Couple of months ago I upgraded myself with 8mm silicone hose and the ECs couplings. Since I have had constant leaks in my vacuum. I haven't bothered too much and have run with the pump on. Last month I decided to examine where the leak was. First I checked the couplings attached directly on the pump. All good. Then attached the hose with couplings in each end. Leakage! 
First I tried to replace the tape on the couplings, still leaking. Then I bought some specific thread sealant, still leaking. Then on top the hose I put specific hose clamps. Still leaking. Is the conclusion that the couplings themselves are leaking or am I missing something obvious here? 

Which couplings are you using, the quick disconnect ones? These ones have an o-ring inside the female connector that is very easy to tear if you dont connect the two together straight enough. Have a look inside and see if you can see any damage on that o-ring.

It's tough to diagnose without seeing it in person, little leaks like this can be so tough to pinpoint. But to me, if the couplings are sealing on the pump but not on the hose end, it simply can't be the couplings that are leaking. 

So, three questions:

1. What is the actual leak rate, have you measured it? 

2. Have you tried moving the hose around while testing the vacuum to see if the hose position makes a difference to the leak rate?

3. How snug is your silicone hose on the hose-tail? Does it slide in and out easily, or does it take a fair degree of effort, especially to remove? I find silicone hoses can be problematic with leaks, they need to be very tight. And hose clamps dont always fix the issue, because unless you're using Murray style clamps, they dont apply equal tension across the entire circumference (they are much tighter at the drive end and looser on the opposite side). I think the best solution is if you're using 8mm silicone hose, get a 10mm hose tail. It can be a bit of a prick to get the hose on and off, but it seals significantly better.

Just another thought to add - When I used those couplings a while ago I had similar issues initially and was left feeling that it must be the couplings that were letting in air

You mentioned using a thread sealant (liquid type?) but, I found, using a gas-specific  sealant tape around the threads (not the type designed for water pipes) was the best way to ensure a leak-free fit. It seemed to take away the issue, anyway

It only needs to wrap around a couple of time and can be a bit fiddly, but it seems like the best solution to sealing air couplings.