weakforms: BUT

BUT can function as a
- conjunction: "He rushed to the post office, but it was already closed",
- preposition: "I can come any day but Wednesday",
- adverb: "We can but try and do our best" or
- noun: "I don't want to hear any buts from you".
BUT has one or more weakform pronunciations. What you find in the three pron dictionaries is this:

EPD18:
strong form:  bʌt
weak form:  bət
Note: Weak-form word. The strong form /bʌt/ is used contrastively (e.g. ifs and buts) and in sentence-final position (e.g. 'It's anything but'). The weak form is /bət/ (e.g. 'It's good but expensive' /ɪtsˌɡʊd.bət.ɪkˈspent.sɪv/).

LPD3:
but strong form bʌt, weak form bət

ODP:
but strong form bʌt, weak form bət

Jack Windsor Lewis's PhonetiBlog has an entry (no. 441), in which he lists additional weakform variants:

The conjunction, adverb and preposition etc but has only a single ordinary weakform /bət/. However, before a word beginning with a vowel, a form of but reduced to the consonantal cluster /pt-/ may sometimes occur in relaxt style where the original initial /b/ is devoiced to a /p/ which is merely an unreleased bilabial closure. Meanwhile the release of the /t/ is without aspiration [...].

Here are three excerpts from a TV film (credit: BBC4) on the famous lexicographer Samuel Johnson.

1. "/bət/ you can't, as it were, make decisions about which words should or should not be used." (speaker: Prof John Mullan, UCL)

2. "/bət/ in the course of his reading he found that - you know - some words had 20 or 30 or a hundred different applications [...]." (speaker: Henry Hitchings, author)

 3. "/bə i dʌz hæv/ the word retromingency, which means pissing backwards." In this excerpt I don't hear a /t/ at the end of "but".