Winterization: Unable to measure air pressure, despite evidence of it’s presence?

MadCityJack

Today I winterized my recently purchased TAB 320, 2017 for the first time.  I followed all the guidelines on the forum on winterization, choosing the “blow out” method.   A brass fitting with a tire type valve was used to provide air to the city water intake.  I used two different compressors, 12v Ridgid, and a Bostitch, and set all air pressure at or below 45 psi.   

Here is the curious thing:  When measuring from the valve as soon as I met the desired air pressure, the reading of the pressure, both with a digital and mechanical gauge was zero.  Yet, when i opened the sink faucet, air pressure blew off the aerator at the tip with a loud pop, evidencing significant air pressure.  Also, when I left a rear Alde drain open and provided air pressure, I could see that water was moving out of the drain. 

Any clues to why I couldn’t measure air pressure at the air intake valve when all faucets/drains were closed?  I wonder if I have a small leak in one of the water hoses/fittings that is letting the air out quickly.  But if so, why would the sink faucet “shoot” out the aerator at the tip with tremendous pressure?  Is there a valve somewhere I need to check?  Next step?  

Thank you for any tips.

Pintoplumber

There would be a check valve in the line you’re pumping air in. Once the air enters the trailer it can’t get back out to the point where you are measuring.

MadCityJack

Pintoplumber said:

There would be a check valve in the line you’re pumping air in. Once the air enters the trailer it can’t get back out to the point where you are measuring.

@Pintoplumber, Thanks so much for the clarification.    I didn’t think a check valve would just work with air pressure as well as water, but that makes sense.   So interesting to learn about how this TAB works…

Denny16

Normally when you blow out the water lines with air, you open one of the faucet vale’s first, then pressurize the line, to blow water out the vale, you,do,this one valve at a time, with Alde bypass set to prevent air from going into the water tank, which is gravity drained anyway.  
Pressurizing your water lines with the faucets closed, could allow too much pressure to build up in the line, and damage them.
cheers

Horigan

It really depends on whether you can control the air pressure.  If you can set it at 40 psi, that would be fine for blowing out the lines and not hurt the plumbing.  10 psi is probably sufficient.  The key is having enough volume in your compressor to have enough air flow to push out the water.  I personally don't use any anti-freeze and just blow out the lines, including the outdoor shower.  Air doesn't freeze and any little bit of water left in the lines has enough room to expand without damaging plumbing, etc.

MadCityJack

Denny16 said:

Normally when you blow out the water lines with air, you open one of the faucet vale’s first, then pressurize the line, to blow water out the vale, you,do,this one valve at a time, with Alde bypass set to prevent air from going into the water tank, which is gravity drained anyway.  
Pressurizing your water lines with the faucets closed, could allow too much pressure to build up in the line, and damage them.
cheers

That makes good sense and hopefully no damage was done.  

Sharon_is_SAM

We leave the valves closed and watch the pressure gauge.  Our little compressor takes it’s good old time getting up to 50 psi.