I have an Ultegra UST wheelset and tubeless tyres on them. After suffering some punctures I decided to run them with a tubeless sealant. The problem is that the sealant's manual instructs you to remove the valve cores and pour the liquid through the hole but my rims came with standard integrated valves. I ended up removing part of the tyre off the rim and pouring the liquid through the slit and then quickly using the pump to seat the tyre back in the rim. However, for some reason this is apparently not quick enough as it results in some considerable amount of the liquid spilling out.
Is there any weird trick to doing it effectively? Note that "buy different valves" is not going to be considered an acceptable answer (even though I may consider upgrading eventually.)