The majority of boiler pressure gauges are defective or inaccurate.

When dealing with boiler pressure problems, the FIRST thing that must be done is to verify the accuracy of the pressure gauge. 

A test gauge is easy to construct.

What you need:

1. A 0-30 (best) or 0-50 (OK) PSI pressure gauge. Higher pressure gauges are useless for this application as they lack the resolution needed for a proper reading. Gauges can be purchased for $10-$25. The best place to find a suitable gauge is a REAL plumbing supply, or swimming pool supply house. (or even a Beer homebrewing supply!)

2. A way to adapt this new gauge to a Garden Hose Thread (GHT).

Here is one way:

This is a 3/4" FGHT (Female Garden Hose Thread) to 1/4" FIPS (Female Iron Pipe Size) adapter. This part will run about $6 or so.

Your gauge should have a 1/4" MIPS thread and will screw into this fitting.

If you have a well stocked junque boxe, you can put something like this together.

This is an end of an old washing machine hose and some fittings to adapt the gauge.

When you have your test gauge, simply screw it onto any one of the drain valves and open the valve to read the pressure.