Need to overcome UNION / ORDER BY restriction

I have a table:

id | priority | seniority
-----+----------+-----------
902 | 2 | 271
902 | 1 | 271
902 | 3 | 271
924 | 2 | 581
924 | 1 | 581
924 | 3 | 581
935 | 1 | 276
935 | 2 | 276
972 | 2 | 275
(9 rows)

I'd need to get a result set where rows are sorted according to these rules:

1. first all rows with priority = 1 sorted according to seniority
2. then the rest of the rows sorted by seniority, priority

Something like this:

SELECT * from foo where priority = 1 order by seniority
union select * from foo where priority > 1 order by seniority, priority

but this gives parse error because of the restrictions with ORDER BY and
UNION (I suppose..)

select * from foo order by case when priority = 1 then priority else
seniority end;

goes fine, but it's not quite enough and when I try

select * from foo order by case when priority = 1 then priority else
seniority, priority end;

it's parse error at or near ",".

Any suggestions?

Thanks in advance,
Timo

---------------

CREATE TABLE foo (
id integer,
priority integer,
seniority integer
);

INSERT INTO foo VALUES (902, 2, 271);
INSERT INTO foo VALUES (902, 1, 271);
INSERT INTO foo VALUES (902, 3, 271);
INSERT INTO foo VALUES (924, 2, 581);
INSERT INTO foo VALUES (924, 1, 581);
INSERT INTO foo VALUES (924, 3, 581);
INSERT INTO foo VALUES (935, 1, 276);
INSERT INTO foo VALUES (935, 2, 276);
INSERT INTO foo VALUES (972, 2, 275);