I have an 85' Elektra mcal, I am disassembling it and I am wanting to soak the brass piston in Joe Glo to clean it up. Should I go through the PITA of taking the seals off first or can I just dunk the whole assembly in?

Will Joe Glo dry out the piston seals ?

Yes it will dry out the piston seals. It is, like most coffee oil and residue removers a very strong emulsifier (emulsifiers break down oils) that will strip the lube on the seals right off.

I would make any exposure of the seals to this stuff very brief, only enough to clean off coffee oils and not soak in and remove rubber oils, rinse them thoroughly and reapply the washed away lube (Dow 111 food grade grease.)

I guess that would mean it is preferable to remove the seals for this.

As for the metal parts Joe Glo may not be the best stuff to use on aluminum or pot metal.

It may also be rough on some finishes if left to sit for long periods. I have a pot of Joe Glo at home which I have not used yet but I do have and have used cafiza which I read may be the same stuff as Joe Glo (I don't know for sure) and draw my experience from that.

If you are going through the trouble to work and rebuild or clean the machine
the gaskets might need replacement anyway

just a friendly suggestion
DO NOT disassemble the spring/piston shaft unless you really have to
it will put up a fight to re assemble it back

Oops yes that is good advice. I own a La Pavoni Europiccola and it does not have a spring powered piston. I have read those springs in spring piston machines pack a very very strong wallop.