Question for E_ULY or any other professional Volvo techs: what’s the proper way to seat the o-rings in a P2 chassis heater core?

I replaced the heater core in my S80, which went smoothly until I had to insert the pipes. The new o-rings require a remarkable amount of force to seat properly. The forums state a mixture of push really hard (I tried this) and use silicone lubricant spray (will try this today). Before I have to use a hammer or turnbuckle, what is the actual way to mount these?

UPDATE: a touch of silicone lubricant spray made the pipes and rings mount easily.