Removing cannister caps

[Bricklayer]

Is there a trick to removing the cannister caps? Mine are 4x40. The cap has a ubolt that slides through the cap and cannister locking it in. I've tried everything. I can get it to turn a bit but not budge up. I would assume it's not threaded because of the locking clip. I'm sure the guy who had it before didn't lube the seals when he opened it last.

[mountainvan]

My canister has a plug on top that is removed with an Allen wrench. This removes the suckage and I can pry the top off with a large flat head screw driver.

[Bricklayer]

I finally got one off. Now that I see how it works. I had a strap wrench trying to turn the whole top. I actually used the u bolt to turn the tops a bit then they pulled right out. Membranes were pretty tight too. Used the ubolt on them too to gently turn them around a bit till the suction broke and they pulled right out.