Toyo M145 valve stem core issues

Valve stem cores come in different lengths. It's possible the dealer used a longer one than was originally in the stem. If the stem has a bend or slight curve the core may be interfering with the sidewall of the stem when it's open. If that happens the rod holding the spring in place will bend and not allow it to seal once the pressure gauge is removed.

Here's a link to some information on the types and sizes of valve cores available. Note it's a foreign site with some non-English verbiage. The first group of 17 valve cores are the most common:

http://www.haofeng-ind.com/qmz_f_01.htm