I'd loosen the cap before removing it just because it's held in place.
When I did the single acting ram seals on my 4 post I used Ipswich Hydraulics for the seals, but now I would use Agri-hire - Mervyn is a hydraulics wiz.
There is also FW Stennett not far from you, they have a lot of Ag based stock and are also very knowledgeable
Also Impact Handling in Claydon - fork lift specialists
I've had 2 of those machines and wish I'd kept the full free lift one..........
Steve