I believe Bishop Castro de Meyer held the sedevacanist position starting after the council.  He and Archbishop Lefebvre were of course in disagreement on that point, although as I understand it they were otherwise quite close (friends maybe?).

As for why exactly the SSPX expel sede priests, I am not really sure.  Perhaps they feel that having open sedevacanists in their ranks compromise their position of 'recognize and resist' with the Vatican ??

Luke

Didn't Bishop Castro de Mayer have a reconciliation with the Vatican?

I don't believe the sedevacantist thesis is "obvious".  After all, there is a man in Rome who wears a white cassock (well, actually two men, but that's another issue altogether), calls himself pope, has a small army at his command, and a whole lot of people jumping at his every whim.  He certainly does look like he's the pope!

Just as it is not immediately obvious that raising the price of product does not always increase profits or that a company can often increase its profits by lowering the price of a product and increasing sales, the sedevacantist thesis can seem counterintuitive.

The sedevacantist thesis requires one to consider the nature of the Church and it has always been and think through the implication of a pope who publicly rejects that nature.  The SSPX, like all non-sedevacantists, do not look at the very nature of the Church but, instead, focus on and expand the meaning of individual dogmatic declarations and canons which, to them, indicate that they must accept the claims to the papacy of any man that has widespread and near universal acceptance without competition.  They necessarily ignore the undisputed fact that anti-popes in history have had wide-spread acceptance and have even ruled from Rome itself.  But it is uncomfortable to believe that these historical anomalies could apply in our own present day.

I was at a conference at which Bishop Fellay spoke and he likened the Church to a plane in which the pope is the pilot.  He condemned the sedevacantists of being on a plane in which they had ejected the pilot and the plane would soon crash.  Now I would say that this is a very good analogy for the Protestants who are spiraling toward the ground closer every day.  But I would say that the present situation is more like the pilot has died and an enemy has taken his uniform and convinced the flight crew he can pilot the plane but has actually hijacked the plane and is taking it into enemy territory.  The sedevacantists, at first, accepted this new pilot but after realizing he was an enemy took him out of the pilot's seat and is allowing the navigator to fly.  He may not be able to fly with great confidence, but he is, at least, flying in the right direction until a new pilot is actually found.

Why can't the SSPX see this?  Why can't all non-sedevacantists see this?  One possible reason is that the sedevacantist thesis is extremely distasteful.  Unlike the stereotypical view of the sedevacantist, we don't relish in the papal vacancy.  We are not gleeful at the missteps and the misdirection of the Church.  We don't rub our hands when the Vatican announces a new departure from the Faith.  We don't snicker at Bergoglio's appeasement of ɧoɱosɛҳųαƖs.  We are saddened.  While it does look like Bergoglio will not be ordaining women anytime soon, an official blessing of sodomite couples does appear to be on the horizon.  Only time will tell.  Everything that happens in the Vatican today seems to confirm the sedevacantist thesis while the anti-sedevacantists simply declare that all these actions are simply not the actions of the Church.  While we agree on that fine point, we simply cannot understand how these non-Church actions can keep coming (over and over and over again) from the Vicar of Christ on earth.

I see it like this:  The Jєωs, scribes and those who read the prophesies and etc, could not see Christ in front of them.  Pride was one reason.  Others before Christ, paid more attention to the laws and not the scriptures and did not recognize Christ.

What I see and confront in PIUS X is their reasoning is that they can not judge the pope.  I say, "Christ said that we would know them by their fruits."  I say read Vatican I how infalibility was defined.  The clergy had to define "pope", validity and election.  Some people don't take time to read or to search and just stay where they are.

Pius X wanted jurisdiction to hold up, but it means that they must come under the pope, but yet they say they are separated as traditional at the same time.  If they don't hold up jurisdiction, then are they or are they not valid. If they come under the pope, are they NO?  How is their ordinations?  Are they approved by pope?
How can they be both, tradtional and come under a pope that says that they are schismatic.  Pius the X was founded on what they called jurisdiction from the pope.  If they do not take on the pope, then all their validity as they see it, would not be there.  Then where would they be.

That is why epikea was taken on by Carmona, for that is the logical way and the only way to keep the lineage going.

If the sede position is so obvious, why does the SSPX reject it?

This is a question coming from someone who admits that he would kiss the Koran (!); someone who thinks that the Saint Benedict Center getting their theological position "approved" by the Vatican (which approves of a Mass without any words of consecration to be "valid"; which sends yearly greetings and congratulations to Jєωs, Buddhists, Hindus, Muslims etc. for their special feasts and "holidays"; etc. etc. etc. ad nauseam) actually means something.

Maybe you yourself should answer that question.

Maybe you should explain to us how is it that you were a sedevacantist sometime before, and explain how you went back to being a novus ordo.

Perhaps you should explain to us how is it that you can assert something false, get refuted and exposed, then act like nothing happened, and then assert the same thing all over again.

Perhaps you can explain to us how is it that you can ignore all the facts and all the evidence.

You should explain how and why do you do all these things.

Maybe you should explain to us how is it that you were a sedevacantist sometime before, and explain how you went back to being a novus ordo.

This is very interesting.  I was unaware that the individual who opened the topic was a sedevacantist at one time and returned to the Novus Ordo.  I truly would like to know what brings someone to the realization that the Conciliar church really is the true Church in spite of all the evidence (e.g., the changing of doctrines, the transformation of the sacraments, etc.) against it.

What is it that takes one to the Novus Ordo?  What is it that takes one from thinking that the leader of the Church can be a notorious and pertinacious heretic?  I'd like to know.  Every traditional Catholic I personally know who has told me why they are not sedevacantist has told me that they agree that the pope says heretical things but they "just can't accept that he's not the pope" and they give me legal arguments to demonstrate some technicality that indicates that he can be the pope.  I really don't understand.

Is the SSPX in denial?