Okay I think I solved the problem (and created it too)
After going back to the store and showing him the problem we noticed the wheel cylinder I was using was for the back not the front. I bought all 4 and they all had a brown rubber plug like the 3/8 cylinder so I assumed (wrongly) that all of them had 3/8 inlets.
The other two in my box were 7/16 inlet and he matched them as fronts. Totally my fault and I told him so and took my parts back home. Hopefully it will work out.
