Got a 150mm brand x dropper on one of my bikes, its never been the quickest thing going up or down but noticed recently its slightly slower, pressing the lever it takes near 2 seconds to raise back up. Clamp isn’t to tight.
I stripped it down today, although it was abit dry inside, I’ve greased it but its made no difference. The tubes slide freely with the cartridge out. Does this indicate the cartridge is faulty, you cannot adjust the air pressure. They are only £30 to replace with is a bonus. My oneup post comes up mega fast with a satisfying clunk when its extended

Yeah, a new cartridge would sort it right out – worked for mine. It’s a bit annoying tbh they don’t seem to last that long. At least easily replaced

Double check under saddle. My brand x has schrader valve under rubber bung(pressurise it to 150+ it’s rapid return 🙂

Ill have look under the saddle but I’m sure its got a 10mm nut,

I always think mine is slow but on my other bike I have a Specialized Bollockhammer Command post

Your brand-x post will either have a rubber bung and a schrader valve under it or a 3mm hex bolt.

If its got a valve, put more air pressure in.
If its a hex bolt its a sealed cartridge.

If youve had the cartridge out then you’ll know if it has a 3mm hex bolt or not tho surely?!

The service guides for the newer posts are here

150mm internal routed service guide: https://media.chainreactioncycles.com/is/content/ChainReactionCycles/Catalogue/Press%20reviews%20%26%20additional%20content/Brand%20X/Brand-X%20Internal%20Dropper%20Post%20150%2C%20170%2C%20200%20Service%20Guide.pdf

150mm external routed service guide:
https://media.chainreactioncycles.com/is/content/ChainReactionCycles/Catalogue/Press%20reviews%20%26%20additional%20content/Brand%20X/Brand-X%20Ascend%20II%20150%2C%20170%20External%20Route%20Dropper%20Post%20Service%20Guide.pdf

I didnt touch the top of the post when I took it apart, didnt need to remove the saddle, all I did was undo the collar/seal head, then undo the lower actuator, take the two grub screws out so it comes off the cartridge then the lower part of the post slides off leaving the cartridge in the upper section

Mines never been snappy fast from new but worked.

When it wouldn’t return under it’s own steam, used to stick about 1/2 way up. I replaced the top bushing under the collar with a Igus bushing (grey instead of white) for a one up dropper and it’s worked fine ever since.

I recently had this issue, no option to increase pressure on mine so totally stripped it, cleaned it, regreased it and seems to have solved it. It was amazingly easy, in the spring I’ll service it – kit is £10 and includes new seals/bushes and keys.

I brought a new cartridge, it fixed the issue and post returns alot faster. but I stupidly didnt compair it with the old one before fitted. Its about 15mm shorter so the post is no longer a 150mm dropper. Other difference is its changed the top mount from a 10mm nut to a 3mm Allen key. New one was marked as a 150mm internal, guess its for a 2020 model, there is a shop selling service kits on ebay, they told me the kits are only for the latest posts

Comparison between my oneup and the brand x

Difference in cartridge , new one on the left