After you remove the mushroom assembly, remove the brew valve spring and brew valve using needle nose pliers. Soak all of these parts in descaling solution or white vinegar for about thirty minutes and then rinse all parts under running water, cleaning the mushroom with a green "scrubby".
I poured some white vinegar or descaling solution into the chamber with a blind basket in place and the brew lever in the brew position (this ensures that the drain valve is closed). I let this soak for about 30 minutes and used a toothbrush on the mushroom chamber walls. The result is as shown after draining the solution.
Ensure that the mushroom assembly is rinsed well and that all of the small holes are clear by holding the mushroom up to a light source. There are four holes which feed the gicleur chamber and then, of course, the 0.70 mm gicleur (typical) itself.
With the brew lever in the exhaust position (all the way down), plop the brew valve in place and place the spring over the brew valve. Now screw the mushroom into place hand tight. Tighten with the 36 mm wrench - just snug - maybe 1/12 of a turn maximum.
Place the gicleur screen over the gicleur itself and screw the gicleur chamber nut in place hand-tight. Tighten with the 22 mm wrench - just snug - maybe a little over 1/12 turn.
Flush the machine well - maybe for about 5 minutes - remembering to not run the pump for over 30 seconds or so before giving it a rest for a similar period of time.
This is not meant to be a descaling procedure but the methods proposed will certainly remove a goodly portion of scale (if present) from the areas mentioned. Once you do this (and I can understand the trepidation), you will wonder why it took so many words to describe a simple procedure.