The 156 Lee Chek high-pressure check valve from The Lee Co., Westbrook, Conn. (theleeco.com), weighs only 0.7 gm, and has a 0.156-in. diameter.

To check the pressure, you'll need a simple brake-pressure gauge. They're sold by several sources, including Lamb (PN 882) and Wilwood (PN 260-0966).