W116 Brake Issues

[sailor15015]

My 1980 300SD has been sitting since last fall because of an inoperable servo and when I went to drive it a couple of weeks ago I noticed the brakes were locked up. I could definitely hear the rear brakes dragging but I'm not positive about the front brakes. I was told a good test for the brake booster is to pump up pressure on the pedal when the car is off, hold the pedal and start the car, and if the booster is good then the pedal should slowly sink. I did this and got very little pedal movement, like less than two inches. All of my vac lines except the one going to the key switch were plugged anyway but I went ahead and plugged it as well and verified that I had around 20" of vac at the main line and still no change. I'm not afraid of overhauling the brakes on this car (I just did the front calipers and tie rods on the '84 last week) but I would like to have an idea of what the problem is before I start spending money. Is it more likely that the calipers have seized from old fluid/parts/sitting or could it be the master cylinder, or possibly the booster itself? Thanks in advance.

[kerry]

There's not a lot of pedal movement with that test so I wouldn't condemn the booster yet. I think the problem is probably calipers but it could be as simple as rust on the rotors. Does the car drive but the brakes drag or is it locked up enough so that it's not really driveable?

On a related topic, do you feel a difference in pedal pressure between your D and SD? I don't like the feel of the pedal in my 79SD. Near the bottom of the travel, the pedal gets touchy with very little additional effort applying more braking power. It makes it very easy to lock up the brakes in slippery conditions. I don't know if this is a quirk in my particular car or if there are basic differences in the engineering of the braking system. It looks like the SD has a much bigger booster canister.

[sailor15015]

It'll drive but it takes noticeably more throttle effort to get it moving and when you put it in drive on a level surface and let off the brake pedal it will barely move if it will at all. The rear brakes are audibly dragging but I either can't hear the front ones over the engine or they aren't. I need to raise the front and check.

Originally Posted by kerry

On a related topic, do you feel a difference in pedal pressure between your D and SD? I don't like the feel of the pedal in my 79SD. Near the bottom of the travel, the pedal gets touchy with very little additional effort applying more braking power. It makes it very easy to lock up the brakes in slippery conditions. I don't know if this is a quirk in my particular car or if there are basic differences in the engineering of the braking system. It looks like the SD has a much bigger booster canister.

I read your thread that was never replied to when I was searching on this issue. I didn't drive the car all that much, and its been a couple of months but I seem to remember it being more touchy. It stands out in my mind when I was bringing it home that I did make it lurch a time or two when I hit the brakes.

[rs899]

I'd second Kerry's thoughts, either caliper or just some rust on the rotors. Not likely at all that it is the booster, IMO.

Have you ever replaced the rubber brake hoses? When they fail they do not allow fluid to pass back to the M/C, keeping pressure on the calipers. They may look good on the outside, but can be bad internally. If they've never been changed, its worth doing (and worth doing sooner than this).

Rick