Broken M4 x 0.7 screw in brakeoil reservoir cap

One screw came out just fine, the other I mashed up the screw head like it was butter. Them screws are soft. So drilled off the head then tried to turn the remaining bit out with pliers but ended up snapping the thing off. So here I stopped to have a think first.

Haven't got a picture yet but any advice or ideas are welcome. Found a second-hand master brake unit complete for €40 (£33-ish). Will try myself first drilling with 3mm then retreading.

What else would work? Anyone experienced the same problem?

see if it can be done using a punch tool and a hammer, tap the tool in the right direction and see if you can get the screw to come out.

if you got a welder, you can place a nut on top of the screw and weld together the bottom part of the nut and the shaft of the screw, but just a little, then you can take the screw out using a wrench or some pliers.

if it's too corroded or nothing seems to work you might try something like this Screw extractor | easyout | broken bolt removal - you can browse around on youtube and find different ways of doing it.

I would just get the second hand unit and be done with it. The welding trick is unlikely to work as you'll be more likely to damage or distort the alloy reservoir casing with that amount of heat that close to it. If you're lucky enough to drill out the old screw absolutely straight, then you might be able to recut the thread and reuse it. But the only way to get that lucky is to use a dpillar drill and clamp. 

At least with the second hand unit you'll be on the road sooner rather than later.