Hi, just thought I'd explain a method which seems to work for me with my old HW77k from the eighties.
1) Start with a new seal.
2) Place the cylinder, piston and seal in a pan of boiling water. With the piston with the seal on push the piston in and out of the cylinder. Let the pair rest in the pan for about five minutes.
3)Get a bowl of cold water (best with ice)
4)Quickly use the piston to suck up ice cold water then block the transfer port with your finger and push the
Piston down to create as higher pressure as possible and hold.
This process first softens the seal then opens the parachute seal and cools it in the perfect diameter. This will not work so well with an old seal. Once done the piston can be pulled to the top of it's stroke and let gravity make it fall but then stopped by covering the transfer port.
I hope this is of help.
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