Bypass Valve?

[ditchguy]

Our Buderus GB142 has a DHW circulator with an indirect tank and sends hot water to the radiators with a circulator pump and a zone pump via a Watts Radiant stainless steel manifold with zone valves. The Watts manifold had a Watts bypass valve bridging from Supply to Return. With the bypass valve slightly open, the system has been running for seven years. The bypass valve recently began leaking from under the adjustment knob. I took it apart and was disappointed to find no provision for improving the seal around the stem. With the valve removed, the system works just fine. My only concern is that the heating circulator pumps run for five minutes after the zone valves close and the heating call ends. This five minute circulation is intended to purge heat from the boiler. The boiler circulator can move water through the pipe loop in the manifold, but that hot water can't travel more than about four feet. With no bypass around the closed zone valves, is there any danger to the zone circulator as it pumps against water that can't move? If you think we need a bypass valve, I'd welcome suggestions for a better design than the original. Does it have to be spring loaded? Could I fabricate one using a partly open ball valve?