Like Monty said, your maximum theoretical vacuum for your elevation is 29.38" Hg. You should be able to easily see 29"+ if you are using an oil filled rotary vane pump. You mention that you held vacuum for 1:45. Do you mean you shut off a valve and held the vacuum or you had the pump running the entire time? You need to have the pump running to keep the vacuum level as high as possible. At a 27.5" vacuum, you are only pulling a 93.6% vacuum which translates to removing 93.6% of the air in the wood at best. The more air you remove, the more resin you get back in.
To check for leaks, pull a full vacuum with your chamber and apply shaving cream to any fittings and place where a leak could occur. If there is a leak, the shaving cream will disappear at the leak and you will know what to work on. You can also take the gauge off your chamber and hook it up directly to the pump to see if the pump is pulling full vacuum and or the gauge is good.
In reality, 1:45 is not very long to be pulling vacuum on pen blanks. I have never had any stop bubbling in that short of an amount of time at full vacuum.
