I see you found something while I was writing this, but will attach what I did.
I dug into this for a 67 GP. Was working okay until I tried topping it off, then fluid seeped past pump (small o-ring on shaft) into motor and that was the end of it. Old GM cars (pre-68?) used brake fluid, newers used ATF - I think I used hydraulic (or convert top) fluid but can't find bottle in garage w/quick look.
Ah-found this link from when I discussed this on other forum (almost 3 years ago..).
67 Pontiac convertible fluid type, leak, and add - Hot Rod Forum : Hotrodders Bulletin Board
I used this kit.. $20, cheap.
These guys are good, but the "green" armature o-ring may not be green. It's same size but different material than two other o-rings in the kit. Black one seals, but being on an armature instead of static seal, I'd want the green (viton?) seal. Check around and maybe call vendors to confirm. I'm not sure if these (Hydroelectric) are the guys I got that kit from (whoever it was, changed their ad so it no longer said "green").
Pull the back seat if not already done. Pull armrest/whatever on sides so you can access/see everything and check for leaks. Good to take photos so if you pull all apart, you'll know how it all goes back together/lines are routed.
To "purge" air from lines, all you do is raise/lower top a few times. I can't remember sequence, but you fill it when top's either up or down (of course one or other, but it matters so you need to know..). With back seat removed, you'll see air bubbles in lines disappear as you do this.
To "flush" the pump, cylinders, and lines...
1) I'd remove the pump and rebuild it on a bench. Is easy. Problem with no power is either with motor (fluid leaked into it) or the check ball areas are gunked up. Pull it apart before you order a rebuild kit - if motor is bad, you need a different pump. Test the motor using jumper cables (don't run pump dry for long.. there should be enough fluid in it to test motor briefly - my bad motor growled and got hot fast).
2) I flushed cylinders by removing them, then moving rod up and down manually while collecting fluid (to see if gunk was inside). You can pull clean fluid back into them and flush out a few times. This can be messy - fluid will squirt far... proper way would be attaching lines and using a can to collect old stuff, separate can/ATF bottle to supply new fluid). Pushing or pulling rod makes fluid come out of top or bottom of cylinder, while it'd pull fluid into bottom or top. When removing cylinders, keep track of hardware, plastic bushings, wavy spring washers, etc (maybe stuff a rag into an opening, where if anything fell/bounced into, it'd be hard to get it out
3) Lines can get sediment in them - blow them out with compressed air, then maybe hook a clean tube to one end and suck clean fluid into them, then blow it out with compressed air. Seems if ATF, could use a can of transmission line flush. I wouldn't be concerned about getting these squeaky clean inside. If you pull lines, you could attach and flush them while flushing cylinders.
Curious about your symptoms.
Top doesn't go up at all, Top goes up crooked, leaks, air in lines, goes up but labored, etc
My switch was bad - didn't get enough current to pump motor. You can test this by hooking 12 volts directly to motor (there's a common line, then one each for up and down). Motor also needs to be grounded well in trunk.
I never read anything about anyone needing a more powerful pump. I ended up getting a used Mustang pump for $50/ebay for the GP and just had to change with the mount (maybe transferred pump to motor?? my memory is bad, but it still works fine...).
I had no problems removing hydraulic lines. IIRC, all were brass swivel connections that came off easily (I used a tubing wrench, but a good open end would probably work). These lines can be brittle and crack/snap, but then they needed replacing anyway... not too expensive.
Lots of good info on HydroElectric site.