I’m building up an old steel road bike to commute on. It has a 105 short cage mech that looks in good condition and i have a 7 spd Deore cassette (bike had a knackered 8 spd.

Will they work together??

Should do. I run a 105 rear mech with 32/22 front and 11-32 rear. Works fine, infact FWIW, I think it’s even better than an mtb mech as it keeps everything tucked up and in tension (no chainsuck).

Shimano say that the road mechs have a 28t max rear capacity, but I’ve not seen any problems.

This will have a 39/52 front , does that make any difference?

I’d suggest it’s unlikely, and Brant’s frame must have an unusually long mech hanger. A 32 certainly didn’t work with a road mech on my CX bike. Whilst Shimano specs are conservative, with most frames you’ll only stretch to a 30.

Cheers, dug out an old deore and swapped the jockey wheels across, perfect.

Replace the b tension screw thingy with something a bit longer, and a 11-32 or 34 goes in no bother. Done it with a tiagra and a 105 mech.