Complex sentence: No.2004 was fitted with an experimental butterfly valve blastpipe bypass, manually activated to prevent fire-lifting at high cutoffs, this was later replaced with a plug valve and higher bypass pipe diameter-both designs had problems with sticking due to carbonised deposits.
Split into simpler sentences: No.2004 was fitted with an experimental butterfly valve blastpipe bypass, manually activated to prevent fire-lifting at high cutoffs. This was later replaced with a plug valve and higher bypass pipe diameter, but both designs had problems with sticking due to carbonised deposits.