Clean and check the valves and pistons.
WARNING: This page is about a different car, the 2002 Volvo C70, 2001 Volvo C70, 2000 Volvo C70, 1999 Volvo C70, and 1998 Volvo C70. However, it is still accessible from the selected car via links, so may be relevant.
Clean all metal parts carefully in cleaning fluid, type gasoline, paraffin or trichloroethylene. Dry with compressed air or lint-free cloth.
Check that the valve opening is not worn or damaged.
Check that there are no signs of wear or damage on the valves and that the valves run easily in their bore. Valves can possibly be carefully ground using very fine emery cloth.