if they are really sludgey you may want new ones. you can get aftermarket ones now at a reasonable price

if they are clean you may want to only bleed them. stick em in a vice ( soft jaws and something to go up the inside) and gently compress them 2-3 times

to service:
theres a couple different types

early ones have a tin retainer that hold inside in.you can gently pry it off with a hook. then pull the inner out. i think there the ones with the little cap on top of the inner.take the inner and push the tiny ball off its seat with a small piece of wire and compress it a couple of times to bleed out oil. wash in solvent. reassemble with the little cap on top of the inner,if it wont stay there use a tiny bit of grease as an adhesive. squirt a little oil into the body. push the inner into the main body. done

late ones have a wire retaining ring on the top of the inner and dont have the little cap inside. to disassemble you need a flat clean steel bench and you strike the lifter on the bench dead square. it hurts your fingers and it can take a few shots. you may damage the bottom of the body so check them. inner will then come out bleed and clean as above. reass as above.

lifters must be squishy when you refit or the valves will hang open and the motor wont start. pistons may hit valves too

dont rev engine until lifters quieten down. they can take a while to bleed all the air out. let it idle for a while.

if you have any dramas pm me