86' 300SDL Brakes Hard ONLY in COLD. - PeachParts Mercedes-Benz Forum

Putty · #1 12-12-2008, 08:49 PM

I have had a problem with my 300SDL from the day I bought it but never put much thought into it...Ive had it over a year and I was always wondering why it did this...

In the warmer part of the year...my brakes will be fine all day all night.

In the cold months...when my car has been sitting overnight in weather colder than say...35 or so, the brakes will be HARD like not having ANY boost. When the car is running for a little bit and the vac pressure builds up, the brakes will be fine for all day.

The next day on a cold night, HARD, like I have no pressure!

I thought it was the check valve but why only when cold?

I thought it was a failing vac pump, but when the booster charges up, I get a good 20" at the pump...and, again, ONLY when sitting overnight in COLD weather.

Perhaps I thought it was the switchover valve going wild until I realized the brake booster has a direct line.

Anybody had this happen to them? Any insights?

Cr from Texas · #2 12-12-2008, 08:52 PM

Did you try flushing and bleeding the system?
I suspect brake fluid contamination.

Putty · #3 12-12-2008, 09:15 PM

Quote:

Originally Posted by Cr from Texas

Did you try flushing and bleeding the system?
I suspect brake fluid contamination.

I thought that might be the case as well...but I inspected the fluid and it seems fine. The vac pressure seems to be gone in the cold mornings.

And it doesnt do it when warm...like I dont lose pressure when the weather is warm.

Putty · #4 12-09-2009, 10:58 PM

For some reason, getting back into the cold months...the same problem is happening...in the cold mornings, the brake pedal doesnt seem to have any pressure. I have to let the engine idle for a minute or two before the brakes seem to get pressure. Its bizarre because it ONLY does it when its cold outside. It never happens in summer.

jameson · #5 03-31-2010, 12:35 AM

Very same problem with my '86 sdl. The absolute only time i loose boost is on cold mornings before the engine revs up good. i can idle for 10 min, reverse around the driveway and go to put on brakes and have no assist! (very scarry since i back in front of the shed where the corvette is!) shift into drive and go, no problems for the rest of the day. I still have the original vaccum pump on the engine(300,000mi). Thats on the to-do list. Thats all i can think of.

bustedbenz · #6 03-31-2010, 12:52 AM

I wouldn't worry much. It's fairly easy for me to believe that the cold weather makes everything cold, contracted, brittle, less flexible, and less inclined to form tight seals.

I'd suspect a simple vacuum leak down that is prevented in warm weather by the fact that the extra warmth tends to expand things, make them more flexible, make things seal tighter.

Not even worth tracing down; likely just a minor, minor little leak that would be about impossible to locate. Once the pump catches up, it's exponentially faster than the leak so there's no more issues.

vstech · #7 03-31-2010, 02:13 AM

them thar vp's are known to grenade and take out the motor...
there is a seal at the back of the MC pull it and make sure it's not cracked.
aside from that, check all the little vac hoses and tees. i bet you have quite a few that are worn out.

2.5Turbo · #8 03-31-2010, 02:45 PM

Mine does this from time to time, usually after running for a minute or two it's fine. My VP is creating plenty of vacuum at this point on the accessory fitting. I took apart the vacuum hard line and it was OK, no leaks. I took the check valve out of the pump and it seemed a little sticky. The valve is ridiculously expensive and I'm going to replace my old style pump this summer anyway (and new pumps come with a new valve) so I'm just going to wait.

jameson · #9 05-06-2010, 09:15 PM

I installed my new vp yesterday with no problems, but we wont have weather down here anymore till next january so time will tell. wont let it worry me though!

btw, original vp looked just as good as the new! ha ohh well