Connector

The Breville BES860 was not getting any pressure and was leaking water due to broken coupling connecting heating element to the pressure gauge. You will likely need to buy replacement gaskets as well as they're probably damaged. I don't have exact measurements unfortunately but was able to get a proper one fitted by bringing in the replacement parts to my local True Value hardware store.

Remove water retention box and trays, unplug the device
Remove 4 Torx screws in back and two phillips head screws on bottom of the back plate.
The top plate slides forward towards the front of the. This takes quite a bit of force usually but no un-screwing is necessary.

There is a ground wire connected to the top plate, remove screw and washer
Remove two screws holding the left side (when facing front) of the main body of the machine, this will allow you to move it slightly out and remove the plate housing the control buttons. The control plate is secured with two Phillips head screws. There is a tube running from the heating element to the pressure gauge. Remove the hairpin securing this and pull the tube out (watch out for a small black o ring from popping out you'll need this for it to work).
Using the adjustable wrench disconnect the pressure gauge from the front housing and the coulper connecting it to the hose running to the heating element.

Note - the plastic coupler is likely broken or one of the gaskets has worn down causing the water leakage.

In my case the plastic coupler sheered off in the pressure gauge and I needed to use drill bits to remove material and eventually cut the remaining plastic until it was able to be screwed out. This took about 30 minutes. The replacement pressure gauge is for sale here for around $20.

For reassembly:

Place large gasket around threads of the coupler, return the pressure gauge to its location if you removed it, secure the coupler to the pressure gauge (note if the gasket buldges out you will need to use a smaller size).

Place small gasket around the tube, insert tube into replacement coupler, secure with pin. Hand tighten everything withe the adjustable wrench.

Re-attach ground wire, plug in machine and stay clear

TEST THE SEAL - if this works complete assembly

Unplug the machine

Return faceplate to its location and secure it's screws

Re-secure the wall that was moved to allow removal of faceplate

Re-secure top

Re-secure back

Have an espresso

- Remove back cover (4 torx screws on back, 2 phillips on bottom)
- Remove top cover (hit it forward with your palm to clear the catches
- Ran machine to determine leak location. Found two plastic connectors that were leaking. Both were damaged upon removal (broke apart).
- Keep internal and external O-rings if possible. (internal o-ring was about 1/8" inner diameter, outer ring was 1/4" O-Ring.
- Ordered new metal connectors. The connectors were better, but didn't include any of the new O-rings.
- Ordered FDA, food grade O-rings for outside of connector.
- Threaded in new connectors after installing O-rings inside and outside the threads, then buttoned the espresso machine back together.