed after pressing it in myself. I've been monitoring brake fluid level closely and it hasn't budged so far.

If it was my car, I'd suck it up and replace the master cylinder.

In fact I did exactly that during my restoration. I bought a new MC. It leaked. I took it out and replaced it with an ATE. I didn't bother returning the faulty unit either, as I did not want the possibility of it going to someone else's car. I just tossed it.

Zach