Greetings! This is my first post, and I come from the Honda Civic world. I have spent a lot of time reading about the NSX ABS system since it operates in mechanically identical fashion as the ABS system in my 1994 Honda Civic EX. I am currently replacing rusted brake lines and have the ABS modulator unit out of the car. I also have a few spare units, and I will be rebuilding the modulators soon with new o-rings and cleaning everything up. When comparing my modulator units, I noticed that some of them have this empty hole in the valve/piston housing like @
spacemaster notes in the original post. However, the unit I removed from my car has a rubber plug on both sides where this hole is located. See photos below:
When I removed one of those rubber plugs for the photo, brake fluid squirted out of the housing. I was surprised by this, since from the ABS diagram in the service manual, this "Chamber D" is simply designed as an air chamber and I would assume should be free of fluid. I suspect what @
spacemaster found (that the valve/piston assembly has an internal leak via the o-ring) is why brake fluid came out of my "Chamber D" when I removed the plug. It's the only way I can see brake fluid entering "Chamber D", unless there's a design flaw in the system where in certain conditions brake fluid can seep into "Chamber D" with air bubbles or something. I'm not sure, but I tend to lean toward the valve/piston leaking. Maybe they're even designed to leak a little I don't know, but I would assume not since that would affect the hydraulic pressure created by the system.
So what about the rubber plugs? My original unit (pictured) had both plugs. One of my spares was missing one, and one of my spares was missing both. It seems like from the factory these came sealed up with the plugs, but perhaps under severe ABS modulation there's enough pressure sent into "Chamber D" to blow the rubber plug out? The one I removed did not appear degraded or cracked in any manner, so I presume they don't fail due to integrity but rather must be expelled from extreme pressure. But this does bring up the point of contamination entering "Chamber D" via this "breather hole" (for lack of a better term). Dirt, dust, debris, and water can all penetrate "Chamber D" if the rubber plug is gone.
It remains a mystery to me whether the plugs are just from the factory and are assumed to be expelled or whether they are designed to remain in place. I had to use a pick to remove it, and there's an obvious shoulder on the plug to help it seat, so it seems like it's designed to remain seated in the hole. I am surprised more folks who have opened their unit to replace O-rings have not discovered more contamination in "Chamber D", especially if their plugs were gone. Perhaps the placement of the NSX unit (high in the front bay of the car) is more favorable than where the Honda Civic is placed (low in the passenger firewall of the engine bay). I plan to open one up that was missing a rubber plug, and I suspect I will discover some form of contamination but perhaps not.
@
spacemaster- did replacing the 2mmx6mm o-rings in the valve/piston assembly solve your brake fluid leaking problem into "Chamber D"? It also seems to me that if this O-ring does go bad and the plugs are gone, brake fluid could leak out onto the modulator unit itself. This makes it look like the typical "solenoid leakage reservoir overflow problem" but perhaps some leaks have been due to this problem as well.
