Hmmm, excellent question.

The bleed screw is listed as Volvo p/n 678155 in Volvo parts catalogs, so that's the cross reference you're looking for. Although there are numerous references to that p/n, I can't find the size listed in the Volvo brakes green manuals, aftermarket manuals or anywhere on the usual sites selling Genuine Volvo. You're presumably looking at the various Dorman ones listed on Rockauto, so let's work through this together.

First off, if you stop to think about it, your front brakes are dual piston Girling -> Girling is Lucas -> Lucas is British -> so definitely an SAE bleed screw rather than metric. Having now determined that, be wary of a couple of Ebay listings I spotted.

Referring to the online Dorman brake parts catalog (same thing their tech support people use, easily found with Google), you can go at this two ways. You can either use the Volvo p/n in the parts interchange field or you can do a vehicle lookup. There are two front bleeder screws listed for Volvo 240 front Girling applications that include your year: 13904 and 12703. Both are 1/4-28 X 15/16 for size and thread. 13904 is listed as a brass screw, while 12703 is listed as a steel quick bleed screw with a smaller nut face and 1/16" longer. These should actually fit any Volvo 160/240/700/900 Girling caliper.

Quick bleed screws, also called speed bleed screws, have a check ball valve inside (as noted in the Dorman catalog) so air isn't sucked back in, such as can happen when doing a one man pedal bleed, also a steel screw is less likely to shear off. On the other hand, a softer brass screw makes a better seal when tightened to the lighter torque spec (perhaps making opening easier next time, although issues of having dissimilar metals may be involved), also a larger nut face means you can get a larger handled wrench on it. When opening frozen bleed screws, after a hit with penetrating oil, a combination wrench is by far preferred over using a socket ratchet wrench in order to avoid sheering the head, so I'd probably opt for the standard screw strictly for the sake of future use. BTW the reason we can use metric wrenches on these is that the nut sizes are close enough for a six point wrench. Also, a flare wrench is best for loosening brass fittings, plus you can slip it over an attached bleeding tube and leave it hanging there during bleeding.

Going back to the Rockauto catalog and clicking on Info, 13904 cross references back to the currently listed 41417 screw. Also, there's only one speed bleeder left at a bargain closeout price and I'd want to replace all four front bleeders if I went to speed bleeders.

For future reference, the rear brakes in Volvo RWDs can be either ATE or Girling, mostly ATE in the later models. ATE is German, so you're looking at a metric bleed screw for those, in this case an M7-1.0 bleed screw.

I've only ever broken one bleeder, the first one I ever tried on my old 122S Sport Amazon when I was young. Now that I recall it, I'm fairly sure I traded in that car for my brand new 144 with the broken bleeder still there. You can do a half-baked job of bleeding with a sheered bleeder by tilting the caliper, tapping on it to move air up into the line as you retract the piston and then loosening the hose on the caliper for bleeding fluid -when you're a starving student and don't own a screw extractor it's a good alternative.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now