Bad servo? '79 300TD - PeachParts Mercedes-Benz Forum

babyjames · #1 10-16-2006, 10:55 PM

Pulled the master cylinder out of the way, and the booster had a nice pool of motor oil at the bottom. Used the mityvac to suck the oil out, installed a new o-ring, and put the master cylinder back where it belongs.

Ordered parts and rebuilt vac pump. Better, but still not a ton of assist on the brakes.

Booster holds vacuum, but drops by about 5 in of Hg when the brakes are applied (i.e starts at 15, then drops to 10, and holds, when the brake pedal is depressed and released).

Think the booster is shot? The brakes feel great until the car warms up....

Thanks,
Jay.

P.S. I tried disconnecting everything vacuum (plugged the line) except the brake booster and drove the car (before I overhauled the pump) and got no assist. Needless to say, I want to eliminate EVERY possibility before I shell out the cash for a new booster.

kerry · #2 10-16-2006, 11:37 PM

I'm not clear on why you bring up the servo? How did you determine it wasn't brake fluid as opposed to motor oil?

unkl300d · #3 10-17-2006, 12:20 AM

see your other post....slow down! get things on the table and checklist away before tossing your bucks for parts!
Cheers

babyjames · #4 10-17-2006, 11:53 AM

Brake fluid is rarely pitch black, and also has a very distinctive smell. The brake fluid reservoir is full, and I saw no evidence of leakage.

Slow down? What more can I do to diagnose this? The fact that the brakes feel great until the engine warms up leads me to believe the master cylinder and calipers are o.k (the car stops fine, just with a WHOLE lot of pressure). I'm pretty confident in my ability to overhaul the pump ($15.24 in parts); good insurance, anyway, as a vac pump failure can be catastrophic. Everything points to the booster - - the prime suspect from the start. Just looking for a little reassurance on this one....

Thanks,
Jay.

JimmyL · #5 10-17-2006, 12:04 PM

Have you isolated the vac line from the vac pump to see how much vac the pump is pulling? Plug the end to the brake booster, and check the vac coming off the T in the line (if two T's, plug one). If you are getting around 22 'whatever units' of vac, then the pump is OK. I know you plugged the lines and ran vac to brake booster only, but this info would help you with a direction.
Oil in the lines and in the booster can sometimes be a rupture in the shutoff valve on the back of IP. Any oil dripping our of ignition assy? Bypassing all vacuum except brake booster would tend to make this unlikely, however if bad, the oil will migrate to brake booster, and can cause it to fail.

babyjames · #6 10-17-2006, 12:15 PM

No oil inside, yet, but I know the shutoff is the source of the oil, as it doesn't hold vac. The pump was pulling in the low 20's before the overhaul. There was oil in the front of the pump, but probably from the shutoff rather than a ruptured diaphragm (sounds ouchy!).

I haven't checked the vac pump output since the overhaul.

Jay.

babyjames · #7 10-17-2006, 02:39 PM

Ran a line into the cabin so I could watch the vac gauge as I drove around. When I apply the brakes, the vacuum goes to zero. Started plugging through the FSM, what about the check valve? Any possibility that is the culprit?

Jay.

unkl300d · #8 10-17-2006, 09:02 PM

Your call, if you feel confident that you have no observable vac leaks from lines, t's, etc., the maybe you can pick up a cheap used/working booster and install. Observe the results. You seem handy. I know that this idea is redundant cuz if it is the booster you'd maybe want to find a better one. I dunno....

is your engine shut off still dieseling? maybe it is the shut off valve....???
good luck.

babyjames · #9 10-17-2006, 10:46 PM

I know you're trying to help, but if you read the previous posts, you'll notice I completely isolated the booster by disconnecting the small line that branches off the main vacuum supply and plugged it. If it were just the shutoff leaking (and it isn't), then the problem would have gone away when I did this.

I'll be installing a used booster tomorrow. I'll let you know how things went.

Jay.

kerry · #10 10-17-2006, 10:49 PM

I agree. It's the booster.

JimmyL · #11 10-18-2006, 01:14 AM

I've heard oil from a ruptured shutoff valve can ruin the brake booster. Sounds like you are confirming that bit of info.
Let us know how it goes. Also, I'd like to know how bad a booster change is.........

babyjames · #12 10-18-2006, 10:01 PM

Got a used booster and master cyl. from an '82 300D. The two are not directly interchangeable, but my trusty hacksaw helped change a bushing into a spacer, and voila! POWER BRAKES!

Had the functional booster and a functional shutoff servo in place in less than two hours. Didn't have to diconnect the brake lines from the m.c., so no bleeding necessary. Keys to this operation: an air ratchet and a wobble extension. If you don't have wobble extensions, wait until the set goes on sale at Harbor Freight and buy one. For $10 you get nine or ten extensions (1/4, 3/8, AND 1/2") that allow the socket to wobble a few degrees (nine, I think, to be exact). These aren't pretty, and don't stay on the ratchet all that well, but they are INVALUABLE! You will thank yourself ten thousand times over for bying them! Especially if you have air, 'cause you can't always get a straight shot at the nut/bolt with a bulky air ratchet and hose to deal with.

Anyway, the two nuts you can see get the m.c. out of the way. After that, remove the d/s kickpanel and the cruise brain, then the bolt (or pin on later models?) that attaches the piston to the brake pedal arm, and the four bolts on the inside of the firewall that hold the booster in place.

Getting it back in was a little tricky, as there is a reinforcing plate on the inside that wants to fall down so the holes don't line up. Take a 90-degree pick and use it to hold the bracket in place while you get the bottom two studs on the booster started in their holes; get your pick out of the way, and get that booster in there. Tighten everything back down, and your done.

If you are using a later-model booster on an older car, p.m. and I'll tell you what modifictions are necessary.

Jay.