Water seems to be leaking up thru the electrode. is it an o ring prob or the elecrode itself?

Replace the O-ring or seal.

the low pressure issue is most likely the expansion vessel needs repressuring because the valve has leaked and is not seating properly so causing water leak outside.- with the pressure on zero, drain about 1/2 bucket out, mine had a exit tap near the inlet tap, then attach a car pressure gauge/ pump to the top of the orange exp vessel. pump it up to about 1.5 bar, that should do it..you may have to do this 2 or 3 times..
re loud rumbling sound, mine had this too, and at around the same time ! heres how to fix it..
BGas engineers are lazy, and unless you watch em like a hawk they lie about changing a part...mine had this loud noise problem and its caused by the gas being deflected out, because it has not been ignited by the spark electrodes, and the rubbish boiler doesnt detect this quick enough, so some gas gets out ..You may be able to smell some gas too. The cover is supposed to stop this, but it dont.

with power isolated, all you have to do is carefully remove the fan, and then the burner, and then dry it, clean it, and adjust the electrodes, 5mm gap between the pair of spark electrodes, and 7mm gap to the cone. and 7mm for the sensor electrode. Mine had a bit of water, i think its from the condensation coming back down the flue ? because the exchanger looked ok.

Also be careful of knocking the sensors and wires, and also check if these are ok as well.
This is a very common issue and nobody tells you

You may have an intake leak between the carb and head. Make sure the carburetor is fully seated in the intake boot and that the intake boot has no cracks in it. If the carburetor is a flange type, make sure that the gasket or "O" ring, (whichever is used), is in good shape. You can test for a vacuum leak by spraying WD-40 around the carburetor mount. If the engine changes speed, you have a leak. Running the engine with an intake leak can cause serious engine damage.

ok you got some possibles.1 check cabin filter if signs of water than drain plug on fire wall cloged.causing water to enter by over filling and no were to go.but thru the filter.easy fix.also if you losing anti-freeze than that will point at your heater core leaking from line or core itself.need to test it,turn on heater and check floor with a towel. lay it on floor going up to firewall inside truck.go for a good drive so pressure can build and heater on.while engine hot look at floor towel top part by firewall wet prob heatercore.also check if door is wet to eliminate possbility of door seal leaking.let me know.

Basically, the electrode that causes the spark to jump from it to the gas ring/top is shorting out because of the water. It has to be bone dry and clean for it to work. With the rings off, make sure they are dry and use say kitchen towel to clean and dry the electrodes. Be careful, the electrodes are fragile and will not take any punishment. If in doubt you could use a hairdryer to dry the electrodes but do not have it too hot - the force of air alone may be sufficient to clear away the dirt and damp.
Watch the spark on the one that does work to see what it should do!

replace the wter conduit that runs up to the top and center spinner arms,after while these plastic conduit outlets get worn by water going thru them and leak past the o-rings installed on them

the water pump attaches to the motor, that is the olnly thing that could be leaking below, hope this helps

when installing new o-ring on tube, lubricate o-ring and stick it in water pump. Then pull pressure back on tube and then tighten hold down clamp. problem solved

If you find any coolant loss, likely the head gasket is beginning to leak or you have a small crack in the cylinder head that is opening up when it warms up. If you have it checked, you will save yourself large problems down the road. Sometimes you will see a slight greenish residue on spark plug electrodes. Sometimes even shop tests are inconclusive if leak is very small. But, I can't see the smoke from here and really can't advise further.
Good Luck

sounds like the rubber seals have dry rotted prob from not oiling it enough, but you should be able to go on the Bostitch web site and order new rubber seals for the gun