Brake Troubles w/ a 450 SL - PeachParts Mercedes-Benz Forum

79450sl · #1 11-08-2003, 05:20 PM

The brakes were fine on the car. I then changed the front subframe mounts, steering gear box, tie rods, vacuum modulator, and two check valves in the vacuum system. After putting the car back together and bleeding the brakes (had to remove the brake lines to do the subframe mounts), the car had no stopping power. I have since bled the brakes twice more, and even went to the extent of replacing the pads and rotors (thought I may have gotten something on the old pads).

My problem still persists. Am I missing something when it comes to bleeding the brakes? Could I have altered something in the vacuum system after fixing the other vacuum leaks that would cause this new condition?

HELP!!! I am going insane!!!!!!!!!!!!

-Rich
'79 450 SL 185K

psfred · #2 11-08-2003, 09:43 PM

Hard or soft pedal? Any funny noises?

Peter

79450sl · #3 11-08-2003, 11:24 PM

The pedal seems to travel farther down than it used to. The brakes used to grab earlier than now. I have to put the pedal about 3 inches further before there is any noticable grabbing of the brakes. I can put the pedal down to what seems to be as far as it will go, and the car will coast to a stop. But its funny......the pedal is not really soft. If I pump the brakes while the car is running, there is a change in RPM( very slight, but noticable).

Any ideas???

-Rich

psfred · #4 11-09-2003, 07:50 PM

Air in the system or a leak.

Bleed the front brakes again -- you can use gravity by attaching a plastic hose to the bleeder and running it into a container. Just open the bleeder a quarter turn or so and keep the reservior full.

Most likely you either allowed the reservior to get too low or didn't bleed long enough to get all the air out.

Bad vac will give you a very hard pedal, but the brakes will work fine if you apply enough pressure to the pedal. Soft pedal with excess travel is air or a leak.

The last possibility is that when you bled the brakes and the pedal went all the way down the master cylinder seals failed -- the intake bores on the master cylinder aren't chamfered on MB master cylinders, so a bit of rubber from the piston seal can get nipped off. Ditto for dropping the piston to the bottom of the bore -- any roughness there form ancient brake fluid will chew up the piston seal, and the pedal sinks.

Quick test is to rapidly pump the pedal -- if it suddenly gets firm and the brakes work, but goes soft again after release, the master cylinder need a rebuild kit or replacement -- some have a kit readily available and some don't.

Peter

79450sl · #5 11-10-2003, 09:36 AM

Thanks for all the info. I'll try to get at it this week, and post the results when done.

Much appreciative,
-Rich

haasman · #6 11-10-2003, 10:32 AM

Check also the flexible brake lines. If your '79 450 SL with 185K miles has never had them replaced, I would do that.

I read somewhere on this site someone was having similiar problems and after many repairs, it was internal deterioration of the hoses.

Keep us posted,

Haasman

79450sl · #7 11-22-2003, 06:22 PM

Well guys, I bled the brakes again and it is still the same. I then pulled the master cylinder out, tore it down, cleaned it (the seals looked like they were brand new) and put it back together. Still no improvment.

While test driving the car I could hear what sounds exactly like a vacuum leak (could not hear it before due having the top down.......its getting cold in Chicago, so the hard top is on it now). I stuck my head down by the pedals, and as the brake pedal is depressed I could hear the "hissing" associated with a vacuum leak. The sound would continue until the pedal was returned to a resting position. If I were to hold down the pedal and slightly separate the rubber boot surrounding the "brake unit pressure rod" (as the shop manual refers ro it) the hissing became louder. I removed one end of the boot from the booster side (closest to the firewall) and depressed the pedal and the "hiss" was much more noticable.

Another thing I noticed is that when the brake pedal is depressed, the RPMs begin to rise. The farther I push the pedal down, the higher the RPMs get. Its only a difference of idle to idle+300 RPM, but definatly noticable.

My questions here are

1. Should the RPMs change that much when in park, at idle when the brakes are applied??

2. Should there be a consistent "hissing" while the brake pedal is depressed??

3. Is the consistent "hissing" a vacuum leak causing flutuation in engine RPM????

This is driving me insane!!!!!! I have to get the car in for emmisions testing by Dec. or they suspend my liscense.

HELP!!!!!!!!!!!

Thanks guys.....
-Rich

psfred · #8 11-22-2003, 10:01 PM

Sounds like the diaphram in the booster is shot, so it leaks when you step on the brake. The booster is supposed to be evacualted, and when you step on the brake, it is progressively vented. This applies pressure on the brake master cylinder.

If it hisses when you apply the brake, it's busted -- this happened on my old Dodge Aries in Canda in bitter cold weather -- engine would die when I stepped on the brake (manny tranny). Sure enough, when I listened carefully, it was hissing when brakes were applied. New booster fixed it.

The rise in idle is a givaway, too -- it means you have more air going down the manifold, in this case from the booster and not the idle circuit.

Peter

79450sl · #9 11-22-2003, 11:37 PM

Does anyone know of a good source to get a rebuilt booster?

I dont think its possible, but can I rebuild this myself??

Dont want to shell out $400 for a new one. I've spent enough $$$ this summer on parts.

-Rich

79450sl · #10 12-10-2003, 01:46 PM

Well guys, the problem is solved. It ended up being the booster which was bad. I guess that the diaphram had ripped when I had bled them the first time. Damn 25 year old cars!!!!!

Thanks for all your help guys!!!!!!!!!!!

-Rich