Alpha blending a complex model

Hi,
I've modified the Toon Shader to add an X-Ray effect whenever my characters goes behind the walls. It works fine when _XColor's alpha is 1 (255), but overlapping polygon starts to show through when alpha is lowered.

How can I get a clean alpha blended solid color without the overlaps? Thanks

Edit: I've tried adding ColorMask 0 as shown here http://wiki.unity3d.com/index.php?title=AlphaVertexLitZ but the outcome is still the same

Edit2: Further testing shows that ZTest Greater in X-Ray pass may be interfering with the ColorMask. But I'm not sure how to resolve it.

Code (CSharp):

1. // Toony Colors Pro+Mobile Shaders
2. // (c) 2013,2014 Jean Moreno
3.  
4. Shader "Toony Colors Pro/OutlineConst/OneDirLight/Basic X-Ray"
5. {
6.     Properties
7.     {
8.         _MainTex ("Base (RGB)", 2D) = "white" {}
9.         _Ramp ("Toon Ramp (RGB)", 2D) = "gray" {}
10.    
11.         //COLORS
12.         _Color ("Highlight Color", Color) = (0.8,0.8,0.8,1)
13.         _SColor ("Shadow Color", Color) = (0.0,0.0,0.0,1)
14.         _XColor ("X-Ray Color", Color) = (1.0,0.0,1.0)
15.    
16.         //OUTLINE
17.         _Outline ("Outline Width", Range(0,0.05)) = 0.005
18.         _OutlineColor ("Outline Color", Color) = (0.2, 0.2, 0.2, 1)
19.     }
20.  
21.     SubShader
22.     {
23.         Tags { "Queue"="Transparent" }
24.         LOD 200
25.  
26.        Pass
27.        {
28.            ColorMask 0
29.        }
30.  
31.         // X-Ray pass
32.         Pass
33.         {
34.             Zwrite Off
35.             ZTest Greater
36.             Blend SrcAlpha OneMinusSrcAlpha
37.             Cull Back
38.             ColorMask RGB
39.  
40.             Color [_XColor]
41.         }
42.    
43.         // Toon pass
44.         CGPROGRAM
45.  
46.        // NOTE: Toon shader pass redacted because it's bought from the asset store
47.  
48.         ENDCG
49.    
50.         UsePass "Hidden/ToonyColors-Outline/OUTLINE_CONST"
51.     }
52.  
53.     Fallback "VertexLit"
54. }