I have one of these and it works great. I set it at about 10 lbs. of pressure and it does the trick on my Speedster with front discs and rear drums.
Ditto what Gordon said about getting the right cap. Look at the manufacturers website first after measuring for cap size to get the right model number.
There a re a number of sources for these, not a great deal of price difference but a couple of places offer free shipping. I just don't remember which ones.