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You can use Dot 3, 4, and 5.1 - not the ".1" on the Dot 5.1. DOT 5 is silicone fluid, and the system must be moticulously cleaned prior to switching, but 5.1 is 'super' regular fluid, and can mix with the old. Basically, as you climb the number scale you mainly climb in boiling pont as well. 5.1 adds more chemicals to neutralize water contamination (you get water in the system no matter what as heat cycles in the engine compartment cause condensation in the brake reservoir in 95% of the climates). Some varieties of DOT 5.1 claim you can leave it in for up to 10 years, but I'd still flush with fresh fluid every 3 or 4 years. Water contamination lowers the boiling point (boiling brake fluid is very, very bad - causes temporary brake failure) and eventually causes your system to corrode from the inside out.
Stuck and broken bleed valves are endemic on old cars in many climates. The previous post was your best bet - remove the cylinder, soak the bleeder with PBlaster for days, then give it a good try with some vice grips. But don't hold out too much hope, it's probably well fused. You might try a local parts store for new wheel cylinder, several years ago I asked on a whim at one and they had them in stock for about $15 each. They obviously weren't in the business of stocking Volvo PV parts, but they apparently cross-fit some 60's American car. Otherwise the usual vintage Volvo part sources should have them for about $50 each. Before you start bleeding the new cylinders, remove the bleeder and coat it with antiseize. It probably wouldn't hurt to put a glob of grease over it when done too. Ups your chances of it working 4 years from now when you re-bleed to flush out the old fluid.
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I'm JohnMc, and I approved this message.
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