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.