92 300D strange brake issues -booster or pump?
 

My power assist on the brakes has been acting up. Sometimes good, sometimes not.

Checked vacuum. Pump by itself does 23" HG. With all accessories connected there is 8" Hg at idle. There are no leaks - checked the VCV and shut off and the other one that goes to the cabin - they all hold vacuum.

The check valve on the booster line is fine.

However, the metal nipple on the vac pump (accessory connection) makes a buzzing noise and lets a vacuum through when applying vacuum with the mityvac. Should it do that?

The brake booster holds vacuum up to 20" hg (that's as much pumping on the mityvac my forearms could handle.)

Does that mean the booster is OK? Or could it be intermittent?

As far as I know, it's either the brake booster or the vac pump, but both seem to be behaving how they should. Or maybe the vac pump can't deliver as much as the booster needs?

Help! I need brakes...

Paul,maybe you could rig up a gauge and mount it where you can see it while driving then you may be able to see whats going on.When i was fooling with the waste gate mod i taped a pressure gauge to the outside of the windshield with clear package tape. Don

With a little extra hose you've could have ran it into the cabin. LOL

Where in FLA?

Vacuum Pumps
 

Don't you have 2 vacuum pumps... One mechanical and one electric under your rear seat??? Does it get really hard to pump up your brakes... Usually when it gets hard you have a bad booster.. Jim

Yes, but the electrical is just for the central locking.

I'm in Location: Fernandina Beach, Florida - in my profile info at the top. It's above jacksonville.

I have another 92 300D and it has 23" hg at idle, so there is a problem, no need to have a gauge - either the booster is screwed or the vac pump. How do I tell which one?

What is "Acting Up"?
 

When it misbehaves?
Are you experiencing limited power assist or none at all?
(which w124 is this [how many miles?] )
Is the fluid level (In the front or the back reservoirs) depleting [vanishing] ?

The brakes went out for the first time (no power assist) 2 days ago. Then it was OK, then slowly became consistently no power assist except for the first pedal push, then it would take a bit of driving to get the power assist back. So, ATM, it is first pedal push after a while of driving is power assist, then no power assist until a while (20seconds) of driving.

This is the non-aquatic car.

Fluid level at the reservoir on the booster hasn't moved - still between max and min. Back reservoir? Didn't know there was one. But the low vacuum in the lines leads me to believe it is not fluid leaking issues.

I'm leaning towards it being the vacuum pump. The booster held vacuum up to 20hg with my mityvac. Occams razor is telling me it is the vacuum pump unable to deliver enough vacuum under stress. But I don't know how to test that.

197K miles.

Yes, thanks, that's the one connected to the brake booster - and it holds vacuum fine.

When I apply vacuum to the other connection on the vac pump (that goes to the tranny and cabin), it doesn't hold it. Not sure if it should, or even if that would cause a problem or even whether that is part of the rebuild kit, or if storks really bring babies, or why the tooth fairy doesn't pay me for my teeth anymore..

VP
 

Olddiesel's idea with a twist...Cheap Vacuum gauge duct taped to the outside of
the windshield (WD40 will get all that goo off later)...run the line into the
engine compartment and "T' off the Brown line that "T"s into the Brake Booster
line.

Plus you've got a "Control" for the experiment.
(The other W124,with good brakes.)

The accessory nipple will not hold a vacuum with the engine off.

(Oh,Yeah the "Back" reservoir is really, really hard to see if it's full.You see it
best leaning over the Master Cylinder and looking as if you were on the passenger side with a flashlight shined from the driver's side.
It only fills after the front is full,sort of a "slop over")

I reread your Original post
 

If the vacuum decreases (With the engine running) when you hook up all
the accessories [from 23"Hg with only the pump to 8"Hg with everything]
something's bleeding off your vacuum(or the pump's bad as you suspect).

Whoops! That Brown Vacuum line that "T"s off the Brake Booster line...
will it hold Vacuum???

"something's bleeding off your vacuum(or the pump's bad as you suspect)."

Yes, exactly. And the bleeding off vacuum suspects have all been eliminated - accessories hold vacuum, so does booster...

And the brown line T'd (to ignition and then shut off valve) holds vacuum.

Above tests all done with car off. Perhaps I should have it running and see. Something is dumping vacuum. I'll check them all with car running / key on and post back.

Mine does this from once in a while. Almost always happens after a cold start in the morning. For about the first quarter mile driving through the neighborhood, about three or four stops, I have no brake assist at all. Then on the fourth or fifth stop, it magically starts working again (and I hang myself in my seatbelt ;)) and will work for the rest of the day.

I'm pretty sure the booster is good because there's still residual vacuum in it after sitting overnight. I think I've narrowed it down to the check valve on the fitting between the big vacuum line and the pump. I took it out and it seemed a little sticky. Haven't tried a new one to see if it fixes it...probably something I'll do when I get home this summer.

I hope it's when and not if. I drive my car out with all sorts of stuff in disrepair but never the brakes :eek:

Sixto
87 300D

Yep, something I'll do when I get home. While I'm at school the car's 2750 miles away and I drive the Subaru ;)