if using a heat gun, keep it far from it, its a surprise melting. you just want to warm it, not melt it.

i really think a lighter will do it in like seconds if its that brittle. with a lighter, you want to hold the lighter under the hose, dont let the flame touch the hose, just the heat from the flame, keep the flame moving and spin the pump over to warm all sides, with some gloves on, pull the hose off when its still warm. then maybe push it on the new pump while still warm.