86 300sdl Brake pedal sinking at idle
 

Good evening all,
a new perplexing problem with my 86 300sdl. First of all I have new rotors, brake pads, sensors, and rubber brake hoses. Fluid level in the master cylinder reservoir is normal and not changing. Brakes work excellent EXCEPT occasionally at idle at a red light my pedal will sink slowly to the floor almost.
At first I thought master cylinder, but no leakage and no fluid level drop.
In my driveway with the car off, I can pump the brake pedal two or three times to release all of the built up vacuum in the booster and the pedal sits there hard as a rock. So I am thinking not the master cylinder.
Today at a red light it did it again so I pumped the brake pedal 4 or 5 times and the pedal comes all the way back to normal, but I hear a whooshing sound at my feet. Could my booster be leaking? Could I have a vacuum leak somewhere? My transmission shifts perfectly so I was ruling out the vacuum source for it, BUT... a Climate control vacuum leak? Could that affect your brakes?

Thanks in Advance for all comments.

ok.
if you come to a stop, and hold your foot lightly on the brake, and the pedal drops to the floor, your seals in the MC are shot and you need to either rebuild yours or put a new/rebuilt one in.
period.
sometimes they will leak internally and not lose any fluid.
sometimes they will leak into the booster, and you will lose fluid.

You know, my common sense told me that, but I was just hoping it wasn't the master cylinder. Time to buy a new one. I have read that you must bench bleed a master cylinder prior to installation,. I have never done this before. Is there a kit you can purchase?
Thanks for the quick reply.

What he said.

If it is sinking, step down harder. It often will stop sinking if it is a bad master cylinder.

Sometimes a brake job will cause this failure, some feel that it is due to pumping the brake pedal further than the normal travel to bleed, which can damage the seal as it crosses a wear-ridge in the bore.

Yep, new MC time.

most MC"S come with a bench bleeder kit.
for some reason, the MB MC's don't. I think if you put a rag under your mc, install it, with the lines loose, then fill the reservoirs, they will bleed themselves. when clear fluid weeps out the line nuts, tighten them, then proceed to bleed the brakes at each wheel normally.

Awesome! That is the way I did it on my old Toyota Corolla ( 1974 ) way back when I was in college. Believe it or not that was the last master cylinder I ever had to install. This MC in my 86 is the original one with 24 years and 122,000 miles on it. I guess I have had good service out of it. The MC in my 85 380se with 257,000 miles on it is the original. I am a firm believer in yearly brake fluid changes. The sdl does not get driven as much and this probably had an affect on my calipers and brake hoses which I just replaced.
My old brake hoses were almost swollen shut.

Thanks for the input and advice!

& if you are real careful when you disconnect the lines from the old MC, when you put the new one in, if the MC is bled you can get away with not bleeding the whole system. its worked for me.
You do need to change out your BF regularly though, if you havent done that in a while, best do the change.

1.Power brake booster seal is bad.
2. Napa parts sells a metric bleeding kit ( reusable ) .. about $10 when I got one last.
3. Do not try to install a MC which calls for bench bleeding without bench bleeding. I have that that in my younger days and the work taking it back out to do it correctly is totally wasted energy.