126: Brake Pedal Hard on start
 

Hello,

I couldn't find the answer for this searching so I will post a new questions.

My 300SD is having problems with a stiff pedal after 2 times when leaving the driveway after being parked. The pedal is good intially, but then becomes hard after 2-3 tries. Seems much worse than the other (older) 300SD.

Here's what's been done:
1. Vaccuum Pump - dealer replaced about a 1 year ago so I don't suspect it currently. Getting about 20" Vaccuum.

2. Brake Booster and Master Cylinder - just did this myself -- using parts from fastlane. Didn't help much with the cold start problem, but car stops better once running. Brakes bled using Motive Bleeder. Valvoline Syn Dot 4

3. Vaccum Leak testing - swapped the entire booster line from the other car - no help. SO hose and check valve must be okay

4. Leaks in lines - I can't seem to hold good vaccuum on the brown (IGN?), or red/green (AC?), or the door lock. The white (TRANS) line is holding 15" vacccum.

Question#1: Is the brown line supposed to hold vaccuum once car is on? Car shuts off fine... door locks...working. Trans, shifts fine. EGR - temporarily bypasssed for diagnostics. A/C Vents - working, although I had leaks in the dashpots last year.

Question #2: Could leaks from these "small" vacccum lines cause the pedal to become stiff. Has anyone thought of putting one-way check-valves in on these auxillary lines?



:confused: :confused:

To test the condition you describe I would install a fitting we built instead of buying the real tool. It is a fitting about an inch long that screws to the booster and the line screws to it. It also has a section of steel brake line welded to the tube in the center making a test port for the vacuum gauge on the booster side of the plastic line's check valve.

It is most likely that you are at very low vacuum when the condition exists. There are two possible causes. Too little source or too much leak. When shut off a gauge placed at the point I describe should hold vacuum for significant time. Fully charged with vacuum three applications of the brake pedal can deminish the vacuum (if not replenished) to the state causing hard brakes.

You need to check for leaks, evaluate the use rate of several brake applications with motor off, and then watch with no vacuum how quick it builds after starting. The only other thing to evaluate is whether the same finding take place when the event occurs.

The system is designed with enough vacuum capacity that all the small source tees from the line could be left open and the size of the hole would still be small enough that the pump will still quickly supply the more important braking vacuum.

The brown line is the shut-off vacuum and if there were vacuum in it when running you wouldn't be running. You would be shutting-off. There are actually two brown lines one has a tracer. The source line off the booster line will have vacuum if the pump is working. That line goes to the steering/ignition lock/switch and when shut down the switch allows the vacuum to flow through the other brown line to the shut-off diaphram on the back of the injection pump.

Question: has there been a modification in design of the brake booster line? I have picked up a replacement line...the line resembles the old one, including the white check valve, except that the two accessory ports for the IP and Ignition/ACC system are located outside the firewall closer the vaccuum pump on this "new" line whereas the original line has the two accessory connections right next to the brake line.

Since I really am interested in just changing the white check valve, does anyone know if the brake hose disassembles...the fittings seem really tight and I don't want to break it...Are they glued together?

Thanks

maybe I am expecting too much....here's what I notice still with the new master and booster (ATE)

1. Three applications of the brake will turn the pedal hard...is this "normal" while driving at city speeds in that will 3 quick sucessions cause the pedal to become hard, until the vaccuum catches up and improves it in about 20-30 seconds.

2. The vaccuum connector at the door lock vaccuum connection reads -18 PSI and holds the vacccum steady while car off...I capped off the ACC vaccuum line and that seemed to help slightly (I stilll have some leaking ACC dashpots I believe, but would that affect brakes?)

3. Also, what fluid do you guys recommend in terms of fluid? I put DOT4 Valvoline Synthetic...I'm wondering if I'll have better performance and braking power by going with genuine mercedes fluid....

4. I did accidently introduce air during bleeding after changing the booster and master in the left front wheel bleeding. I believe I have got it all out with the power bleeder (bled all four wheels again RR, RL, RF, adn LF)....if there's residual air it would causing a sinking pedal adn not a hard pedal right?


:confused: