Link these two compressors?

[Seadog]

Yes. It's to prevent pressure loss back through the pump head when not running.

 

[Chris152]

Is it this one? There's one on each end of the Hyundai.

View attachment 439198.

Still not finished! I'm struggling to undo this blank - could it be a reverse thread?

 

[carbon]

Very unlikely, but it could have thread lock on it. Warming it may help release it and allow it to be undone.

 

[angellonewolf]

Very unlikely, but it could have thread lock on it. Warming it may help release it and allow it to be undone.

as he said i had one that would not budge but a good bit of heat and did not fall out but did turn with heat and welly

 

[lewlux]

Heat and a breaker bar with a suitable exctension like a scaffold pole should shift it easy, prollty wont evenn need heat tbh, worth a try first anyways.

 

[indykid]

Yes. It's to prevent pressure loss back through the pump head when not running.

That, but also it allows the unloader valve to vent the pump when it cuts out so that when the pump starts next time, it starts with no pressure load.

The unloader valve venting the pipework is the 'pshhhh' you hear when the pump cuts out at full operating pressure. Since the OP seems fairly new to compressed air, it's why you should always stop a compressor by pressing the button on the pressure switch, not by turning the power off at the wall - the button on the switch operates the unloader too. turning it off at the wall leaves the pipework pressurised (until it bleeds down through the pump)