Condensate pump draining back into reservoir despite non-return valves

[Offcumden]

I'm interested to see if anyone on here has a solution to the problem of condensate from our CH boiler draining back into the pump reservoir.
Diagram shows the arrangement which I came up with to replace the plumber's, which ran through the loft (so used to freeze).

Now the flexible pipe from the pump runs straight up onto the top of kitchen cabinets, then drops down behind them to floor level, then rises enough under the sink (about 500mm) to a connection with the sink waste pipe.

After pumping out, a small quantity of condensate always drains back into the tank, with a loud glugging noise. It does this despite the check-valve (ball type) at the foot of the flexible pipe.

I've tried fitting another one-way valve (non-ball-type) at the other end of the flexible pipe where it discharges, but that made no difference.

Not a big problem in the scheme of things, but I'd like to find some solution!

Not a plumber, but based on the diagram I would guess that the bit draining back into the reservoir is the condensate sitting in the bit of pipe between the top of the reservoir and the non-return valve. Because gravity.

Is there any way to re-route it so that it drains from the bottom of the reservoir?

No that's not the case, that valve is right at the bottom of the pipe and prevents water returning by gravity (it's a simple ball valve). A certain amount of water seems to be sucked out at the other end, creating negative pressure, which I want to stop.

Electrician here, why don't you just fit a trace heater and use the gravity flow route through the loft.

Waste pipe check valves are notorious for blowing a slow back flow, they are common on saniflow instals and are the cause or random starts.

You could do away with the pump by running the drain through the back or behind the kitchen cabinet and directly into the sink drain. It can be run in 21mm overflow pipe.