No power brakes and car won't shut off.
 

I have been trying to tackle this issue for 6 monthns now and I am at the brink of giving up. I have with me 1984 300D with 410k miles. It all started when I noticed the MC leaking I had it replaced with a Cardone MC from Amazon it came with a pre installed o ring seal. I picked up a brake booster from a part sales men whom I frequent when I need parts.


Anyways I installed the booster, master and bled the brakes of course. I started the car up the pedal dropped a tad bit indicating boosters I then tried turning the car off but it just continued running
(Am I loosing vacuum?) The only way I could get the car to shut off is when I press and hold on the brake pedal the engine gradually would shutdown.

I plugged unplugged the brake booster main line and clogged it with a Vacuum cap low and behold car shut off. (mhmm Bad brake booster???)

FYI

I tested the vacuum it pulls well over 21HG with my finger on the line. (Vacuum pump is good!)The transmission shifts like BUTTER! I disabled the door locks. Now when I measure the vacuum on the main line it reads 10hg low vacuum.


After tinkering with this issue I decided to check the vacuum again. It's 100% on sync and working I went ahead and bought another brake booster this time from this forums and the seller is a pretty reputable member He swore it holds vacuum.

I installed and yeild the same results. I don't know what to do!! Two defective brake boosters?? How in the lords name is that possible. Inspection is due in 2 months I really need to tackle this issue. Hell I have literally tried everything I don't know what to do.

If anyone can give me some insight on what it could be?



https://www.amazon.com/Cardone-Select-13-2326-Master-Cylinder/dp/B000EPZTPW/ref=mp_s_a_1_3?keywords=master+cylinder+w123&qid=1580160297&sr=8-3

is this a vacuum pump test?

if yes then where exactly did you take that measurement?

I forgot to mention towards the end of the sentence. It measures 30hg with my finger over the main hose. When I plug in the main brake booster line in the booster the checkvalve on the line measures 15hg

check 3 things:


1. Did you install a rubber o-ring between the master cylinder and the brake booster?


2. It maybe hard to find a fitting but try to measure how much vacuum you have at the end of the big hose that fits into the booster

3. how did you bleed the brakes/master cylinder?

.

1. Yes it came with the master cylinder


2. I put my finger over the hose and it measured 30 hg with my mityvsc


3 I bled it through calipers

Since you mention in your first post that it all started when you did all the work at once (master+booster) and from reading what you diagnosed (good vacuum at the end of the pipe and o-ring at master cylinder) AND you tried 2 boosters then the only thing that I can think of is air in the hydraulic system.

I could be that the no shutoff you're experiencing is caused by a leak in the shutoff system that gets worse when the booster is hooked up.

did you use the foot method? gravity? a power bleeder?

did you bleed the master cylinder before installing it?

I didn't bench bleed the master cylinder I only bled through calipers. I had a helper help by pushing the pedal

Intermittent vacuum failure is commonly caused by a broken check valve at the vacuum pump inlet. The valve components rattle around in the pump inlet, causing the vacuum issue to "come and go."

If you measure solid vacuum with the booster line capped but weak vacuum with the booster connected, odds are good there is a leak.

The master cylinder could be bad, are you losing fluid? Did the old boostedpr have any fluid in it?

This would be my third master cylinder testing. The one I put on came with a pre installed seal.

new check valve for the pump already put on

Guys - I thought it was always essential that the MC be bench bled. Years ago I had a similar problem and had forgottten to bench blead the MC. Took it out, bench bled, and reinstalled. Instantly problem solved.

Insure that nothing is restricting the pedal from sitting where the MC wants to place it, i.e. not bottoming out on the brake switch or such. If so, that is like the pedal is slightly pressed, which would let cabin air into the booster (how the booster works). If so, insure the output rod to MC adjustment is correct. I can't recall on my M-B, but many boosters have an adjustable tip on the output rod. You are supposed to adjust that until it barely touches the MC piston when bolted up (or even a slight gap). Remove the O-ring and feel for that as you press the MC up to the booster. If you must push in the output rod a bit before it touches, that is likely your problem.