1984 380 SL brakes won't bleed
 

I have been trying to bleed the brakes in my '84 380 SL after the system drained itself. I don't yet know why the system drained itself. I have had the same thing happen on other cars, but I would put fluid back in, bleed the brakes, and everything was fine. I put fluid in the reservoir that same night.
But this one won't bleed.
I tried putting a hose on the bleed nipple with the other end in a bottle with a little fluid, open the nipple, then pumping the brakes. Nothing came through.
Next I put a vacuum bottle on the open bleed nipple and got nothing. But the vacuum held. I also tried pumping the brake while it was on the bottle. Still nothing.
Both rear wheels have been tried with nothing to show for it. I'm stumped and thinking that the master cylinder is to blame somehow.
Where do each of the three lines from the master cylinder go? The front line go to the front brakes and each of the opposing rear lines to a rear wheel?
And where did the brake fluid go in the first place? There are no stains on the floor or in the wheels?

The bleeders might be clogged, remove one and check for obstructions.

If the rear seal of the master cylinder leaks, fluid will enter the brake booster vacuum chamber.

Do you know of a way to check for fluid in the booster without removing it? A car guy at work suggested feeling the bottom of the booster for fluid, but I can't get my hand past all the hoses to get under there.

Euro designed cars typically have booster vacuum exposed to the rear of the master cylinder so any fluid leakage ends up inside the booster canister not outside it.

US designed cars have a vacuum seal on the booster and a drilling on the master to allow fluid to leak externally. This is likely the type of system your guy was speaking of.

All you need to do is remove 2 nuts from the MC to booster mounting and pull the MC forward, the lines can be left attached.

I'm no super tech but I'm betting that the rear portion of your brake reservoir is empty!

MB of this vintage have a reservoir with two sections, the rear section is filled by overflowing the divider between the two sections.

Get a good flashlight and you can more easily see the fluid level in that rear section of the reservoir. Sometimes wiggling the plastic reservoir left and right helps the rear section fill a bit faster.

Give the reservoir a good examination first before you start jumping to conclusions that "might be" despite any lack of direct supporting evidence.

The three brake lines from the MC, two go to each of the left and right front brakes. The third goes along the bottom of the chassis to a splitter located somewhere around and above the rear axle. From that splitter through the two lines to each of the rear brakes.

Your bake fluid likely leaked out through a brake line that has corroded, check the under chassis line where it passes through the rubber cushioned brackets, that is the usual spot that these lines corrode. Salt and dirt get in between the line and the surrounding rubber over time and vibration you end up with the line leaking.

Fluid is most likely in your brake booster. Had the same thing happen to me. Take off the 2 nuts and disconnect the hoses. Can be done in 15 minutes easily. Keep a bottle of water handy to wash off any wayward brake fluid.

Essentially means both your brake booster and your master cylinder are now bad and need replacing. If there's no fluid, then it means you should consider replacing all of the brake components like calipers and hoses since something leaked.

6was9 also has a good point. The rear chamber of the brake fluid reservoir can be empty. Make sure you fill it up tight until the bubbles stop. You really need to fill it up, do it to the brim!

Of the booster is filled with fluid, don't think you can continue using your brake booster without replacement. The internal diaphragm is now compromised and will cause a hard pedal.

Don't fret however, these are cheap and easy parts to replace. Remove both from your car and take them to your nearest NAPA autoparts. Most likely they will have both the MC and brake booster rebuilds in stock (check their website ). Swap them both for your core charge and reinstall.

It is possible to rebuold these yourself, but with rebuilds being so cheap I don't see the point in the added effort.

Thanks for the ideas.
I just rechecked for an empty or low rear reservoir. I can see fluid sloshing against the top all over the reservoir, so I think the rear is filled.
Thanks if6was9 for the clue to finding the splitter. I'll check it over the weekend. While I'm under the back I will reapply the vacuum pump to see if the vacuum holds with the bleeders open. I believe they did when I tried before, 97_SL320. I didn't know NAPA stocked parts for this vintage, okyourabeast, thanks for those tips.

Glad to help!

You'd be surprised what they have in stock for these cars. Always check first if you break something and don't want to wait. They sometimes price competitively with the online sellers too.