83 SD Hard brakes. Signs of a bad booster?

[netboy]

My 83 SD has the opposite of soft pedal caused by air in the lines. The pedal gets hard but will not stop the car. So far I have bled all 4 calipers, replaced master Cylinder, changed the front passenger caliper because it was frozen, VAC SYS is producing 18 inHG and holds it on shut down. I removed the booster but it sounds like the bellows are not torn. I have another booster from a 300D but not sure if it will fit on the SD correctly. Looking for directions to test a brake booster or any ideas on troubleshooting direction. Rear calipers?

[Rogviler]

I've never had to actually test a brake booster, as they've always made noise when they go bad for me. But basically, there should be a change in the pedal if you're pressing down on it while you start the car, and again after you turn it off.

I will also say that when I was testing my vacuum system I had the brake booster disconnected for one drive and it stopped just fine (I have a wagon, I would guess that's a comparable weight), but of course required more effort. My booster was bad (hissing), but still provided assistance. So if your car just plain doesn't stop then that wouldn't be my first suspect, although since you've replaced the things I would suspect more then you might as well replace it if you think it's bad. It's way easier than people say it is, especially if you have several long extensions to snake through to get to the top nut.

-Rog

[tyl604]

Even with a bad brake booster, you should be able to brake the car; it is just quite a bit harder. You can even drive the car for a while if you are careful but it is somewhat dangerous.

This sounds like something else but I am not sure what.

[Rogviler]

Oh, also, the boosters between the 123 and 126 are basically interchangeable. The 126 is thicker, so going to a thinner one might require some lengthening of the main vacuum line. Other than that they swap. The 123 is firmer, the 126 is softer, but both will stop the car as quickly as you want if they're in good order. If you have a spare I'd throw it on.

-Rog

[BillGrissom]

When the booster in my 65 Newport failed, it felt like trying to stop a freight train. I even rolled thru a stop sign even though not going fast, and that car has 11" drum brakes (less effort than disks). That car does weigh more than my 300D's (~4000 lb vs ~3500 lb). I was surprised how hard it was without the boost. BTW, when I opened the booster to inspect, it had a hole in the rubber diaphragm right where the vacuum port attaches, and the rubber was brittle there. Must have been from gas fumes, even though I installed an in-line charcoal filter. You won't have that concern in your 300SD.

Yes, you can test a booster off the car. Just hook it up to a vacuum source. If not your diesel, use intake manifold from a gas car that is idling. As you push in the pedal rod, the output rod should follow your motion, and you should hear no steady hiss from leaking, though it will hiss as it moves since it lets air in by the pedal rod to move. I did this on my 1960's Chrysler boosters. But, your 300SD booster is different. Like most pancake style ones, it requires a vacuum seal between the booster and MC, so you need an MC bolted up. You then can't see the output rod move, but you should feel the booster trying to follow your input and give little resistance like it would without vacuum.

[netboy]

I've got some extra brake boost vac lines from other MBs. I'll hook that up to my vac pump with the MC install and see if it assists.

[Zacharias]

I am having a similar issue with my '85 300d but the car stops okay. It's just way too hard to mash the pedal.

I have done two calipers and I have about 19 mhg vacuum.

Have also completely flushed the fluid and replaced with new ATE.

That made the pedal slightly better but just slightly. Compared to my wagon it's almost like manual brakes.

[Diesel911]

Originally Posted by netboy

My 83 SD has the opposite of soft pedal caused by air in the lines. The pedal gets hard but will not stop the car. So far I have bled all 4 calipers, replaced master Cylinder, changed the front passenger caliper because it was frozen, VAC SYS is producing 18 inHG and holds it on shut down. I removed the booster but it sounds like the bellows are not torn. I have another booster from a 300D but not sure if it will fit on the SD correctly. Looking for directions to test a brake booster or any ideas on troubleshooting direction. Rear calipers?

Unscrew the check valve from the body of the Vacuum Pump and see if the parts are still in it. If it is hollow you need another valve.

If it is gunked up hose it out with WD-40. Don't use Brake or Carb Cleaner as the valve inside is plastic and either of those may soften it making is sticky or eat it up.

For more info check the repair links: http://www.peachparts.com/shopforum/diy-links-parts-category/

[netboy]

Hooked the original SD brake booster up to a vac pump and it ops ckd good. Found out that a SD booster will fit a D, but a D booster has too large of diameter and won't fit on a SD..The only thing I haven't checked are the rear calipers but the front brakes are independent from the rear and should be able to stop the car..May have to rebuild the rear calipers but still puzzled about the pedal...

[Rogviler]

I never thought of the diameter being a problem. What's in the way?

-Rog

[moon161]

Your car is a danger to yourself and others like this, and you have to fix it before driving more. It won't get better by itself, you can't supply the force needed to stop in a reasonable distance.

Vacuum wise, you need in the 20's at least. I think spec is 21-23 in Hg.

Revving the motor can bring this up, but it's not a fix.

You can also pull all the accessory (EGR, and vacuum to the pax cabin) lines and plug the T with golf tees or 12 penny nails. This can help a failing VP if your locks/EGR, vacuum reservoir are leaking, but not a fix, a healthy VP overcomes this leakage.

Vacuum Troubleshooting:
Mercedes Vacuum Troubleshooting

Check the main vacuum line to see if it holds vacuum, replace if you need to.

When I had this problem, I think it was replacing the check valve, or pulling the VP from a parts motor that fixed it.

Some people raid JY cars (some Volvo's?) with electrical vacuum pumps to replace (not a direct fit) the Mercedes pump with an electric one.

I'd get a check valve in the mail right away. If you can hit a parts yard locally, get a vacuum pump and check valve from there. You need to buy or cut a gasket if you replace the VP.

Whatever you do, remember you're on a lead sled till you fix it.

[vstech]

when my SD began showing stopping issues, it also displayed slow shut down issues...

then the timing chain exited the valve cover.

fix it before you drive the car!

[netboy]

Thanx Mom