Here goes:
Remove the cover next to the left lever, this reveals the brake reservoir. My SL had the comms control unit there.
Remove the two screws and the lid, this reveals a rubber concertina thingy which seals the reservoir and keeps the fluid free from air moisture. remove this carefully and have an absorbent rag handy. Paint really doesn't like brake fluid.
Under here you'll find the fluid. A transparent turkey baster is good for sampling it and checking condition and, if the fluid is brown the baster can be used to get as much out as you can. Keep it in a jar for disposal later, you'll need the jar soon. Once the reservoir is empty you can wipe out residual crud and add fresh Dot 4 fluid. Replace the rubber concertina or the fluid will squirt out when you try to bleed the brakes.
You should do the rear caliper first but I've only got a pic of the front.
Locate the bleed nipple (ooer Matron) It's the nut with a rubber cap (ooer Matron)
Next you need to create a set up like this:
Make sure you have the correct sized combo spanner or you're guaranteed to round off the nipple. Fit the spanner first then attach the tube. Its easy to snap the nipple off so place your little finger at the open end of the spanner and your thumb at the ring end to give more sensitivity when you first crack the seal. Quarter of a turn should be enough to release the brake fluid into the tube.
The tube you see in the pic is one of these:
https://www.ebay.co.uk/itm/Motorcycle-Car-Brake-Bleeder-Clutch-Bleeding-Tool-One-Way-Valve-Tube-Kit/363062076787?hash=item54882fa573:g:Y~EAAOSwWuxfInM-
(others are available)
If you now pump the brake lever repeatedly, the fluid will come out in spurts (matron) If the brakes haven't been bled for a while, the fluid will be brown, like coffee and you will probably get bubbles in the tube. This is what is making the lever spongy because the air compresses in the brake line, nice clean fluid won't. You might not get bubbles at first but sometimes they hide, keep trying. The valve in the bleed kit stops the air and coffee from being drawn back into the caliper. Every five or six pumps of the lever, check inside the reservoir and top up the fluid or you'll end up pumping air in which kind of defeats the object. The job is done when the fluid runs clear and there are no more bubbles. Tighten up the nipple (little finger at the open end so you don't overdo it) At some point the tube will miss the jar and squirt you in the groin and you'll jump and knock the jar over onto your favourite suede loafers and you'll say rude things. Be prepared for this inevitability.
Now (having done the rear first) do the front. When you're happy with that, top up a final time and replace the reservoir lid and cover. Now check the other front calliper (right hand lever)
You can do this job with a simple length of plastic tube and no bleed valve but it needs more patience and probably an assistant unless you have orangutan arms like me. An old battery breather tube is about right for the Job.
The process is the same but requires you to open and close the nipple to prevent air returning into the system.
We'll do this by numbers as it's important to get the sequence right
1. Pull the lever.
2. Open the nipple and wait for the fluid/bubbles to stop.
3. Close the nipple
4. Release the brake lever.
Repeat the process checking the reservoir level as you go.
If you can persuade an assistant to do the lever work whilst you do the spannering it helps a lot. Make your instructions to pull/release/check level clear and be nice or they won't help again.
And that's about it.
Oooh... If you've been through this and you're still not happy that the brakes are really firm: Try wrapping a bungee tight around the brake lever and leaving it overnight. I don't know why it works but it sometimes does.
Ooh... and dispose of the crap in the jar in an environmentally sound manner. Get rid of the extra fluid too as it absorbs moisture from the air rendering it unfit to use.
Hope that helps.
And, come to think of it, I think the caliper is on the wrong side to bleed with the wheel off which doesn't help. I'll sort some pics out this evening as I can't find a definitive guide to bleeding the linked brakes here. I may not be looking in the right places of course. 
