Replace the cap with a stock oem cap, then look for other problems.
Check the coolant reservoir level when cold, then observe it when hot, it should be higher when hot, then as it cools it should suck the coolant back in to the same level as when cold.
When ever you flush the coolant system, some air will always remain, this will be noticeable when you check the coolant reservoir, because the level of the reservoir will be slightly lower when cold. Then as the air is purged, more coolant will be sucked back in, and it will seem like the the reservoir level will become lower and lower until all the air is purged, only then will the coolant reservoir level become stabilized when cold.
P.S. try not to mix different coolants, flush it and start from scratch if you don't know what's already in it.
P.P.S. When I say flush I mean with demineralised water, not that shitty "radiator flush" that some shyster like autobarn will try and sell you.