[Borges]

Until 1873 Brahms had written chiefly for the piano, the instrument he knew best, and for chorus and orchestra

Not really correct. The majority of his pre 1873 work was chamber music and songs.

[Publius]

And what chamber music!!

• Piano Trio in B, Op. 8 (original 1854 version)
• Sextet in B-flat, Op. 18 (where he first tries solving the formal problem of string sextet writing)
• Piano Quartet in G minor, Op. 25 (one of the greatest crowd pleasers in the chamber repertory)
• Piano Quartet in A, Op. 26
• Piano Quintet in F minor, Op. 34
• Sextet in G, Op. 36 (where he solves the formal problem of sextet writing)
• Cello Sonata in E minor, Op. 38 (with that great uncompromising fugue in the finale)
• Horn Trio in E-flat, Op. 40 (with one of the greatest slow movements ever written by Brahms)

These are the Cadillacs of chamber music.