Using my Dad's old Skil 728 roto hammer. Hasn't been used for 20 years but has been cased and protected. Drill runs smoothly but the hammer function only engages intermittently. Repeated trigger pulls will eventually engage the hammer function and then it makes concrete seem like butter! So does anyone recognize the symptoms and are you able to suggest a possible corrective action. This tool has sentimental value so I'd like to keep it running and hand it down to my grand nephew. I was lucky enough to find some bits online. Thanks.