Pro-Tip New metering valve leaks

Some really good info here- take notes!


Well-known member
Supporter 2021
This post is for anyone using a new metering valve or combo prop/metering valve, usually when retrofitting a front disc brake kit while keeping rear drums. Since this issue seems to be present on EVERY new metering valve as well as numerous online accounts of leaks from them, I'd like to humbly suggest this be made into a sticky.

I installed a Scarebird front disc brake conversion kit onto my '69 E200 along with a brand new "GM style" combination proportioning, metering, and warning valve. The same is used on many old trucks/vans with slight variations, but finding anything new other than the GM style is a bit difficult. Most complete retrofit master cylinder kits come with a GM style combo valve attached. And when you use any of them (or just a metering valve alone), you will probably find that it leaks from under the little rubber cap. Even if it doesn't happen immediately, it probably will after some time. At best you might just have a slow leak and slightly mushy brakes. I drew up this diagram over a picture of a cutaway of the valve:

The first one I installed was a new, ebay special iirc. The second came new from NAPA. I got a third, separate metering valve insert from CPP. They all were crap, and after doing some research found many reports of the same, along with this video suggesting that the issue was due to poor manufacturing of this plastic washer. Be sure you check this washer if using a new metering valve.
As you can see, the first pic shows extra plastic flash from the mold which wasn't trimmed (this was the CPP version), and the second one (harder to see) has a gap in the flat sealing surface (the ebay special, no pic of the NAPA part, but it also leaked). This fits into this rubber, pan shaped seal to seal against 2 surfaces, the spring retainer between the spring and the seal, and the inside bore of the brass block, on an angled surface (shown below with a darker region where the angle is):

I went to clean up this plastic washer to fix the leak, but after looking at the brass plug that screws in behind it, I saw the big section of bare brass and thought, "why use this silly plastic washer when you can simply mill that out of the brass (as long as you lube the brass to prevent the seal from binding)."
So I chucked it up in my drill, grabbed a flat file, and got to doing a poor man's lathe milling job on it. I made sure to measure things so they would fit up properly. I marked the inside diameter of the seal on the top of the brass. I cut the brass plug back straight up, then I matched the angle of the inside seal surface of the bore as best I could and cut the plug back at that angle until I reached the diameter mark.
Now that the plug sits further down in the block, you'll need to make sure that you deepen the stop inside the bore of the plug by carefully drilling it with an appropriate sized bit. If you don't do this, it won't ever open to allow the front brakes to work. There's plenty of meat there to work with though, as long as you're careful not to go too far.
No more leaks now! Hope this helps someone else. Don't forget to lube the seal so it doesn't bind. If you have something like a drill press with a bigger chuck than a typical hand held drill (or an actual lathe), it would be a lot easier. I feel like this is a much better fix than trying to get the plastic right. Filing the huge gaps in the surface seemed like it would make it too thin. Trying to cut out the plastic flashing seemed like it would be easy to mess up too.
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